Research Record Terrain Evaluation Highway Planning and...

36 Transportation Research Record 892

ties and llmitations of an area of grounil in rela-tjgq to q particular !in{ of latld use. Tle wordIterrainr refers not merely to the shape of the landsurface but to aLl the factors that, act ín co¡nbina-tion to mold the land itself, nanely the relief,geology, soils, and water conditíons. The advan-tages of using a terrain evaluation approach toengíneering surveys are ãs follows:

I. By studying a problen withín its region, itis possible to see all available options for routes.

2. À terrain evaluatíon wilI indicate pointswhere the terrain changes from one type to another.ft is possible to devise a sampling program to coversítes that are typical of large areas an¿l thus toextrapolate information from known to unknown groundlwith some degree of confidence. A regional approachto napping also helps to draw attention to particu-larly inportant indívidual sites that nay influencethe design.

3. The study of sites in relation to the terrainemphasizes the fact that data should not be treatedas a series of isolated points but as being repre-sentative of a continuous nedium that varies in waysthat can be defíned.

4. By relating infornation to a classíficationof terrain units, it is possible to use the sameunits from the early phases of planning through tothe final stage, so that data can be carried forwardand amplified as necessary in each succeeding stage.

For the híghway engineer, the terrain evaluationprocess shouli aíd such díverse activities as deter-nining the alignment of a highway, ídentifying ile-posíts of gravel or rock for u6e in construction, orarranging a soil sampling prÕgran. A terraín evalu-ation is able to do this effectively when baseil on aterrain classification, or ¡nap of terrain types,where each type has its own engineering characteris-tics to which survey work can be related. The pro-cess of terrain evaluation takes place in twostages, of which the procluction of a terrain classi-fication map represents the first stage. The eval_ua-tion stage is completed when engineering character-istics are related to each of the terrain uníts inthe classificatíon, which are gathered from informa-tion collected during the fiel-d survey stages.

Certain technigues have proved themselves to beparticularly suitable for terråin evaluation, es-pecial-ly those that provide information to establishan appropriate form of Èerrain classification. Thesemainly involve the use of imaging renote-sensingsystems, which convey a large aÍìount of informationabout the terrain, especially when supported bybasic thematic mapping such as geology or soils maps,

Aerial photographs are the oldest form of remote-sensing imagery, and they are still the most impor-tant forn for highway enqineers. New forms of imag-ery have appeare<l in the past two decades, whichcreate images that yiel<l new types of i.nformatíon,but which can still be interprete¿l in the tradi-tional way provided due allowance is made for theiríndividual characteristics. One of the first sys-tems to be available was infrared line scan, andthis has had some Ii¡nited application. Airborne¡nultispectral scanners have also been used, hut byfar the greatest use is that made of small-scalemultispectral data from the Landsat satellites.

A potential source of imagery is radar, which is

Terrain Evaluation for Highway Planning and Design

PJ. BEAVEN AND C.J. LAWRANCE

Studies of the terra¡n to ælect route locat¡ons and 3ítes for dstailed geotech-nical investigations are esæntial prel¡m¡naÌies to highway rurvey and design.ThE Þrocers of using all sources of iñlormàtíoìf incluiliirglemoæ senaiñg,b derivs an understanding of the engineering characteristics of an area isknown a! terra¡n eyaluatíon. The basi¡ of terrain evaluation is a terrain classi-fication, whose pufpose ¡3 to act as a gsograph¡cal indexing 3yltem into wh¡dtall terrain*elated data, collected throughout the duration of a pro¡ect, maybe fitted. A terrain class¡f¡cation can be baæd on land form patterns, wh¡chare ¡elated to the underlying geology, soils, and water cond¡t¡ons. A landform clåssif¡cat¡on ha3 the advantage of being readily mapped from aerialphotograph! or remote-sensíng images. lt can be used to pred¡ct changes inso¡l cond¡t¡ons and to relate data from individual sampling s¡tes to areãs oftera¡n. The two most useful forms of remote*ensing imagery are aerialphotographs for detailed work and Landsat imagery for regional studies.Pandrromatic photography ¡s available in many parts of the world, and thereis increasing experience of its use in the trop¡cs. lf more detailed photographyusing special films is required, the use of a light aircraft that carries small-format camelas can provide high{ual¡ty photography at low cost. Landsatimaçry has proved useful for preliminary suryeys, Photographic prints ofimagery are inexpensive and easy to produce. Deta¡led analysis by us¡ngdigital images can be carried out with a computer, nornully in con¡unctionw¡th an ¡nteractive procesþr, and the use made of these systems w¡ll prob-ably increase as the¡r cost decreases. The principal aspects of terrain evalua-t¡on for h¡ghway surveys are illustrated by êxamples of work carried out inAfrica and Asia.

The objective of highway planning and design is toconstruct a road at the appropriate standard to sat-ísfy an expressed need for transportation. Consider-ation of the terrain is an íntegral part of theplanning process an¿l contributes nuch to the successof the design. The desígn stage itself is con-troLled by codes of practice anil nanuals that setout specifications ând procedures to be followed. Incontrast with this, the early stages of a project,notably the location of initial tentative alignmentsand the choice of sites for investigation in thefield, are poorty supported by guides to practice.The reason is that the appraisal of terrain requiresan assessment of nany related natural factors, sorne-times over very large areas, for which the writíngof a simple manual is impractícable.

It is important that decisíons taken during theearly stages of a project be based on some reason-ably accurate and comprehensive assessment of ter-rain conditions. The object of this paper is toshow how aerial photographs and remote-sensing tech-niques can be used to gain an appreciation of theimportant engineering characteristics of the terrainanil to devÍse a representative sampling and site-in-vestigation procedure along routes that are con-sidered to be feasible. The process of using allsources of infornåtion, integrated within a singleschene that can be apptied throughout the durationof the project, is known as terrain evaluation. Ter-rain evaluation techniques are of rnost immediatevalue in areas where information is timited or notavailable, although they can equally well be appliedin parts of the world hrhere terrain informatíon isavailable but not coortlinated.

The examples given of work carrieil out in Africaand Àsia each illustrate a particular sÈage or as-pect of the projects they describe, rather than theprojecÈ. as a whole. At the end of the paper, thevarious techniques are brought together in a tablethat shows which techníque is considered to be nostrelevant to each indivíduat stage of the survey.

