Tm kim v phn tch d liu khng gian. y l chc nng ng vai tr rt quan trng trong GIS.

N to nn sc mnh thc s ca GIS so vi cc phng php khc. Tm kim v phn tch d liu khng gian gip tm ra nhng i tng ho theo cc iu kin t ra hay h tr vic ra quyt nh ca ngi dng GIS 1. Tm kim v phn tch d liu khng gian C rt nhiu cc phng php tm kim v phn tch d liu khng gian, cc phng php khc nhau thng to ra cc ng dng GIS khc nhau. Sau y l mt s phng php c dng ph bin nht: 1.1. Buffer (tm kim d liu trong vng khng gian) Buffer hay cn gi l truy vn khng gian trn c s cc quan h khng gian gia cc i tng. Cc quan h ny thng thng ni ln v tr tng i ca i tng ny vi i tng kia. Phng php buffer c chia lm nhiu loi (php ton) khc nhau, nhng cch thc x l th lun tun theo cc bc c bn sau y:

Chn ra mt hay nhiu i tng trn bn , gi l cc i tng gc. p dng mt quan h khng gian tm ra cc i tng khc m c quan h c bit vi cc i tng gc. Hin th tp i tng tm thy c trn d liu khng gian v thuc tnh

Mt s php ton buffer thng dng Tm cc i tng nm bn trong cc i tng khc. Php ton ny xc nh quan h bao kn gia cc i tng khng gian. ng thng bao gm nhiu im, mt a gic (polygon) c th bao gm nhiu ng thng hoc gm cc a gic con khc. Tm cc i tng ct cc i tng khc. Php ton ny xc nh cc i tng c giao im hay nm chng ln cc i tng khc. Hai a gic giao nhau nu chng c mt min chung. Hai ng thng ct nhau nu chng c mt im chung. Mt ng thng giao vi mt a gic khi n nm mt phn hay ton b trong a gic. Tm cc i tng lin k vi cc i tng khc. y l kiu tm kim trong cc i tng c chung ng bao (bin). Quan h ny ch p dng cho ng thng hoc a gic. Tm cc i tng nm bn trong hoc bn ngoi mt khong cch xc nh. Kiu tm kim ny c s dng trong vic xc nh cc i tng xung quanh mt hay nhiu cc im mc. Qu trnh thc hin bao gm vic to ra mt vng m quanh cc im mc ny v sau xc nh cc i tng cn c vo v tr ca chng so vi vng m to ra. Mt bi ton rt in hnh cho phng php buffer ny l bi ton v Nh my ho cht v cc bnh vin. Mc ch ca bi ton l xc nh cc v tr thun tin nht trn bn cho vic di di cc bnh vin trong trng hp nh my ho cht gp s c. Cc nh my ho cht v bnh vin c biu din trn bn bng cc i tng im (points). Mi nh my bao gm cc thng tin chi tit v loi ho cht sn xut v mc pht tn cht c ra mi trng trong cc iu kin thi tit khc nhau. Khi c s c, vng nguy him cn di di s c th hin trn bn . T , chng ta c th bit c nn chuyn bnh vin n vng no l an ton v thun tin nht.

1.2. Geocoding (tm kim theo a ch) Mt i tng trn bn bao gi cng c biu din bng mt kiu d liu ho. Phn ho ny c th thu c bng cch s ho hay qut nh bn . Tuy nhin, khi ta c bn (bn s), chng ta cng c th xc nh c phn ho biu din i tng hay l v tr, hnh dng ca i tng thng qua cc d liu m t v tr ca n v d: s nh, tn ng, tn qun Geocoding (hay address matching) l mt tin trnh nhm xc nh cc i tng trn c s m t v tr ca chng. y l mt k thut rt ni ting, c mt trong rt nhiu ng dng ca GIS. Ngi ta gi mt geocoding service l qu trnh chuyn i ton b m t thuc tnh v v tr sang m t khng gian. tm c v tr thng qua a ch, geocoding service phi tham chiu n t nht mt ngun d liu bao gm c thng tin v a ch (thuc tnh) v thng tin khng gian (v tr, hnh dng). D liu ny c gi l d liu tham chiu. Cc gocoding service c th thao tc trn nhiu kiu d liu tham chiu khc nhau. Sau khi geocoding d liu tham chiu (tc l nh x m t thuc tnh vo m t khng gian). Ta c th nhp a ch ca i tng cn tm. Quy trnh x l tri qua cc bc sau:

Chun ho gi tr a ch va nhp vo bng cch tch n thnh cc thnh phn a ch nh. Geocoding service sau s tm trong ngun d liu tham chiu xc nh cc i tng c cc thnh phn a ch tng ng vi d liu nhp vo. Mi kiu geocoding service s quy nh cc nh dng ca cc thnh phn a ch ny. Tp kt qu tr v s c gn cc trng s (im) tm ra kt qu gn ng nht. Geocoding service s nh du i tng va c tm thy trn bn bng mt i tng ho.

1.3. Networks (phn tch mng) Networks l k thut c ng dng rt rng ri trong giao thng, phn phi hng ho v dch v, vn chuyn nc hay xng du trong cc ng ng di, trao i thng tin qua mng vin thng Trong GIS, networks c m hnh di dng cc th mt chiu hay mng hnh hc. Mng hnh hc ny bao gm cc i tng ang c hin th trn bn , mi i tng ng vai tr l cnh hoc nt trong mng. Trong GIS thit lp nn mi quan h gia nt - cnh v cnh - cnh ta cn to cc topology cho c s d liu. Topology c hiu l mi quan gia cc i tng trong bng d liu. Quan h topology gia cc i tng gn ging quan h gia cc bng (relationship). Chng ta c hai kiu lin kt l nt - cnh v cnh - cnh. Nt - cnh l lut lin kt c thit lp gia mt nt ca i tng kiu A vi mt cnh ca i tng kiu B. Cnh - cnh l lut lin kt gia mt cnh ca i tng kiu A v mt cnh ca i tng kiu B qua mt tp cc nt. Khi to topology v xc lp lut lin kt, mt mng lgic c hnh thnh. Lc ny ta c th p dng cc thut ton v mng gii quyt cc bi ton t ra.

1.4. Overlay (ph trm hay chng bn ) y l k thut kh nht v cng l mnh nht ca GIS. Overlay cho php ta tch hp d liu bn t hai ngun d liu khc nhau. Ngi ta nh ngha: Overlay l qu trnh chng kht hai lp d liu bn vi nhau to ra mt lp bn mi. iu ny tng t nh vic nhn hai ma trn to ra mt ma trn mi, truy vn hai bng c s d liu to ra bng mi, vi overlay l gp hai lp trn bn to ra bn mi. Overlay thc hin iu ny bng cch kt hp thng tin mt lp ny vi mt lp khc ly ra d liu thuc tnh t mt trong hai lp. Ngi ta chia overlay thnh ba dng phn tch khc nhau:

Point-in-polygon: chng kht hai lp point v polygon, u ra l lp point Line-in-polygon: chng kht hai lp line v polygon, u ra l lp line Polygon-in-polygon: chng kht hai lp polygon v polygon, u ra l lp polygon

Mt bi ton rt in hnh cho k thut ny l bi ton v kim tra tnh hnh ngp lt ca cc tha t trong mt vng c thin tai. y chng ta thy c hai lp: mt lp cho bit tnh trng l lt trong vng, mt lp thuc v t ai. Thng thng hai lp ny s nm trn hai bn khc nhau v mc ch s dng ca chng khc nhau. Khi cn bit tnh trng ngp lt ca tng tha t, ngi ta tin hnh chng kht hai lp bn . Lc ny thng tin v tnh trng ca tha t s c ly t lp l lt ch khng phi ly t lp tha t v lp tha t khng cha cc thng tin ny. V d ny m t bi ton thuc loi polyon-in-polygon. Qua bi ton chng ta c th thy mt iu rng hai lp m ta a vo overlay phi c s thng nht vi nhau. Thng nht v h quy chiu, thng nht v t l, c c iu kin ny ta mi tin hnh overlay c. Qu trnh overlay thng c tin hnh qua 2 bc:

Xc nh ta cc giao im v tin hnh chng kt hai lp bn ti giao im ny Kt hp d liu khng gian v thuc tnh ca hai lp bn .

