Phan 1_Phuong Phap Phan Loai DHDT

S dng phn mn eCognition cho phn loi nh hng i tng

Trung tm Quc t Nghin cu Bin i Ton cu (ICARGC)- HQGHN 1

PHN I. PHNG PHP PHN LOI NH HNG I TNG

1. Mt s vn nguyn l

Nguyn l ca phng php ny c xy dng trn khi nim cho rng tp hp cc pixel ca nh s hnh thnh nhiu i tng chuyn m mt ta c th nhn bit. Bc x l c bn trong phn tch i tng nh l cc phn mnh nh (segment) ch khng phi pixel. mt ngi nhn bit c i tng c phn mnh (segmentation) th hng lot thng tin c x l. Cc thng tin ny dng m t mt s c im hnh dng (shape) kin trc nh (texture), cc quan h khng gian (topology) ca i tng c phn loi v cch tch hp cc thng tin ny chnh l cc quy tc cn c xy dng phn mm c th phn bit cc i tng. Trong qu trnh phn loi chng ta cn c th s dng cc lp thng tin chuyn ngoi d liu vin thm nh m hnh s cao, bn th nhng, bn a cht, bn s dng t v.v. Vic tch hp cc thng tin ni trn trong PLHT da ch yu vo logic m (fuzzy logic) (Ravi Chauhan, Nitin K. Tripathi et al. 2004). y chnh l qu trnh xy dng c s tri thc (CSTT) trong PLHT m chng ti s m t phn cc bc phn loi. Vi CSTT ny, mi i tng c th c phn loi bng mt thut ton khc nhau ch khng phi bng mt thut ton thng nht nh trong phn loi Pixel-based.

Mt c im na ca PLHT l nguyn l phn cp (hierarchy) i tng; Th d: cc i tng thc vt c gp li thnh mt lp (class) v di l cc ph lp (subclass) thc vt c th. Ly th d: lp cy trng Duy Tin bao gm hai ph lp: la v mu; bn thn ph lp mu li chia thnh cc ph lp ng, u tng. Trong th d ny, ph lp mu va l ph lp ca lp cy trng va l cp trn ca ph lp u tng v ng v.v. Cch phn chia nh vy nhm m bo rng mi i tng c phn loi

theo mt thut ton khc nhau nhng cc i tng thuc mt nhm s c th k tha cc c trng chung ca nhm.

2. Cc thng s s dng xy dng c s tri thc trong PLHT

xy dng cc c trng s s dng vo phn loi chng ta cn hiu r cc vn lit k sau y: c trng ph ca d liu vin thm, la chn t l thch hp cho vic

phn mnh nh, xc nh bi cnh (context) v mi ph thuc c tnh phn cp gia cc i tng, tnh bt nh(uncertainty) ca bn thn cc d liu vin thm, d liu chuyn

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v ca khi nim m (fuzzy concept) s dng trong PLHT. Xut pht t c im hin trng s dng t ca khu vc nghin cu v mc ch ca ti th cc yu t ny cn c tnh m n mt cch y v phng din phng php lun v phng

din th nghim phn loi ng dng. Trong phn di y chng ti s trnh by chi tit v cc yu t ny ngoi tr yu t c trng ph ca d liu vin thm l vn c trnh by phn phn loi Pixel-based.

3. Vn chn v phi hp t l

T l l vn quan trng trong vic l gii thng tin trn nh v thng c hiu trn c s kch thc pixel. Tuy nhin, trong thc t bn thn cc i tng c sn t l ca n v vic nh ra t l phn tch s quyt nh s lng cc lp ca php phn loi. Cng chnh v vy m chng ta s da vo mc ch phn loi xc nh t l s s dng chit tch cc i tng khc nhau (Ursula C. Benz, Peter Hofmann et al. 2004). Cn phn bit r s khc nhau gia phn gii v t l khi phn loi. phn gii l khong cch gia hai ln o sung lin tc ca b cm hay cn c hiu l kch thc ca mt pixel. Trong khi , t l li l quy m hay mc khi qut i tng m chng ta cn m t v ph thuc ng thi vo mc ch phn loi v hin trng ca khu vc nghin cu. Vi cch hiu nh vy, khi p dng PLHT vo khu vc Duy Tin chng ti la chn mt s t l khc nhau phn mnh nh. Trn hnh 6 l th d so snh kt qu phn mnh nh vi cc t l 20 v 30. Nh ta thy trn hnh a, i tng t trng kh ng nht khi nh c phn mnh vi thng s t l 30.