PRINCIPLES AND TECHNIOUES OF TER.RAIN EVALUATION

Terrain evaluatÍon is the appraisal of the capabili-

Transportation Research Record 892

currently available as an airborne syslemr althoughin the future satellites wiII carry versions câpableof a 20-m groun¿l resolution. The great advantage ofradar is its independence of vteather and time ofday, which indicates that it v¡ill become of increas-ing i¡nportance in areas where persistent cloud coverprevails. Airborne radar surveys have been carrieilout in parts of central and South America,9lestAfrica, and Southeast Asia, but for co¡nmercial orstrategíc reasons the imagery is often difficult toobtâin.

Aerial, photography and Landsat imagery have foundthe widest acceptance of all remote-sensing systensby virtue of their versatil"ity' wicle coverage' andretatively low cost. It is the interpretation ofthe inagery fron these systems that has providedl thebasis of most terrain evaluation studies that havebeen ¡nade.

TERRAIN CLASSIFICATION

The basis of a formal- terrain evaluation is the rec-ognition of land surface patterns that can begrouped together into a terrain classification. Mostengineers recognize the repetitive nature of terrainin areas that they know weÌÌ, and they can see theassociaÈion between landforms, with their tyPicalsoils, drainage conditions, and vegetation, and theunderlying rocks on which they occur. Terrain clas-sification sets out to define these relations in a

systematic vtay. There are many different systems ofterrain classification in existence' as illustratedby Èhe range of pubtished thematic maps such asgeology, geomorphology, or soils naps. These clas-;ificâtions nay be used as the basis of a terrainevaluation for highway engineering where they êxist.For ân engineer, it would be advantageous to co¡nbineseveral types of napping (e.g., geology, topography'and soils rnapping) to produce a classification thatconsists of composite units that reflect theengineer's interest in a variety of aspects of theterra in.

In devel-oping countries' iletailecl thematic map-ping is often not available, but a form of rnappingthat offers a convenient compromise is landform map-ping. Prelirninary resource surveys often contain¡naps of this type, and rnany parts of the world havebeen mapped in this way (I'2). Landforn mapping hasthe advantage in that it consists of units thatrepresent a combination of geological, soils, anilhy¿lrol-ogical factors' and that theY are relativelyeasy to map. Lanclform classification can be used

where no other forn of survey exists, although sucha classification ís considerably improvecl if it isaugmented by infornatíon from existing thenaticmaps. A landform ctassification can be extracgedfrom topographical naps, but it is much more effec-tively compiled from renote-sensing images.

Landform Mapping: Land svstems and Land Facets

There are strong línks between Patterns of landforn,drainage, and vegetation with the underlying geo-logical fornations ancl hydroJ.ogical regime. There-fore, a lan¿lforn map !o so¡ne extent reflects theproperties of the terrain beneath an¿l' in adõlition,takes account of aspects of the terrain bhat relatealirectly to the engineer's sphere of interest'namely relief and slope, bedrock, materials of thesubgrade for use in construction, an¿l surface andsubsurface water flow. The mapping units are rel-a-tively easy to map from the aír and space' and largeareas can be covereil in a short tine.

A version of landform mapping, which incorporatesaspects of geology' geomorphology, ancl soil distri-bution, has been formalized in England after a peri-

37

od of research and comparison vriÈh sinilar work car-ried out in South Afríca and Australia (-3.). This issi¡nilar to the use of physiographic units in theUnited states (!) but exten¿ls the classification toinclude smaller areas of ground. It was recognizeilthat landscapes fa1l naturally into hlerarchical as-sociations, in which grouPs of snâll terrain unitscombine to form larger ones. The two most importantunits are the land systen and its constituent landfacets (see Figure 1). A 1an¿l systen is a largearea of characteristic landform, ilrainage Pattern,and associations of naterial-s developed on a singlegeological unit or sequence. It is typified by a

distinctive scenery and is generally nappable atabout a 1:250 000 to 1:I 000 000 scale. The conpo-nent parts of a lånd system, called land facets, arectefinetl in a simiLar way, but they are s¡naller antlIess variabLe, such that an engineer would norrnallyexpect a single tlesign to be appropriate for sec-tíons of road. built on each facet. The number ofland facets in a lanil syste¡n is generally few, andthey ah¡ays occur in the sane relation; the lanclsystem is matle up of land facet associations re-peaÈed over a wiile area. Figure 1 also shows how

lantl facets may be subcliviiled into land elements--Èhe smallest features of the terrâin. These woul¿lonly be rnapped where they are of particular impor-tance (e.g., an outcroP of gravel) , but their Pres-ence would be íncluded in the description of thelancl facet.

At the other end of the scale' Iand systens ¡nay

be grouped into larger units called land regions,comprising land systems that have similar geologyand topography. Àt this level of detail, a land re-gion inevitably contains a considerable variety oflandfor¡ns and a lower ilegree of homogeneity thanwould be expected of snaller units. Nevertheless,general engineeríng châracteristics, such as therange of soil types, depth Èo bedrock, subgrademoisture conclitions, and recurrent problens, oftenremain consistent over these large areas. Land re-gion classification is useful during the earliestreconnaissance stages of an investigation to iden-tify the principal characteristics of an area. It isequivalent to the "section" in the Arnerican classi-ficâtion (4).

Examples of Terrain classification in HighwayEng ineer ing

Reconnaissance Survey: Trans-African Highway

The Trans-African Highway project is part of theAfrican international network of roadsr the firststage of which is to provi¿le an all-weather road be-tereen Lagos and llombasa. To identify the agreedroute, a prefeasibility study of the whole area was

commissioneil to inventory existing roads and iden-tify sections that needed improving. To assist inthis vtork' the Transport and Road Research Labora-tory (TRRL) preparecl a land region map of the area(Figure 2), which covers all of the routes that were

IikeIy to be considere¿l. vÍhere possible' thís map

was prepareil from existing lan¿l systen napping but,where this was not available, Èhe best topographicand geological naps vrêre used' assisted by airphotograph mosaics in a few places.

The map defines regions with distinct topographicand foun¿lation condi!ions, within which it can be

assuned that the costs of roa¿l buil¿ling are reason-ably consistent. A map prepareal in this way, with-out the assistance of fiêId work to check it, islikely to require some revision before a final landregion nap can be published. However' the proposedboundaries provided workable survey units for co1-lecting anil classifying engineering anil other land

38

Figure 1. Relat¡on among land system, land faeet, and land slement.