Cc php ton overlay bao gm: php hp (Union), php giao (Intersect) v php ng nht (Identity) Php hp Hot ng nh ton t Or u vo l hai lp bn kiu l polygon Kt qu u ra l mt lp bn mi bng cch overlay hai min d liu u vo v d liu thuc tnh ca chng. iu kin: min d liu phi l polygon

Php giao Hot ng nh ton t And To ra mt vng bao ph mi bng cch overlay hai tp d liu u vo Kt qu u ra bao gm phn d liu thuc vo c hai tp d liu u vo

Php ng nht To ra mt vng bao ph mi bng cch overlay hai tp d liu u vo. Kt qu u bao gm ton b phn d liu ca lp u tin v ch nhng phn no ca lp th hai c chng kht.

1.5. Proximity (tm kim trong khong cn k) Proximity l php tm kim trn c s o khong cch quanh hoc gia cc i tng. Khong cch ny c tnh theo khong cch Euclidean. C 3 phng php phn tch proximity: Phng php th nht l tm kim ni dung trong vng: trong vng tm kim c xc nh bi xp x ti hin tng c sn, chnh l phng php buffer. Vic tm kim ny c thc hin trong vng to bi m rng i tng cho trc theo mt khong cch cho trc. Trong GIS vng ny c gi l vng m, n c xy dng xung quanh i tng im, i tng ng hay i tng vng. Trong cc h thng trn c s raster th vic to lp vng m dc thc hin nh chc nng spread. Phng php th hai ca tm kim cn k l tm ra cc vng ni trc tip vi i tng xc nh trc: chng hn nh tm cc mnh t lin k vi mnh t s xy dng nh my. Phng php th ba ca tm kim cn k xy ra khi cn phi tm kim nhng vng gn nht ti tp cc v tr mu phn tn khng u. Cc mu thng l cc im. Tm kim ny thc hin bng cch to lp a gic Thiessen, n xc nh cc vng xung quanh mi im m gn im ny hn mi im khc. S a gic Thiessen cn c gi l s Voronoi. Chng c s dng lp ra bn s dng t cc mu t cch bit

2. Vector v Raster trong GIS C hai phng php chnh lu tr thng tin bn : GIS lu cc i tng bn trong nh dng vector v trong nh dng raster. Trong nh dng vector, cc i tng bn c biu din bi cc i tng hnh hc c bn point(im), line(ng), polygon(vng). Point dng xc nh cc i tng khng c hnh dng kch thc c th, hay c kch thc qu nh so vi t l bn . Line xc nh cc i tng c chiu di xc nh. Polygon xc nh cc vng, min trn mt t. Trong nh dng ny, thng tin c m t c tnh chnh xc cao ng thi tit kim khng gian lu tr. Thng tin lu trong nh dng vector ch yu c ng dng trong bi ton v mng, h thng thng tin t ai. Trong nh dng raster, cc i tng bn c biu din trong mt chui cc im nh trong mt li hnh ch nht. Mi im nh c xc nh thng qua ch s hng v ct trong li. Trong raster, point s c biu din bi mt im nh n, line c biu din bi mt chui cc im nh lin tip nhau, v polygon xc nh bi mt nhm cc im nh k st nhau. D liu c lu trong nh dng ny rt n gin nhng li i hi dung lng b nh ln. Raster ph hp vi cc dng d liu c ng bin khng r rng. Raster c ng dng nhiu trong phn tch b mt lin tc.