Hnh 6. So snh kt qu phn mnh nh vi cc t l khc nhau

Khi ta thay i thng s t l th kt qu s cho cc i tng nh c mc khi qut khc nhau. Ni hm ca php gp (merging) s c chng ti trnh by phn quy trnh to i tng nh.

Cn nhn mnh l vic thay i t l phn mnh nh khng h ging vi vic thay i phn gii khng gian ca pixel v cng l u th ca PLHT. Cc t l

ny s c phi hp vi nhau chit xut i tng theo nhn bit ca mt thng v theo hiu bit ca chng ta v i tng. Trong qu trnh phn mnh nh , ti mi t l th ta u c kh nng iu chnh cc tham s lin quan n tnh cht hnh thc ca i tng nh hnh dng.

4. ngha chuyn ca i tng v quan h qua li gia cc i tng

Nh trnh by trn, trong PLHT, vic tnh m n bi cnh l rt quan trng; cc i tng c cng c trng bc x li c th c ngha chuyn khc nhau ty thuc vo bi cnh. C hai loi bi cnh: tng th v bi cnh cc b. Bi cnh thng th gip m t khung cnh thu nhn nh v bi cnh cc b m t mi quan h qua li gia cc i tng vi nhau. Vi phng php PLHT th s nhn bit i tng bng mt thng l quan trng v cn c vo cc nhn nh v bi cnh m ngi gii an s quyt nh cc thng s s dng trong phn loi. c c cc thng tin v bi cnh th cc khu vc trn nh phi c t vo mt s quan h. Ly th d c th Duy Tin: c hai gii tha cng c c im v hnh dng v v mu sc ging ht nhau; mt gii tha nm bi gia sng Hng s l bi du, gii tha kia nm trong ng v l rung ng. Mt th d khc v vai tr ca bi cnh, l v tr ca cc . (hnh minh ha).



5. Vn tnh bt nh (uncertainty) v tnh gn ng (vagueness) trong PLHT

Tnh bt nh c th lin quan n b cm, n phng php thu nhn v nn, lc d liu v c bit l khi nim v hin trng quan st c trn t liu nh. Di y chng ti xin ln lt trnh by tng nhm yu t gy ra tnh bt nh c th gp trong kt qu phn loi.

Bt nh do b cm gy ra ch yu lin quan phn gii bc x bin dng hnh hc ca d liu. phn gii bc x s nh hng n khong cch gia cc lp trong khng gian ph m chng ta khng th khc phc mt cch tuyt i ngay c khi p dng cc m hnh nh chun (calibration) cho d liu. Cng vi hn ch v phn gii khng gian phn gii bc x l nguyn nhn chnh l xut hin cc pixel hn hp. Nh ta thy trn hnh 7, gi tr ph C ca pixel cha 60% thng tin ca t trng A v 40% thng tin ca thc vt B. Kt qu cho ta mt pixel cha thng tin v thc vt. Nh vy 60% thng tin v t trng b ln vo thc vt.

Hnh 7: S ln ph v thng tin cha trong mt pixel (MicroImages Inc. 2004)

6. Khi nim m (fuzzy concept) v ng dng trong phn loi nh vin thm

S m ny lin quan ch yu ti vic phn chia cc lp thuc v hin trng lp ph (land cover) v s dng t (land use). Trn thc t kh c th vch ra ranh gii chnh xc gia cc lp nh: rng tha, rng rm, trng c cy bi, la tt, la km, khu dn c dy c, khu dn c tha tht v.v. Cc ranh gii ny tht s l m ngay c trn



thc a cha cha ni ti kh nng vch chng trn d liu nh vin thm. Hin nay c mt s cch tip cn theo hng phn loi mm (soft classifier) trong c tnh m n tnh bt nh ca cc kt qu. Tnh bt nh ca cc i tng c phn loi chnh

l mt phn ca kt qu phn loi theo logic m v c nhiu tc gi cp n trong lnh vc x l nh vin thm (Benz 1999; Nedeljkovic I. 2000; Jaeger and Benz 2000 ; Suzuki, H. Matsakis et al. 2001; Geneletti D. and Gorte B.G.H. 2003; Shackelford and Davis 2003; L. Wang, W. P. Sousa et al. 2004; Ursula C. Benz, Peter Hofmann et al. 2004; JEONG CHANG SEONG and USERY E. Lynn 2001).