Figure 2. Land prov¡nces of Trans-African Highway.


Alor Gajah land system Gentle h¡lls w¡th broadterraced r¡ver valleys

LAND SYSTEM

LAND FACETS

æ\\:J?dil I I\tr--t-l

li,o ri,,,t""t'""

LANO ELEMENTS

wwI

¡ 4b Abandoned

I channel

¿lc Streem

I

Slope

llv s¡de

4â Floodptain lc Gully

| 1b Gully

d Stream

floor


use data for a prefeasibilty survey. Soil changesnoted during the ground reconnaissance were relatedto boundaries drawn on the map' and from this baôk-groun¿l it was possible to forecast conditions onroutes that v¿ere put forward as alternatives afterthe conpletion of field v¡ork.

Feasibility Survey: Rurnpi-Chiweta Roadr Malawi

For the next stage of work--a feasibility study of a

setectecl section--more detailed mapping would beneeded over a narrower corridor. Land systems map-ping is suitable for this, provided air photographcover can be obtained, although in some areas Land-sat cover night provide an alternative source ofimagery for this purpose, so long as changes intopography are sufficiently pronounced.

An exanple of terrain evaluation used in more de-tailed surveys is a materials survey undertaken fora road project in Mal-awi. The consulting engineersresponsible for the design of this project wÍshed touse the information collected during field work car-ried out for the feasibility study to prêpare tenderdocuments without further field investigations. Theproject involved two alternative alignments thattotaled 130 kn (Figure 3). Although the two routesrun quite near to each other, the topographic andgeological conditions are quite distinct on each.The rnateríals survey commencecl before the finalchoice of alignmenti therefore, it was necessary todevise a sampling and testing program of the soilsin such a r{ay that a pavement design could be pre-pared for a road located ín any part of the wholearea.

Tvro possible routes were identified. Before fieldwork comnenced, the area that contained the tworoutes was studied to establish the pattern of Landsystens, facets, ancl elements. Having iclentifiedall the terrain units, the field survey was arrangedto collect aÈ least one major soil sanple from allunits that would be crossed bv either of the aI-ternative routes. where the soil or moisture condi-tions of any land facet appeare¿l to be variable'extra samples r.¡ere collected for testing in thelaboratory.

The laboratory testing program wâs arranged todeternine the strength of the soiLs for all 1ikelyconditions of subgrade density an¿l noisture content.From the test ¿lata, together with measurements ofactual noisture content, a pavenent design was prê-pared for every land facet traversed by the road,including a provisional estimate of the quantitíesof materials needed for cÕnstruction.

AIR PHOTOGRAPH INTERPRETÀTION

AeriaL photographs contäin â cornplete record of theland surface and can be interpreted to predict sub-surface conditions and provide information on manyfeatures ímportant in engineering design. Althoughsone knowledge of the natural sciences is helpful íninterpreting aerial phoÈographs, an engineer neednot be deterred fron attempting to use them due tolack of experience or knowledge of geology. It íspossible to carry out useful v¡ork with air photo-graphs after only a basic tuition, and expertise ininberpretation râpidly cones with practice. Airphotographs should always be viewed under a stereo-scope to achieve the significant benefits offered bythe three-¿limensional linage.

There are many standard textbooks (5) and reports(!) tnat describe the practice of air phoÈograph in-terpretation, which draw on North Anerican and Euro-pean examples. The technique for organizing an aírphotograph study does not differ significantly whenapplied in the tropics' although tropical têrrains

Figure 3. Rump¡-Ch¡weta transportation corridor land systems.

'' / .,"' i

39

a^-.(

! ,,u,.,, /) )/ ^r

/'"..f lrcJ'¡i:\. ,4.ò\s' I r-t-,t/l{l***^:

(-/ t'oùs" ,/ç,'

f-¡ i\r*

RumNa

\ u,rro '-- \

nuvpi Y))--

LUZI ¡tlrrl0 2 t, 6 I lokm

themselves are generally very ¿lifferent from thosein temperate zones. Interpretation is often basedon photography taken for phoÈoqran¡netric purposes'typicatty 9-in panchromatic filn in large câmeras ata scâIe of beth'een I:20 000 and l:50 000. where ¡îorespecialized photographs are needled, a sinpler canerasysten can be used in a light aircraft to obtainhigh-quality nonmetric photographs at the requiredscale, using color fil¡n if necessary (7). The useof conventional and specialist photography is illus-trâted by lwo separate road schenes in Nepal.

The Butwat-Narayanghat section of the East-WestHighway in Nepa1, which was funded by the Britishgovernment, provides an example of the use of airphotograph interpretation to align a road and orga-nize the search for materials on a rational basisbased on a terrain evaluation. The eastern sectionof the road (Figure 4) runs roughly east-wes! for 56km; it is constrained on the north by the foothillsof the Himalayas and on the south by the NarayaniRiver. Between the lower slopes of the mountainsand the floodptaín of the river lies a conplex ar-rangement of terraces with a well-defined piednont-fan sysÈen that exten¿ls southward fron the foot ofthe mountains.

An original alignment, lying to the south nearthe Narayani River, traversed low-l-ying terraces andfloodplains of fine-grained plastic soils used forwet paddly cultivationr and it involveil the crossingof wide ill-defined watercourses. It became clearthat a relocation of the road 1-3 kn to the north ofthe original route would ptace the road within thezone of gravel terraces' where mâterials are plenti-ful and the river courses are better defined. Exíst-ing black-and-white air photographs of the area at a

scale of 1:12 000 were exanined to identify the main

40

Figure 4. Part of Butwal to Narayanghat Road, Nepal.

lOmiles

terrain types, potential sources of gravel, oldstream channels, and areas liable to inundation.Particular attention was paid to identifying thebest crossing points of three large and severalsmaller rivers in this section of the route atpoints where the floodplains are narrowest and noststable. In or¿ler to compare the retative merits ofthe tvro routes in this section, rough quantitieswere taken off by scaling from the photographs. Àr-bitrary unit rates were applied to these quantities,which showed a gross cost re¿luction of the order of30 percent in favor of the northerly route. Althoughthis figure was only a crude estimate of the poten-tial cost difference between the two routes, it wasfelt that there was anple justification from thepurely engineering point of view to direct subse-quent survey and design effort on Èhe northerlyaligrunent. The road was opened to traffic on thisline in 1975.