2.1. H to a l v h to quy chiu V tr ca vt th trong khng gian u phi gn lin vi mt h to . Trong GIS, biu din d liu khng gian ngi ta thng dng 2 h to : h to a l v h to quy chiu. H to a l l h to ly mt cu ba chiu bao quanh tri t lm c s. Mt im c xc nh bng kinh v v ca n trn mt cu. H to quy chiu l h to hai chiu thu c bng cch chiu d liu bn nm trn h to a l v mt mt phng. 2.1.1. H to a l H ta a l s dng b mt hnh cu xc nh v tr ca mt im trn tri t. n v o ca h l . V y l h ta gn lin vi trc tri t nn xc nh v tr ca i tng ngi ta chia b mt tri t thnh cc ng kinh tuyn v v tuyn. Kinh tuyn l cc ng cong cch u

nhau chy qua hai im cc Bc v Nam, v tuyn l cc ng trn song song c tm nm trn trc ca tri t. Giao im gia kinh tuyn v v tuyn to thnh cc li. Trong s cc kinh tuyn v v tuyn c hai ng quan trng nht c ly lm gc to l: v tuyn c bn knh ln nht - chnh l ng xch o v kinh tuyn chy qua vng Greenland nc Anh. Giao im gia hai ng ny l gc to . Hai ng ny cng ng thi chia tri t lm 4 phn bng nhau: na Bc v Nam nm pha trn v di ca ng xch o; na ng v Ty nm pha bn phi v tri ca kinh tuyn gc. Mt im nm trn mt cu s c hai gi tr to l kinh v v c xc nh nh trong hnh v trn. Gi tr ny c th c o bng theo c s 10 hoc theo , pht, giy. Min gi tr ca v : -900 900 kinh : -1800 1800

ch : ch trn ng xch o th khong cch mt ca v tuyn mi bng khong cch mt trn kinh tuyn. Trn cc v tuyn khc khong cch ny khc nhau rt nhiu. Ngi ta tnh rng mt trn kinh tuyn di khong 111,321 km trong khi 600 trn v tuyn ch c di 55,802 km. Chnh v s khc nhau ny nn ta khng th o chnh xc c chiu di v din tch ca i tng khi d liu bn c chiu ln mt phng. Mt cu v mt Ellipsoid Trong h to a l c hai b mt hnh cu c s dng l: mt cu (tuyt i) v mt Ellipsoid. V b mt ca tri t ca ta khng phi l hnh cu tuyt i m n gn vi hnh Ellipsoid nn mt Ellipsoid thng c dng biu din. Tuy nhin i khi ngi ta cng s dng mt cu cng vic tnh ton d dng hn. Khi t ln bn rt nh < 1:5000.000, t l ny s khc bit gia d liu biu din bng mt cu v mt Ellipsoid l khng th phn bit c bng mt thng. Lc ny, mt cu c dng. Nhng khi t l > 1:1.000.000 th ngi ta cn thit phi dng mt Ellipsoid m bo chnh xc. Do , vic la chn mt cu hay mt Ellipsoid ph thuc vo mc ch ca bn v chnh xc d liu.

Nu mt cu da trn hnh trn th mt Ellipsoid li c c s l hnh Ellip. Hnh Ellip c xc nh bi hai bn trc m ta hay gi l: bn trc ln v bn trc nh. Ta cho Ellip xoay quanh bn trc nh ta s thu c hnh Ellipsoid. Kch thc v hnh dng ca Ellipsoid c xc nh bi bn trc ln a v bn trc nh b, hay bi a v h s dt f f = (a - b) / a. V h s f rt nh nn ngi ta thng dng gi tr 1/f A = 6378137.0 m 1/f = 298.25722563 2.1.2. H to quy chiu thun tin cho s dng ngi ta phi nghin cu cch th hin b mt tri t ln trn mt phng ca bn . Do phi thc hin php chiu b mt cong ca tri t ln mt phng v h to quy chiu ra i. H to ny lun ly h to a l lm c s. H to quy chiu c c trng bi hai trc x theo phng ngang v y theo phng thng ng. Gc to l giao im ca hai trc ny. Hai trc giao nhau ng thi chia mt phng lm 4 phn tng ng vi 4 phn trong h to a l. Mt im trn mt c xc nh c xc nh bi cp gi tr (x, y).

C rt nhiu php chiu b mt cong ca tri t ln mt phng song v c bn ta c th hiu nh sau. Ly mt mnh ba cun xung quanh b mt cu trong h to a l theo mt hnh tr ng. T tm ca b mt cong ta v cc tia ct cc im giao gia kinh tuyn v v tuyn, ng thi ko di ct mt tr. Thc cht ca vic ny l chiu cc li ln b mt phng. M t ba ra ta c kt qu ca php chiu. Nhn vo tm ba ta nhn thy, cc li thay i kh nhiu (bin dng), co li hoc dn ra. Cng xa ng xch o th s bin dng cng ln. iu ny gy

nn s thay i v hnh dng, kch thc, khong cch ca d liu khng gian.