7. i tng nh v c im ca i tng

Nh trnh by, trong PLHT th yu t c bn s l cc i tng nh. Ngi ta phn bit hai loi i tng: i tng nguyn thy (primitive) v i tng chuyn (object of interest). i tng nguyn thy c dng trong cc bc phn loi trung gian v chng ta ch chit xut cc i tng chuyn . Vi cch hiu nh vy th i tng nh nht s l pixel (Denfinies 2007). Cc i tng nh c c kho st trong mi lin quan vi cc i tng khc trn nh theo mng phn cp (Ghassemian and Landgrebe 1988 ). Chng ti s trnh by vn phn sau trong qu trnh ng dng. Di y chng ti s lit k cc c im ca i tng nh v cch to i tng v s dng cc c im phn loi i tng.

8. c trng thng k v kin trc ca i tng

Vi mi i tng ta u c th tnh c cc c trng Pixel-based ca bn thn n v quan h ca i tng ny vi i tng khc cng c th o c; khng nhng th ta cn c th so snh cc c trng ca cng i tng trn cc knh nh A v B khc nhau.

)(1

)(1

n

n

xn

n

xn

rAB PB

PAf (1)

Trong : n l s pixel x nm trong i tng v P(x) l gi tr ca pixel ti v tr x.



Vic dng mi c trng Pixel-based ca i tng cho php tng cng tin cy ca kt qu phn loi. Trong phn loi Pixel-based, chng ta ch c th s dng mt trong cc c trng Pixel-based ca cc pixel phn loi; th d nu ta mun s dng gi tr xm trung bnh hay gi tr lch chun phn loi th ta phi chy 2 ln ring bit, Tng khi o, PLHT cho php ta k hp ng thi nhiu c trng v chnh l nguyn tc m m chng ti cp n trn.

9. Hnh dng ca i tng

y l mt c trng quan trng ca i tng gip chng ta nhn dng i tng

nhanh chng v hiu bit ca chng ta v hnh dng i tng s c dng to cc quy tc phn loi. Cc c trng c ni n y l kch thc, t l chiu rng chiu di, s cnh ca i tng v.v. Nh s khc bit v hnh dng m nhiu i tng c cng c trng ph c th c tch ra mt cch d dng. Ly th d: hai i tng nu c c trng ph ca nc nhng c kch thc khc nhau s thuc lp sng hoc lp h,

ao.

Ao, H

Sng, Sui



Hnh : So snh c trng hnh dng ca Sng sui v Ao, h

10. c trng v quan h khng gian ca i tng

c trng ny cho php nh gi quan h ca i tng vi min ln cn trn mt mc v c cc mc phn mnh nh (segmentation scale) khc nhau. Ngi ta c th kho st cc quan h ny thng qua tiu ch khong cch. Nh trn hnh 8. y chng ta cp ti khong cch gia cc i tng (s 1 trn hnh 8) v khong cch gia cc mc phn mnh nh (s 2 trn hnh 8). Trn hnh ny chng ta cng thy i tng nh c th thuc cc mc khc nhau v i tng bc cao hn s bao hm i tng thuc cp thp hn.

Ao, H

Sng, Sui



i tng nh

Mc phn on nh

Pixel

Hnh 8: Quan h topo v khi nim khong cch dng trong PLHT

(Denfinies 2007).

Mng li phn cp ni trn cn cung cp cho chng ta mt lot cc c trng khc c o c t:

Phn tch kin trc (texture analysis) da trn ph i tng (sub-object),

phn tch dng tuyn da trn ph i tng,

c trng ca cc lp lin quan trong h thng phn cp (class hierarchy)

Cc c trng ny u c th c s dng trong PLHT v c th nghim trong khu vc nghin cu huyn Duy Tin, H Nam v s c trnh by phn sau.