À second roatl project in Nepal, for which spe-cialized aerial photography was flown, lies in theeast of the country. The road runs from Dharan, onthe plains, to Dhankuta, situated in the foothillsof the Himalayas. The road is 5l km long an¿l in-volves a cumulative ascent and ctescent of sone 3000m.

The geotechnical problems encountered in thisarea are severei they involved widespread instabil-íty and the location of a large bridge site across anajor river that is known to scour its banks and bed(Figure 5). There are no roads and fe$, najor foot-paths in Èhe areat novenent across the country isnade sl-ow by dense vegetation and intensive terracecultivation, and the slopes are extrernely steep andofLen dangerous. Under these circunstances, anelgineering survey cannoÈ aclequately be carriecl outby fielcl work alone. Àir photograph interpretationproved vital in evaluating features (especially un-stabLe ground) thât were either inaccessible or tooIarge or too diffuse to be recognized at groundIeveI.

Black-and-whiÈe aerial photography at 1:25 000scale was available, but for sorne sites, particular-Iy the major bridge site, the scale was too sma1l to


Figure 5. Detail of br¡dge s¡te to be located on rock bar, Nepal.

identify all the salient features of the locaIity.To have the ârea reflown at a larger scaLe w'ouldhave been difficult, owing to the isolated nature ofthe area and the very variable weather condítionsthaÈ prevail. However, at a tíme when possibleroutês were being considered by the consultantsresponsible for the desígn of the project, TRRL lrasdeveloping a rânge of camera systens to provide apackage that can be taken overseas easíly and fittedsinply to a light aircraft. A lightweight bracketallows the cameras to be mountecl in a housing fixeilto the side of the aircrafÈ (Figure 6). This carnerasysÈem was useil to acquire large-scale multispectralphotographs of the area through which the proposedroad would run. In Nepal, a pilatus porter aircraftwas available that hail an internal nounl for can-eras, thus sÍnpJ-ifying the nounÈing and operaÈion ofthe cameras. It r,tas possibte to use trdo camera setsat one time, so a comparison was ¡nade of photographstaken with an inter¡netliate formât camera that used125-n¡n film and a cluster of four 70-mm fornat ltas-selblacl cameras. The larger size of camera covered a

N

Io


Figure 6. Pod to carry up to four 70-mm câmeras fitted to Cessna 172 ai¡craft.

Figure 7. Digital Landsat ¡mage of Sekoma Pan area, Botswana, which showspart of pan and trace of road.

bad weather or on occasions when it is necessaryphotograph the terrain at specifíc tines.

T,ANDSAT SATELLITE IMAGERY

Since the launch of the Lan¿lsat progran in L972,three successive satelliÈes have returned nore thanI ¡nillion images of the earth's surface to receivingstations in the United states, Argentina, Australia,Brazil, Canacla, India, Italy, Japan, South Africa,and Sweden. The orbit of the satellites is so ar-rangedl that they pass in a north-south directÍonover the sane piece of grouncl every 18 days, andl oncoÍunand they can trans¡nit an inage to one of Èhesereceiving stations. Uost of the information hasbeen collected by the nultispectral scanner (MSS),which builds up an l85x178-km image of the ground byrecording successive west-east scan lines as thesatellite moves south. The inages are issued tocustoners either as a computer-conpatible tape (CCT)

or as photographs at a scale of I:1 million.Within each scan line, reflectance is neasured at

four separate wavelengths equivalent to green andred light antl two bancls of infrared. The scan lineis sampled at a frequency Èhat is equivalent to amovement of 57 n along the groun¿l, and the data arereproduced conventionally as picture elements(pixels) 79x57 m in size. Within a pixel, smallfeatures such as buildings or roails cannot be re-solved individually, but they contribute to thetotal reflectance of that pÍxel and nay bias it.Thus, the road in Figure 7, âlthough less than l0 m

wide, alters the reflectance of the pixels thatcover it so that they stand out fro¡n the background.The line of tbe roaal can be seen, but there is noway of determining its witlth fron Landsat dataalone. (Note in Fiqure 7 that the inclividual ele-Íìents are visible. The original scale on a televi-sion screen was 1:22 000.)

A higher resolution is achieved with the return-beam vidicon (RBv) camera on Landsat 3. ThÍs systemuses two cameras, each covering one-quarter of thearea of the MSS scene, and has a resolution of 30-40m. Currently, the data are usually made availablein photographic forrn at a scal-e of 1:250 000, butCCTS are now being produced. The high resolution ofthis system generates such a large amount of datathat it has been necessary to restrict it to onechannel; thus, it is not possible to produce an RBVcoJ.or image. However, experiments have been nade toadd the MSs ¿lata to the higher-resolution RBv datain order to obtain the benefits of both systems.The thematic mapper (TM) on Lan¿lsat D, due forIaunch in 1982, will combine the advantages of falsecolor representation anal 40-m resolution.

INTERPRETÀTION OF LANDSAT IIqAGES

The interprêtation of Lan¿lsat data consists of twophases: data preparation and extraction of informa-tion. The preparation of an image in a form suit-able for interpreÈation (as a false color conposite'for example) is a necessary first step to facilitateÈhe interpretation process. An interpretation, car-ried out at iÈs simplest level, can be ¡nade ín asimilar way to the interpretation of small-scale airphotograph ¡nosaics.

Landsat Photographic Techniques

The basic Landsat data are held on four photographicnegatives that can be usefully enlarged to a scaleof about 1:250 000. Larger prints may be useful tocorrelate with maps but give no extra infor¡nation.These images may be interpreted individually but, totake full advantage of the system, it is necessary

4l

u r¡Efütt

Sekoma Pan

greater area of ground for a given sca1e, but onlynegative film was available for this camera size,It was thus very suitable for proclucing largeprints. The range of film types for 70-nrm camerasis very extensive, and processing is more widelyavailable. A range of 70-mm films was tested, in-cluding high-resolution fil¡ns suitable for smaIl-scale photography, natural and false color fitms,and panchromatic films with filters. Three of thepanchromatic images can be co¡nbined by using a coloradditive viewer to recreate a normal or false colorimage. For general-purpose investígâtions, thenatural color filrns proved nost useful. False colorfilms do show up minor variatíons in vegetation thatare very hard to see in natural color, but in Nepalthe -hills cast strong shadows that are accentuatedby the high contrast of infrâre¿l film. The addedconplexiEy of manipulating the multiban¿l panchro-rnatic photogrãphy has not been justified by theextra information obtained from the interpretatíon.