Sau ngi ta dng cc cng thc ton hc tng ng to ca b mt cong ln to mt phng chiu. Cc php chiu khc nhau gy ra cc bin dng bn khc nhau nn vic s dng php chiu no l da vo mc ch ca bn v chnh xc ca d liu. Cc php chiu c bn Trong phn ny ta s tm hiu ba php chiu c bn v thng c s dng nht l php chiu vi mt chiu: mt hnh nn, mt hnh tr v mt phng phng v. Bc u tin khi tin hnh php chiu ny l to ra mt hay mt tp cc im tip xc. Cc im tip xc ny c gi l cc tip im hay l tip tuyn (trong trng hp l ng thng). Cc im ny c vai tr rt quan trng, v bin dng ca php chiu trn nhng im ny bng khng. bin dng s tng khi khong cch gia im chiu v im tip xc tng. Mt hnh nn thc hin php chiu ny ngi ta cho dng mt mt hnh nn p ln b mt cu. ng thng tip xc gia mt nn v mt cu l mt v tuyn v c gi l v tuyn chun. Cc ng kinh tuyn sau khi chiu mt nn s thnh nhng ng thng ng, cc ng v tuyn s to thnh nhng ng trn. Sau khi thc hin php chiu, ngi ta s ct hnh nn dc theo mt kinh tuyn bt k, lc ny ta s c kt qu ca php chiu trn b mt nn. S giao nhau gia nhng ng thng v cung trn s to nn mt mt li. ng thng i din vi ng ct c gi l kinh tuyn trung tm. Cng xa v tuyn chun bin dng cng tng. Do tng chnh xc ngi ta ct b phn nh ca mt nn hay ta khng tin hnh chiu ln vng ny. Php chiu ny thng c dng

cho vic chiu cc vng c cc v tuyn trung bnh chy qua v hng theo chiu ng - ty.

Mt hnh tr Ging nh php chiu mt nn, php chiu ny cng c mt ng thng tip tuyn. Khi s dng mt tr, ngi ta phn lm 3 loi tu thuc vo v tr tng i ca mt tr so vi mt cu:

Hnh tr c t theo phng thng ng v tip xc vi mt cu theo mt v tuyn, thng l ng xch o. Gi l php chiu Mercator. Hnh tr c t theo phng nm ngang, ng thng tip xc l mt kinh tuyn. Gi l php chiu Transverse. Hnh tr t xin v tip xc vi mt cu theo mt ng trn c bn knh ln nht (bng vi bn knh ng xnh o). Gi l php chiu Oblique.

Php chiu thng c s dng nht l Mercator. Trong php chiu ny, cc ng kinh tuyn s c chiu thnh nhng ng thng ng cch u nhau, cc ng v tuyn s tr thnh nhng ng nm ngang khong cch khng u nhau; tng dn v pha hai cc. Do bin dng s tng dn v pha hai cc. Sau khi thc hin php chiu, ngi ta s ct mt hnh tr dc theo mt kinh tuyn, tri ra trn mt phng ta s thu c kt qu.

Mt phng phng v L php chiu d liu bn ln mt mt phng tip xc vi mt cu. im tip xc ny c th

l: nm ti hai cc, ti ng xch o, hoc ti mt v tr bt k nm gia. V tr ca im tip xc cho ta bit v tr tng i ca mt phng chiu vi mt cu v to nn ba kiu chiu khc nhau: polar, equatorial v oblique.

Mt phng chiu tip xc vi cc ca mt cu l kiu chiu n gin nht v cng hay dng nht. Trong php chiu ny, cc ng kinh tuyn s c chiu thnh mt chm ng thng giao nhau im cc, v tuyn l cc ng trn c cng tm l cc ca mt cu. Gc gia cc ng kinh tuyn c bo tn.