11. c trng ng ngha (semantic features)

Ta c bit cc nh vin thm c to nn bi mt ma trn cc pixel c h thng v tinh quan st Tri t thu nhn bng cc phng thc vt l khc nhau (th ng hoc

ch ng), trong cc gii sng khc nhau (quang hc hay siu tn). Vic bin cc pixel ny thnh cc lp c ngha chuyn l mt trong cc mc tiu ca phn loi nh da trn pixel. Xt v mt c ch thu nhn, bn thn cc pixel to nn i tng nh ch mang ngha c t cc kha cnh vt l, sinh hc ca cc pixel. Vic xc nh i tng nh l g trn thc t li l vn gn ng ngha cho chng v l mc tiu ca PLHT. Vic xc nh ny cn tnh m n c bi cnh (context) trong ta quan st i tng. Cc



thng tin ng ngha ny u c s dng rt tt trong PLHT v cng l u vit ca phng php phn loi ny (Ravi Chauhan, Nitin K. Tripathi et al. 2004).

12. To i tng nh

i tng nh c to ra thng qua phn mnh nh cn c vo vic cn i c tnh u tin mt s tham s: bt ng nht (heterogeneity), cht (compactness),

mu sc, hnh dng v.v. bt ng nht c tham chiu ti c trng s cp (primary feature) ca i tng nh lch chun ca xm , hnh dng i tng, kin trc. Phng php phn mnh nh s dng bt ng nht thng ch gip to ra cc i

tng nguyn thy (primitive) m cha c lin h vi thc tin (xt v kha cnh ng ngha). Tuy nhin, cc i tng nguyn thy ny s c gn thnh lp chuyn trong bc phn loi u tin sau s to ra mt lp i tng bc cao hn v st vi thc

t hn (Ghassemian and Landgrebe 1988 ; Baatz M. and Schpe A. 2000; Denfinies 2007). Trong qu trnh phn mnh nh th thng s t l l thng s quan trng. N s quyt nh i tng c chit xut s c lch thc ln hay nh v cha ng nhiu hay t ph i tng (sub-object) bn trong. Di y l m t chi tit vic to i tng nh trong phn mm eCognition l phn mm hot ng theo nguyn l PLHT.

To i tng nh trong eCognition

Phn mnh nh a phn gii (multi-resolution sementation) trong eCognition l k thut gp vng (region merging) i t di ln v bt u t mc pixel . Tng bc cc i tng nh nh s c gp thnh cc i tng ln hn. y l mt qu trnh ti u ha nhm gim thiu s bt ng nht c trng s n h (weighted heterogeneity) ca i tng c to ra vi n l kch thc ca on nh (segment), h l thng s ca bt ng nht. Ti mi bc ca qu trnh phn mnh nh th cc cp i tng lin k s c gp li lm cho bt ng nht tng ln mc nh nht trong gii hn nh trc. Nu gii hn ny vt qua ngng nh sn th qu trnh gp s dng li. Vi cch lm nh vy phn mnh a phn gii l mt th tc ti u ha mang tnh cc b (Benz 1999; Baatz M. and Schpe A. 2000).

Qu trnh phn don nh c bt u t i tng mt pixel. Th tc ny m phng s tng ln ng b ca cc on nh trn ton cnh nhm lu gi cc i tng lin k c kch c v c t l tng t. Chui x l da trn php m nh phn (binary counter) cho php m bo rng s phn b cc i tng c x l s phn b mt



cch ng u trn khng gian nh. Tuy nhin, do n b rng buc bi bn thn qu trnh x l cc pixel v cc i tng nn cch lm nh vy gy ra mt s thay i nh trong kt qu phn mnh nh. V l do m ngi ta phi s dng cc tiu ch ti u ha mc tng th khc phc tnh trng do th tc cc b gy ra ni trn.

13. nh ngha bt ng nht s dng to i tng nh trong eCognition

Trong eCognition bt ng nht dng cc c trng mu v hnh dng lm i tng nguyn thy v c s dng chit xut i tng nh nhiu phng n khc nhau (Ursula C. Benz, Peter Hofmann et al. 2004). S gia tng bt ng nht f khng c vt qu mt ngng nht nh nh biu din trong cng thc di y:

1]1,0[],1,0[,.. shapecolorshapecolorshapeshapecolorcolor WWWWhWhWf (2)

Thng s v trng s ( shapecolor WW , ) cho php ta chuyn nh nh ngha bt

ng nht vo cc ng dng thng qua vic iu chnh hai trng s ny trong qu trnh thc hin phn mnh nh. Trong qu trnh phn mnh nh trong eCognition ta c th b sung trng s cW vo knh c no to ra cc phn mnh khc nhau. S khc bit v

bt ng nht mu (ph) colorh c xc nh trong cng thc sau:

))..(.( 2_,2_1_,1_. objcobjobjcobjmergecmergec

ccolor nnnWh (3)

Vi:

mergen l s lng pixel trong i tng c gp

1_objn l s lng pixel ca i tng 1

2_objn l s lng pixel trong i tng 2

c l lch chun ca i tng knh c.

cc ch s merge dng ch i tng gp, obj_1 v obj_2 dng ch cc i tng 1, 2 trc khi c gp vo i tng merge.