Small-format aerial photography offers a¿lvantagesbeyond the low cost of the survey. A light aircraftcan usually be hired locally andl should therefore berelatively easy to obtain. Many light planes, whichoperate from unsurfaced airstrips, can fly in areasthat would be too dangerous for larger aircraft.Moreover, they can be more easily held on standby,ready to operate at short notice between spelts of

42

to conbine data fron more than one band. The nornalr,ray to study three differenÈ inages is to use acolor conposite, and the sÍnplest rneÈhod is to pur-chase a standard product, either a transparency atI:1 000 000 scale or an enlarged print. The processof naking a master color negatlve Ís relativelysinple but requires experience to obtain gooil colorbalance. An alternative rneÈhod of producing a colorconposite is t'o use a color additíve viewer, wherethe three images are projected Èhrough color filtersand accurately superimposed on the screen. The ef-fect of changes in color filÈers is seen imrneiliatelyand the inage can be interpreted as projected, butit is usual to make a color photograph that can bestudied more conveniently.

fmages may also be combined in pairs, if desired.À sinple technique Ís to view two different spectralbands, or images tâken at clifferent dates, uncler astereoscope. No stereoscopic vievr is obtained, butthe effect is to reduce disturbances caused by scanlines and irnage "grain", thereby leavÍng unchangingterraín features to stand out nore clearly. Pairsof images may be combined photographically as ratioimages, in whích a positíve transparency of onespectral band is superinposeil on a negative trans-parency of another. If the two images are identi-cal, they cancel each other and Leave a neutrâltone. Any clifferences between the bands will showas either darker or líghter areas to give a simpleform of ratio image. More co¡nplex ratíos âre pro-duced by digital processing in a cornpuÈer.

Use of Landsat for Materials Investígations in Sudan

Individual Landsat scenes at scales between1:500 000 and 1:1 000 000 were used in a study ofthe terrain of the Wad Medani-Kosti area of Sudan tolocate materials for road construction ín tvroprojects in the area (see Figure B; scale isl:1 300 000). The projects involved the construc-tion of Èhe Wad Medani-Sennar-Kosti road and the ex-tension of the roa¿l network easÈ of Sennar in con-nection with new irrigation works. In both areasthere was difficulty in locating sufficient quanti-ties of grâvel for road construction. Sone fiel_dwork had been carried out, but the known depositswere inadequate for the needs of the project. Topo-graphic mapping v¿as old and of poor qualíty, and airphotograph cover was limited to very smalI areas.Under these circumstances, satellite imagery af-forded the only cornprehensive view of the land sur-face that was available. Basic topographic detailssuch as towns, major roads, and river systems werereadily identified on the inagery, which inci-dentally also provicled an up-to-date picture of theextent of irrigatíon development.

The area consists of tr{ro contrasting terraintypess the predominant and very extensive blackclay plains through r{hich protrude occasional out-crops of granitic rock, and t.he valleys of the Vlhiteand Blue Nile, which cut across the plains in a widetract of alluvium. Several types of natural gravetoccur in the area, indicaEions of some of which canbe discerned in the images, for which confirmationin t,he field was obtained. One of the most positiveindicators of natural gravel is the aureol_e oflighÈ-toned (sandy) soils that surround even verylow out.crops of rock in the plains. These naterialscontain pockets of weathered rock that can provide ausable gravel. The outcrops thenselves ccrnsist ofhard rock that can be used for quarryíng. The blackclay plains contain a variety of coarser soils thatform contrasting tones in the images. At thewesÈern end of the road, sandy ridges associatedwith the White Nile provide firmer foundations butyield no gravels. LateritÍc aravels are present in


Figure 8. Landsat satell¡te image of Wad Medani area, Sudan, which shows (Alvalley of Blue Nile, (B) black clay plains, (C) areas of red quartz¡t¡c clayeygravel, and (Dl rock outcrop.

Figure 9. Photographic density slice of pañ of same area as in Figure B, whic*renhances contrast of red quartzitic clayey gravels (Al.

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iryhq'.' -.ftthis area, which have a high and <listinctive reflec-tance Ín the red and infrared bands. But their out-crop of up to 300 n2 is generally too small toallow a positive identificatíon to be made fronLandsat. To the east, on each side of the Blue Nilevalley, extensÍve areas of red quartzitic clayeysand occur within the clay plains. The sands con-tain deposits of gravel, but it is not possibte tosepârate the sands from the gravels in the inages.Sands and gravels also occur along the banks of theriver itself, and they are easily identifiable bytheir distinctive shape and position and high con-trast with their surroundings. But, as before, itis not possible to separate the sands from thegravels by interpretation ãlone.

The interpretation of the satetlite images wasassisted by the use of color composites as well asblack-and-white pictures and by a density-sticingtechnique that was used to enhance the appearance ofthe red quartzitic gravels and make them stân¿l outmore clearly. The color composiÈes were made in anadditive viewer. The dènsity-slicing technique wascarried out photographícaIly. The scene was photo-graphed onto high-contrast fj.tm to bring out aItareas of a specified grey tone at the expense ofcontrast in the remainder (see Figure 9). ThreespectraJ. bands of the scene were treated in this wayand combined into a color conposíte. This technique

lransportatfon Research Record 892

can be very effectíve for areas of uniform tone thatcontrast well wlth their surroundings, as in thiscase. However, in conplex lmages it is dífficult toavoid includlng areas in t.he density slice that havea sl¡niIar brightness value (color) on the ground butthat are not, in fact, related. Methoclical fieldwork is required at the right time of year to checkthât surface characteristics of the terrain (soí1,color, vegetation, etc.) are both diagnostic of thefeatures of interest and consistent over a wide areabefore atternpting èo use density slicing to identifyfeatures in the terrain.

Processing of Landsat Data: Digital Methods

The original Landsat data are recorded on a tapethat contains four bands of reflecÈance Clata for theI million pixels that make up a scene, which totalmore than 30 million reflectance values in all.These data may be nathe¡natically analyzed in a com-puter, for example, tÕ count the percentage ofpixels of a certain value for correlation wíth sotneknown groun¿l feature. Alternatively, the pixelsfron three bands cân be dísplayed on a color tele-vision screen !o generate a full-color image thatnay be inÈerprete¿l in the normal way. However, thedata can also be nanipulated rnathernatíca1ty beforeoutput to the screen, thereby enhancing the picturefor interpretation and analysis. Television viewingsystems, when connected to a conputer to enable theeffect of data transfor¡nations to be seen immedi-ately, are known as interactive processors.

The use of an interactive processor for the studyof Landsat data offers two major advantages overphotographic techniques. First, the range of datatransfor¡nations availabte in the processor is nuchgreater than those achievable by photographic pro-cessing, and the results are available almost in-stantaneously. There is thus a grêater opportunityto find an enhancenent best suited to the userrsneeds. Second, the intensity range of the data canbe stretched over the full range of the display sys-tem, which allows very subtle color variations to beobserved. A co¡nputer-generated inage, therefore, isa very high-quality pro¿luct that makes the nost ap-propriate display of the data ín terns of the user'srequirenents.

There are, however, certåin aspects of digital-inage processing that have discouraged its use inthe past in favor of simpler photographic tech-niques. These are

1. Higher costs in terns of both original dataon tape and either the capital costs of a processoror hire of a conputer (the cost of conputer time maybe considerable if many options are explored beforearriving at a suitable image), and

2. The nee¿l to produce a hard-copy picture,either by an expensive color writing process or byphotographing the televísion screen directly, withconseguent loss of image quality.

Despite these disadvantages, the use of interactiveviewers is becorning more common, and this lrend willundoubtedly increase as they becone adapted to mini-computers and nicrocomputers at correspondinglylower cost.

the U.K. Remote Sensing Centre at the U.K. Roya1Aircraft Establ-ishnent operatês a typical largeinteractive processor, which is linked to a main-frane co¡nputer for bulk processing of daÈa. Theprocessing facÍlities are of tr¡ro typess geo¡netric,in which èhe size or shape of the whole irnage isnodified, and spectral processing, in which thêreflectance values of the pixels are enhanced.

43

Figure 10. Classif¡cat¡on of calcrete areas in vicinity of Sekoma Pan, Botswana,

Enlarged trarnrrrg area frj

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1. Geometric transformation of dat,a: Sinpleprocessing can skevr the original rectangular arrayof pixels into an image that Èakes account of earthrotation and that is geonetrically fairly accurate,even at a scale of I:250 000. More elaborate pro-cessing, in conjunction with conlrol points, can beused to resample the pixel array so that it conformswith a map projection. Experinents have been madeto combine topographic naps anil Landsat data to pro-duce an image that has height information super-imposed onto a detailed topographic base.

2. Spectral processing of data: Spectral pro-cessing involves changing the original pixel inten-sity values to a new set by matheroatical transforrna-tion. The range of processes available is thereforevery wide to serve a number of purposes. Often soneform of preprocessing is applied Èo correct atno-spheric and radiometric distortions in the dâta inpreparation for interpretation. At the interpreta-tion stage, nost inages require some color enhance-nent or "stretchingrr, in which the origínal pixelvalues, crowded inLo a narrovr part of the brightnessrange as they nornally are, are redistributed overthe whole range to improve the brillance and separa-tion of the côlors.

In the process known as density slicing, dat.a be-tween certain levels of a single band are displayeilto the exclusion of the renainder. Thê autonaticclassifier uses a form of density slicing to selectpixels that have a range of intensity defined forall bands ând hence have similar reflectance char-acteristics on the ground (Figure t0). Ratioimages, generaLed by dividing the píxe1 values ofone spectral band wiÈh those of another, emphasizethe differences bet\deen bands. Ratio images nay becombine¿l with normal bands to produce new irnages orused in classificatíons to refine groups of pixelsinto tightly defined color popuJ.ations. More elabo-rate processing can be useti to enhance Èhe edges offeatures where strong tonal contrasts are presenÈand thereby to heighten the eilge of sharp features(e.9., fault scârps) that have a linear trend on thegrouncl.

A more general transfornatíon is to calculate theprincipal coÍiponents of the data¡ this is a ¡nathe-matical technique that takes any nu¡nber of data setsand calculates an equivalent number of componentswith the rnore signíficant information containect inthe first few sets. Thus, the four bands of Landsatdata yiel¿l four principal components, of which mostof the reLevant informatíon is contained in the

:,'Ë,,:.¡ û¡j

44

first three. The fourth conponent alnost completelyconsists of noise. The three components can be dis-played on the monítor as a color image and inter-preted in the normal way. For images in which thereflectance data are highly correlâted between t-hefour bands, principal component displays present theinterpreter with weLl-defined color groupings thatare easy to discriminate.

Digital Processing in Search for ConstructionMaterials in Botswana

TRRL has used the inage processor in a study toassist the Mínistry of Works and Communications,Botswana, to assess reserves of calcrete for roadconstruction within a route corridor between Jwanengand Takatswaane in central Botsldana. The corridoris about 400 km long and 40 km wide, and it proviclesaccess to a large part of vrestern Botswana with pos-sible extensions into Namibia. Calcrete is a cal-careous material that forns within the Kalaharisands. It accumulates abundantly in large bare de-pressions (pans) but also in snaller quantities be-nealh the sand itself in areas where no pans occur.Calcrete is the only form of natural gravel in thearea, and this study to determine the distributionof deposits has been undertaken to decide whetherrelocation of the existing sand track through areasthat contain more calcrete woul¿l be justified.

Over most of the corridor, the Kalahari san<ìs,whích cover central Botswana to a considerabledepth, form an almost irnperceptibly undulating plainwith scattered pans. Some of these pans are verylarge (up to 5 km across) and are ãbundant sourcesof calcreÈe. The snallest are less than 50 m acrossand are ¡nuch less reliable sources of the material.Lârge parts of the corridor contain no pans at all,but calcretes can stil-t be found beneath thêKalahari sand, where the only clue to the existenceof the material is a change in the color of Èhe sandfron its normal reddish or brownish hue to a neutralg rey.