Cng theo tc gi Benz (Ursula C. Benz, Peter Hofmann et al. 2004), bt ng nht hnh dng shapeh l gi tr ni ln ci thin v trn (smoothness) v cht

(compactness) hnh dng ca i tng:

smoothsmoothcomptcomptshape hWhWh .. (4)

Vi:

).(.2_

2_2_

1_

1_1_

obj

objobj

obj

objobj

merge

mergemergesmooth b

ln

bl

nbl

nh (5)

).(.2_

2_2_

1_

1_1_

obj

objobj

obj

objobj

merge

mergemergecompt n

ln

nl

nnl

nh (6)

Trong : l l chu vi ca i tng v b l chu vi ca khung ch nht bao quanh i tng. Cn c vo cc cng thc nu trn y ta thy bt ng nht ca trn chnh l t s gia chu vi ca i tng v di ca khung ch nht bao quanh i tng. Cng theo cc cng thc ny th bt ng nht ca cht bng t s gia chu vi ca i tng v cn bc hai ca s pixel to nn i tng.

Cc trng s cW , colorW , shapeW , smoothW v comptW l cc thng s m ta c th chn,

thay i c tch cc i tng nh mong mun.

Thng s t l l tiu ch dng qu trnh phn mnh nh. C th, trc khi quyt nh gp hai i tng lin k th s gia tng bt ng nht f s c kim tra; nu s gia tng ny vt qu ngng t l thng s t l c tnh bng cng thc: t=, th qu trnh gp s phi dng li (Baatz M. and Schpe A. 2000). Thng s ny cng ln th cng nhiu i tng s b gp v i tng c chit xut s cng ln.

14. Cc la chn khi to i tng nguyn thy

Ty thuc vo mc ch ng dng m ta c th s dng cc phng php phn mnh nh khc nhau. Trong chuyn ny chng ti xin a ra hai cch la chn sau y:



Phn mnh nh theo s khc bit v ph ca cc i tng: cch lm ny cho php to ra cc vng c ng nht ln. Nhng vng c khc bit ph nh hn ngng quy nh s b gp li. Ngng ny ph thuc vo t l m ta chn.

Phn mnh cc ph i tng dng trong phn tch cc i tng dng tuyn ch s dng bt ng nht ca tinh theo cht (compactness) ca i tng. Trong trng hp ny ta dng thng s t l xc nh di ti a tng i ca ph i tng so vi min ln cn ngoi i tng v khng c chung i tng cp cao hn

vi ph i tng ang c xt.

15. Kim chng qu trnh to i tng

Cng nh trong qu trnh phn loi Pixel-based, mt ngi l phng tin nh gi hiu qu nht. C mt s cch kim chng chnh xc ca kt qu phn mnh nh.

C th dng cc polygon m ta s ha trc tip trn nh. Nu c s trng kht ca polygon ny vi i tng nh c chit xut t ng th l kt qu tt nht.

C th phn tch bn ca ng bao i tng nh nh gi xem s gia tng ca bt ng nht c ln hay khng. S gia tng ny cng ln th th xc sut gp cc i tng cng nh v c th ni l s phn mnh nh ny qu nhy cm vi cc trng s ( trn, c cht, hnh dng, mu) v vi t l.

Phi phi hp cc nh gi hai im nu trn moi c th a ra kt lun v cht lng phn mnh nh

16. Mng phn cp i tng

Cc i tng to ra vi cc thng s t l khc nhau c th c lin kt li nh trnh by di y. y cng l cch m phn mm eCognition dng lin kt cc i tng cc mc khc nhau trong phn loi v to ra rt nhiu li th cho phng php PLHT. Phn di y s m t vic to ra mng phn cp ny trong mi trng

eCognition nh th no.

To mng phn cp

Cc mc phn mnh nh c cp phn trn theo cch tim cn a phn gii. Trong phn mm eCognition mi thao tc phn mnh nh u c thc hin mt cch ngu nhin nhng mi i tng c phn mnh ra u c th c quan h vi



cc i tng mc cao hn v/hoc vi cc i tng mc thp hn ty thuc v ng dng.

Hnh 9: Mng phn cp i tng v cc mc lin kt (Denfinies 2007).

Nh ta thy trn hnh 9, mc thp nht c th c ca mt nh chnh l mc ca pixel v mc cao nht l mc ton nh. Gia hai mc ny bao gi cng tn ti cc i tng cc mc trung gian v mc ny l mc con (child) ca mc trn n ng thi li l mc b (parent) ca cc i tng mc thp hn. m bo c c mng phn cp s dng cho phn loi th th tc phn mnh nh phi tun th cc quy tc sau:

Ranh gii ca i tng phi i theo ranh gii ca i tng nm mc thp hn tip ,

Phn mnh nh phi c gii hn bi ranh gii ca i tng mc cao hn.

Cu trc ca cc t l khc nhau phi c th hin ng thi v phi c phn loi trong mi quan h vi nhau,

Cc mc phn cp khc nhau s c phn mnh trn c s cc d liu khng phi l nh nh bn chuyn ,



Phi c kh nng sa cha cc i tng bng cch gp nhm cc ph i tng.

V phng din thc nghim m ni th mng phn cp to ra cc c s tt cho vic chit xut thng tin mt khi tn dng c mi quan h to ra t tnh phn cp ny. Ta c th ly th d khi s dng cc c trng ch s thc vt i din cho i tng thc vt trong qu trnh phn lai cc cy trng.

17. Phn loi m (fuzzy classification)

Nh cp n trn, phn loi m cng thuc nhm cc thut ton phn loi mm (soft classifier) v c ng dng kh rng ri trong x l nh c phn gii siu cao (Nedeljkovic I. 2000). Cng cn nhc li l khi phn loi theo nguyn tc i tng nh hng th chng ta cn tnh m n cc im sau (Ursula C. Benz, Peter Hofmann et al. 2004):

Tnh bt nh ca cc b cm vin thm trong qu trnh thu nhn nh,

S bin i cc thng s do nh hng ca php nh chun thit b (sensor calibration),

Tnh m ca cc nh ngha cc lp chuyn m ta s dng cho phn loi v

S ln cc lp do hn ch ca phn gii khng gian.

Phn loi m s dng mt tp hp n chiu cc thnh vin (membership) cho php m t mc m i tng c th thuc v mt lp no trong s n lp.

)(),....(),( _2_1_, objobjobjf nclassclassclassobjclass (7) Phn loi tng minh thng ch cung cp c thng tin nh gi xem mc

thnh vin (membership degree) no l cao nht mc d n cha kh nhiu thng tin v tin cy tng th, n nh v ln cc lp. Phn loi m li i hi mt h thng hon ton m bao gm qu trnh xy dng cc tp m hay cn c th gi l m ha (fuzzification) cc bin u vo to ra cc tp m (fuzzy set), t hp logic ca cc tp m v kh nng phi m ha (defuzzification) cc kt qu phn loi m bin chng thnh c th nhiu c v chuyn cc lp c phn loi thnh cc lp chuyn .



Mt cch khi qut, logic m l loi logic a gi tr v tng c bn l thay hai kt qu false, true ca thut ton Boolean bng dy lin tc [0, ..,1] vi 0 l gi tr false v 1 l gi tr true v mi gi tr nm gia 0 v 1 s biu din s chuyn tip gia false v true. Vi c tnh ny logic m va cho php trnh c cc ngng t ngt v va cho php ta c lng c thc t vn rt phc tp (Suzuki, H. Matsakis et al. 2001). Logic m c th m hnh ha c t duy khng chnh xc ca con ngi v biu c bit hiu qu trong phn loi cc nh v tinh quan st Tri t l loi d liu cha nhiu yu t bt nh nh phn tch phn u. Mi yu t c xt qua tp m c xc nh bi cc hm thnh vin (membership function).

Phn loi m bao gm ba bc chnh : 1) xy dng tp m, 2) xy dng cc quy tc phi hp cc tp m v 3) phi m ha.