The pan features are visible in Landsat digitatimagery down to about 100 m ín dianeter, providedgood contrast exists betvreen the pan surface and thesurrounding sand pIain. Black-an<l-r+hite aerialpholographs were used successfully to map even the

Trånsportation Research Record 892

snallest calcrete-bearing features, but they werequite unable to map the grey sands because thechange in sand color fro¡n real to grey is not de-picted. In contrast, it was found that the Landsatscanner is extremely sensitive to this color change,the principal- conponent ctisplay being particularlyeffective in the discrimination of this subtle fea-ture. Owing to the small size of nany of the greysand areas, they are poorly visible and barely nap-pable in photographic Landsat products. But the Ín-creased resolution of the co¡nputer-generated ímage,enhanced by the principat component transform andcontrast stretching, enables the interpreter to mapgrey sand areaq as s¡nalI as 100-300 m across (Figure1I). (Note: For !'igure 11, the aerial photograph,although able to resolve tiny detaits on the landsurfâce, does not distinguish areas of grey sandwhere calcrete occurs from the surrounding red¿tish-brown sands. The cornputer image has been enhancedtto naximize the difference betr,reen sand colors andshows them clearly in dark tones. Nunbers I-6 indí-cate correspondíng features in each image.) An image$ras produced that depicts the grey sand areas to theexclusion of all other features from which a photo-graphic negative was made by an optical filmwriter. This image has formed Èhe basis of a map ofthe grey sand areas between Jwaneng and Takat-swaane. The computer images have shown thât theèxisting route pâsses few ileposits of calcrete, butthat it could be relocated through areas where manypotential sources of the material exist.

CONCLUSIONS

The application of geotechnical survey techniques tohighway engineering covers a great diversity ofoperations and involves nany environments, but thedevelopment of sophisticated remote-sensing systemshas improved our ability to evaluate ground condi-tions in all- types of terrain. The foregoing ex-amples illustrate some of the r,rãys in which remote-sensing and terrain classífication methods can beincorporated into engineering surveys, although theprecise ways in which they are ernployed depend onthe circumstances of the survey and the types ofimagery available. It ís irnportant to use a scaleof irnagery appropriate Èo the level of detail of a

f,;.1â-¡

Figure 11. Comparison between black-and-white aerial photograph (left) and computergenerated Landsat image at some scale of area near Sekoma, Botswana,

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Tablel. Summaryofroadalignmentsurveyâctivitiestypicallyaugmentedbyw¡delyavailableremote.sens¡ngtechniques.

45

Project Stage Aim Activity

Remote-Sensing Techniques

Landsat MSS and RBVExistingBlack-and-White SpecializedRemote-Air Photography Sensing Techliques

Preproject phase

Reconnaissance

Feasibility

Design

Const¡uctionand post-constructionmaintenanceperiod

To identify mainsources of infor-mation and toput project intocontext withrespect to tel-rain

To identify possiblealternative routesand define strategyfor constructionprogram

To appraise routecor¡idors andselect bestroute

Collect together all relavant published materialrclating to project toassess requirements formapphg and ínterpreta-tion during suweystages

Define project in terms ofsize, political and physi-cal constraints, and geo-technical complexity;examile possible routeson maps and satelliteimages and air photo-graph mosaim if avail-able; undertake broadtenain classification forcollation of regionalinformation; visit siteto check interpretations;report on findings andplan next stage

Make detailed interpretationof conditions on all routesand, if necessary, make amore detailed tenain class-ification of area; interpretfoundation conditions,earthwo¡ks (borrow andspoil areas), drainage, ma-terials sources Gravels),major bridge sites, andhazard zones; carry outsite investigation of alte¡na-tive routes, noting keyphysical and geotechnicalfeâtures; cost comparisons ;selected laboratory andfield testing; recommendbest route and preparereport

Detailed study of Comprehensive site inves-selected route to tigation of selected routeengi¡eering design with full sampling andstanalards testing program; prepare

final design documentsBuild road and carry Road construction activitiesout repairs prior tohanding over

Purchase låndsat MSS imagery ina form suitâble for tequtementsof project; select images fromseve¡al dates or seasons if neces-sary; make false colo¡ compositeimages at l:500 00G1:250 000;purchase Landsat RBV imageryif available, l:250 000-1:100 000

Examine MSS photograph productsin conjunction with maps; scaleas above; interpret influence of ma-jor features on road alignment, e.g.,changing course of major rivers;catchment area of major river sys-tems; extent of flooding of low-lying æeas; possible sources ofwater for construction; posiblesources of construction materials(e.9., alluvial tenaces and fans);pattem of regional instability; ex-tent of erosion; spread of deforesta-tion; æsessment of land acquisitionor site clearânce problems.

Use MSS and RBV as base map if nomore detailed mapping is available;supplement air photograph inter-pretation with color i¡formationfrom MSS

Make inquiries in Europe Find out ifspecializedor of host govemment to air photography orpurchase afu photography other form of ¡e-and air photograph mote-sensing cover-mosaics age has been made

for some previousproject in ârea

Air photograph mosais atapploximately I :100 000used in conjunction withLandsat materiâl

Use afu photographs for all Commission special-detailedinterpretations izedairphotographyandte¡¡¿in-classification (possiblysmallfor-study; scale l:20 00G mat) at a scale ap.1:60 000, as available: (a) p¡opriate to size offoundation condition sur- task and degree ofvey; (b) mlculate catchment ground complexityareas and location of cul- (approximatelyverts; (c) identify spoil areas, 1 : 1 0 000-l :30 000);also possible bonow areas; examine I¿ndsatminimize erosion risk; (d) computercompatibleidentify posible sources tapes in interactiveof construction material; processor (scales(e) location of all possible i :20 000-l : 1 00 000)bridge sites, and (f) identifymajor hazard areas (poorlydrained soils, sprilg lines,unstable areas, erosion inriver courses)

Use air photography tosupport all field suroeyactivities

Use ae¡ial photogÌaphy to large-scale air photog-locate access roads for ¡aphy may be usedconstruction t¡affic in dif- to monitor changesficult terain takilg place at im-

portant sites as con-struction ploceeds;may also be used torecord damage doneby landsliding, eresion, ot flooding inpreparation for de-sign of ¡ehabilitationmeasures

survey, beginning with s¡naII scaÌes to cover largeareas in a general way ãnd moving to larger scalesas the investigatíon proceeds toward the selectionof a final alignment. To enphasize the seguentialnature of Èer,:ain eval.uation procedures and how theyare matched to survey requirements, the main en-gineering activities and appropriate terrain evalua-tion techniques are su¡unarized in Tab1e 1.

Aería1 photoqraphs taken for nappíng purposes re-main the most important form of remote-sensing imag-ery for both general terrain studies and investiqa-tions of specific sites. The use of aeriãl photo-graphs to mâp land systems and tand facets--terrainunits based prinarily on land form--is seen as aneffective way of organízing a field survey to sampleaII relevânt pãrts of a corridor under study. Theuse of a terrain classification saves time in the

field by relatíng the data collected at individualsites to larger areas. Lon-cost nonmetric camerascan provide high-quality photographs for interpreta-tion, and the film type, scale, and tining of lhesurvey can be adapted to the subject under study.

Aeríal photography is now supplenented by Landsatsatellite inagery as a small-scale napping tool. Theinfornation from Landsat may be presented either inphotographíc or digital- forn. Photographic imagesare inexpensive to produce and require littte spe-cialized equipment beyond the facilities of a photo-graphic color laboratory, for their reproductíon, aI-though an additive viewer extends their scope. Thetine delay in obtaíning original material is usuaì.Iyquite srnall, and this position is tikely to irnprovewith the setting up of regional centers that holdstocks of Landsat negatives. For Èhese reasons,

46

photographic processing is likely to remain of greatinportance for surveys in developing countries.

Despite the high qualíty of Landsat photographicinages, Landsat data can only be fully used whenimages âre generatecl from the ¿tigital dlata helil oncornputer tapes. The resolution of these inages ísli¡nited to the 80-rn píxel size of the scanner, butthey convey a large anount of speetral (color) in-formation about the terraÍn. Developnents are tak-ing place in interactive viewíng systems away fronthe concept of large, sophisticated ¡nachines towardsmaller, simpler systems, which often consist of as-semblies of standardl components tinked to a micro-conputer that can perforn functions similar to thoseof a large machine but concede some limitations ofspeed and flexibility.

With greater international interest being shownin the alêvelopment of Thiril World countries, therole of remote sensing to highway engineering, asr.tÍÈh all natural resource studies of terrain, wilIincrease in inportance. Refinernents in sensing sys-tems will enhance our ability to detect subÈlechanges in terrain conditions, and inprovenenÈs indata handling will permit more sensitive interpreta-tions to be made.

ÀCKNOWLEDGMENT

This paper was prepared in the Overseas Unit (J.N.Bulman, unít head) of TRRL. The work describedforms part of the progran carriecl out for the Over-seas Development Administration, but any views ex-pressed are not necessarily those of the Adninistra-tion.


REFERENCES

1. I.D. Hí1I, ecl. Land Resources of Central Ni-geria. Lanil Resources Develop¡nent Centre, ToI-r{rorth, Land Resources Study 29, 7 Votumes, 1978-I981.

2. R.G. Robbins, ed. Lands of the Rânu-Madang Area,Papua New Guinea. Corunonwealth Scientific andInalustrÍaI Research Organization, Melbourne,Australia, Land Res. Series 37, L976.

3. R. Websler and P.H.T. BecketÈ. Terrain Classifi-cation and Evaluation Using Àir Photography: AReview of Recent Work at Oxford. Photogrammetria,Vol. 26, 1970, pp. 5l-75.

4. M.W. Witczak. Relationships Between physio-graphic Units and Hlghway Design Factors. NCHRP,Rept. I32,1972.

5, D.R. Luêder. Aerial photographic Interpretation., Mccraw-Hill, New York, 1959.

6. M.J. Dumbleton and G. West. Air Photography In-terpreÈation for Road Engineers in Britain. RoadResearch Laboratory, Ministry of Transport, Crow-thorne, Berkshire, England, RRL Rept. LR 369,1970.

7. W. Heath. Inexpensive Aerial Photography forHighway Engineering and Traffic Studies. Trans-port and Road Research Laboratory, Department ofthe Environrnent and Department of Transport,Crowthorne, Berkshíre, England, TRRL Rept. SR632t 1980.

Publication of thís paper sponsored by Commíttee on Explordtion and Cløssi-

ficatíon of Earth Mdtefials.

engineers (geotechnical engineers). Eachprofessions focus their work on a slightlyaspect of the earthrs surface.

A terrain classification system was developed ínBritish Co1u¡nbia (I) in 1976 and accepted nationallyin Canada in 1978 by soil surveyors (2); it is alsowidely used by most consultants in British Colu¡nbia(3). The system encourages a conrnon approach toterrain inventory and provídes standard nornenclaturethat has improved comnunications between earth sci-entists (4). This sysÈem also provides the fra¡ne-work for ¡nuch of the soil mappinq in the provínceand elsewhere in Canada, since soils have inheritedmany properties from their parent material-s.

The terrain classification system vras developedfor reconnaissance mapping surveys (scales ofI:50 000 to l:I00 000) by government (5,6) but hasalso been applied for detail-ed surveys (scales ofI:20 000) by consultants (3). The systen enphasizesfeatures that can be interpretêd from aerial photo-graphs and readíIy verified by field checking,thereby enabling coverage of approximately 2590km2lmapper/year at a scale of 1:50 000. Geneticmaterials classified according to their ¡node ofdepositíon form the basis of the terrain map unit.

Terrain Analysis for TransportationSystems in British Columbia

TERJE VOLD

A terrain class¡f¡cat¡on system was developed in Br¡tish Columbia and ac-æpted nat¡onally in Canada. The mapping system emphasizes features thatmn be ¡nterpreted from aerial photographs and readily verified by fieldcheck¡ng. Genet¡c materials classified accord¡ng to their mode of depositionform the substance of the terrain map un¡t. The material's texture, surfaæexpression, and the presence of any geologic processes of modifietion areadditional components of the system. Tlì¡s system also prov¡des the frame-work for much of the soil mapping ¡n the province, since soils have inheritedmany propert¡es from these parent materials. A manual on terra¡n ¡nterpre-tations for roads and linear developments that ¡nvolve shallow excavationshas been prepared, The manual is designed for planners and indicates howterrain ¡nformation may be used to assess capab¡l¡ty for these transportat¡on-related uses. Physical land constra¡nts and natural hazards that affect trans-portat¡on systems are explained, lnterpret¡ve maps that show the distribu-tion of natural hazards and physical land constra¡nts for development cân beprepared from base terrain and soil maps. These maps can be of use toplanners in assessing alternative transportation corridors and ¡n anticipatingpotential trouble spots before construction has commenced.

Terrain analysis refers to the inventory and assess-ment of the physical conditions of land. This is ageneral term that includes both geological and ped-ological (soil) evaluations. Therè are three dis-tinct, although somewhat interrelated, groups ofscientists who study the physical nature of land:geologists, soil scientists (pedologists), and soil

of thesed ifferent

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