Qu trnh xy dng tp m

Vic m ha s m t s chuyn tip t h thng tng minh sang h thng m bng cch to cc tp m cho mt s dc im ca i tng. Cc tp m ny s m t c trng ca i tng thnh cc lp, th d Thp, Trung bnh, Cao. Bn thn cc lp ny li c xc nh bi cc hm thnh vin. Cc hm ny c s dng gn thnh vin (membership degree) t 0 n 1 cho tng gi tr ca c im c s dng m t i tng trong mi lp ang xt. Hnh di y trnh by mt cch khi lc qu trnh ny.

=0.5

=0.0

=1.0

m(X)

A(X)

Hnh 10: M hnh xy dng tp m (m ha c im ca i tng) (Ursula C. Benz, Peter Hofmann et al. 2004).



Nh ta thy trn hnh, hai hm thnh vin dng ch nht v dng hnh thang c s dng xc inh tp tng minh }1,0{)(),( xXM M v tp m }1,0{)(),( xXA A trn gii gi tr X ca c tnh i tng. Ty thuc vo hnh dng ca hm thnh vin th s chuyn t thnh vin y hay khng thnh vin c th l tng minh (vi hm hnh ch nht) hay m.

Mi gi tr thuc tnh ca i tng m c gi tr thnh vin cao hn 0 u thuc tp m. Nhn chung, hm thnh vin cng ln th m ca nh hng phn loi cng ln ; khi gi tr ca thnh vin cng nh th vic gn gi tr cho tp s cng bt nh (Suzuki, H. Matsakis et al. 2001). Mt im cn nhn mnh na l trong phn loi m th ta s khng dng cc gi tr thuc tnh ca i tng m phi dng cc tp m dc xc nh cho cc thuc tnh ca i tng. Chnh v vy tt c cc php tnh ton u da vo mc thnh vin dao ng t 0 cho n 1 v hon ton c lp vi gi tr gc ca i tng. iu ny s gip cho ta d lm vic hn trong 1 khng gian nhiu chiu. Trong ta phi s dng d liu t nhiu ngun khc nhau v l cc d liu c th nguyn khc nhau.

c c kt qu phn loi tt, th vic chn cc hm thnh vin v vic tham s ha. Cc hm ny phi c chn sao cho n thch hp nht vi vic xy dng c quan h gia cc c im ca i tng v cc lp. Chnh v vy qu trnh phn loi phi c thit k trc v phi s dng rt nhiu kin thc chuyn gia. y l bc quan trng ca qu trnh phn loi. Ta cng bit tt v cc i tng trn thc t th cng t c kt qu tt.

Nhn chung c th xc nh dc nhiu tp m cho 1 c im ca di tng, v d ta c th xc nh ti 3 tp m cho 1 c im ca i tng : thp, trung bnh, cao. Khi cng nhiu thnh vin b chm ln nhau th i tng cng tr nn chung cho cc tp m. V vy, kt qu phn loi s tr nn m h hn. Ta ly th d : Vi 1 i tng nh c gi tr x=70, c mc thnh vin gn vi lp THP l 0,4, vi lp TRUNG BNH l 0,2 v vi lp CAO l 0. Nu gi tr x=200 th mc thnh vin gn vi cc lp s ln lt l 0.0, 0.0, 0.8.

18. B quy tc m

B quy tc l t hp ca quy tc m s dng phi hp cc tp m khc nhau. B quy tc m n gin nht ch ph thuc vo 1 tp m l quy tc If Then. iu c



ngha nu mt iu kin c p ng th s c 1 hnh ng c din ra. V d : nu c im x l Thp th i tng nh s c gn vo lp A. Trong ngn ng ca logic m th iu ny c th din t nh sau : NU c im x l thnh vin ca tp m Thp TH i tng nh s l thnh vin ca lp A. Trong v d ca hnh 5, khi gi tr x=70 th mc thnh vin ca i tng thuc v lp A s l 0,4 ; trong trng hp x=200 th mc thnh vin ny l 0.

to c b quy tc m mc phc hp th ta phi tch hp cc tp m li vi nhau bng cc ton t logic And, Or, Not to ra gi tr chit xut t cc tp m ny. Trong phn mm eCognition ta c nhiu kh nng tch hp cc tp m theo ton t logic ni trn. Tuy nhin vic kt hp nh th no l hon ton ph thuc vo hiu bit ca chng ta v i tng trn thc t. i vi khu vc nghin cu ca ti ti huyn Duy Tin th cc hiu bit ny c thu thp t cc ngun Pixel-based, iu tra thc a v iu tra nng h. Chng ti s trnh by chi tit qu trnh xy dng b quy tc cho vic phn loi cc i tng khc nhau Duy Tin mt chng khc.

Hnh 11: Minh ha cho 3 tp m Thp, Trung bnh v Cao thuc c im x ca i tng (Ursula C. Benz, Peter Hofmann et al. 2004)

B quy tc m c s dng to ra cc gi tr ch th cho mc thuc v lp ca i tng (hnh 12) di y l mt th d.



0.6

0.8

0.3

th Nc Thc vt

Hnh 12: Mc thnh vin ca cc lp c php phn loi

(Ursula C. Benz, Peter Hofmann et al. 2004)

Trong th d ny ta c ba lp th, Nc v Thc vt. C th nhn thy l i tng nh l thnh vin ca c ba lp vi cc mc khc nhau : 6.0)( objthi (

th), 8.0)( objnc (Nc) v 3 6.0)( objTv (Thc vt). Mc thnh vin ca i tng

so vi nc l ln nht ( 8.0)( objnc )) v v vy i tng ny thng c gn vo lp

nc. chnh lch gia hai thnh vin cng cao th quyt nh cng c n nh cao. n nh v tin cy ca phn loi u c th tnh c bng eCognition (Denfinies 2007).

Mt khi cc mc thnh vin ca mt i tng so vi mt lp m bng nhau th kt qu phn loi l rt khng n nh. Chng ta cng bit rng trong phm vi mt pixel th ta khng phn bit c hn mt lp. Tuy nhin nu gi tr ca mc thnh vin m cao v nu h thng phn loi c thit k trn nn cc hiu bit tt v i tng th kt qu s cho ta thy ln bn trong lp. i vi cc nghin cu vin thm nh lng th y l li th ln ca PLHT vi vic s dng logic m trong nh gi ni hm thng tin cha trong cc pixel. Nu gi tr ca mc thnh vin thp th i tng kh c th c xp vo lp d kin. Cn xc nh ngng cho cc gi tr mc thnh vin v mt khi gi tr ny khng t ngng th i tng khng c phn loi (Demetre P. Argialas and Angelos Tzotsos 2004).

19. Qu trnh chuyn ha kt qu (defuzzification)



c c sn phm dng bn chuyn t kt qu phn loi s dng logic

m th cc kt qu phi c chuyn sang cc gi tr tng minh. Vi mc ch nh vy ta phi s dng gi tr thnh vin ln nht nh l mt lp tng minh. y l th tc c bit quan trng v in hnh ca qu trnh phi m ha kt qu phn loi. Cn lu l nu gi tr ny nh hn ngng quy inh th qu trnh phn loi s khng c thc hin.

Cng on ny s ch c tin hnh khi cc thng s c tnh ton k lng v vic nhp gi tr thnh vin ln nht vo h thng s xa ht cc thng s khc m ta xy dng trc trong mi trng eCognition.

Nhn chung, qu trnh phn loi i tng c chia lm hai giai on :

- Giai on th nht : qu trnh phn mnh nh(segment). Kt qu ca qu trnh phn mnh nh to ra cc i tng nh. C s cho s phn chia ny da trn nhng thng s do ngi phn loi t ra v d: thng s v hnh dng (shape), thng s v cu trc, kt cu ca i tng

- Giai on th hai: cc i tng nh c ch nh cc lp chuyn (lp thng tin) da theo s m t ca nhng i tng(cc thut ton phn loi) do ngi phn loi thit lp ra.

20. Bng so snh phn loi Pixel-Based v phn loi nh hng i tng

Mu sc/

ph

Hnh

dng

Din tch/

kch

thc

Cu trc

Bi cnh/

ng cnh

Phn loi Pixel-

based

Phn loi nh

hng i tng

(Sun Xiaoxia Zhang Jixian Liu Zhengjun

Chinese Academy of Surveying and Mapping, No16, Beitaiping Rd, Haidian District,Beijing, 100039, China )

Phan 1_Phuong Phap Phan Loai DHDT

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