VECTOR - RASTER CONVERSION

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1 PERIODICA POLYTECHNlCA SER. CIVIL ENG. VOL. 39, NO. 2, PP (1995) VECTOR - RASTER CONVERSION Tran Quoc HUNG Department of Photogrammetry Techncal Unversty of Budapest H-1521 Budapest, Hungary E-mal: hung@epto.bme.hu Receved: May 15, 1995 Abstract Ths paper examnes the characterstcs of vector and raster databases and compares the applcaton of these databases n geographcal nformaton systems. Ths paper shows n partcular a method of converson of vectoral database to raster and the applcaton of ths method to create land cover database of Hungary. Keywords: GIS system, raster-vector converson, landcover database. Nowadays the geographcal nformaton system (GIS) helps to solve a wde range of complex problems. Wth the rapd development of computers we have the possblty to process and vsualze not only vector nformaton but also mages and raster form data. We can magne a raster fle as a data matrx, the elements of whch reproduce the surface as t s n realty. The raster form storage helps the mathematcal processng of the database (e. g.: ponts, local, geometrcal, etc.). In the case of some works (e. g.: area analyss, envronment protecton, etc.) that demand not only arthmetc but also vsualzng the raster form s clearer and more useful than the vectoral. The most GIS system endeavours to handle the vectoral and the raster databases n the same tme (hybrd raster - vector edtng). It utlzes all advantages of both the databases, so the achevement and the effcency of GIS system ncrease greatly [PIWOWAR, 1990]. There are several ways to create a raster fle, one practcal soluton s to convert the extng vector database nto raster n accordance wth the actual task. In the followng a possble soluton of the vector - raster converson wll be presented. 1. Characterstcs of Vector Database We can create the vector database n several ways (e. g.: dgtzng, scannng, deducton from a model, etc.). The element of the vector database

2 136 T. Q. HUNG can be dfferent dependng on the defnton of the system (e. g.: pont, lne polygon, text, etc.). The storng of element happens as follows: Descrpton of element (type, layer, colour, thckness, etc.) - Connecton between an element and the next one (a ponter may show the place of the next element n the fle) The coordnates of the element (X, Y or Z n accordance w~th the actual model) As the vector database conssts of the coordnates of the elements t s complcated to work wth a surface created from more than one element and t s not capable of processng the elements of the surface. To defne the surface t s necessary to carry out more examnatons one after the other. Ths method s based on a model whch s called topologcal data model. Topology s spatal relatonshp between connectng or adjacent coverage features. For example, the topology of a surface ncludes ts nodes and ts left and rght polygons. By storng nformaton about the locaton of feature relatve to other features, topology provdes the bass for many types of geographcal analyss wthout havng to access the absolute locatons held n the coordnate fles (e. g.: connectvty, route fndng and contnuty are all derved through topology). Ths data model s appled n the most GIS systems (e. g.: ArcInfo, Integraph MGE, etc.). The raster database s deducted from the vector database wth the necessary resoluton dependng on the character of the task. The vector database must contan the coordnates of the objects that serve n the converson for the deducton of raster elements. The type of element depends on the groupng of objects, the three most common ones are the followng: pont elements wth the matchng symbols from whch we get to know the poston of the pont on the map and ts type (t can be a lne element for the statng the fnal pont of whch s the same) - lne elements and borders of the surface (lne or polygon element) - text elements that show the character of the surface (surface code) To avod the problems of converson the vector database (nput base) should meet the followng requrements: - every object should be stuated on dfferent layers because ths defnes the type of the object. - the surface should always be closed - that s the sdes of the surface should be ts borders at the same tme. - the place of the surface code should be nsde the borders of the surface. - the rank of the surface code should be defned unambguously.

3 VECTOR - R"ASTER CONVERSION The Characterstc of Raster Database The raster fle can be magned as a data matrx. The dstance between the rows and columns of the matrx depends on the resoluton of the raster. The employment of the database, the scale of the map and the effcency of the computer have a great nfluence on the choce of the resoluton., The number of the objects on a gven terrtory defnes the number of bts charactersng the matrx pont (e. g.: f the number of the objects s 14 then an element occupes 2 bytes or 16 bts n the fle). The bt storng a characterstc of the surface has the value of 1 whle the others have O. A raster element can store more peces of nformaton because for example the electrc lnes and the surface elements underneath the electrc lnes seem to be the same but n the raster fle they appear as two dfferent elements, so both bts belongng to the object have a value of 1, the others O. Ths knd of storng allows us to defne the type of the object wth the examnaton of the bts or by maskng the bt to process the database wth mathematcal methods. 3. The Process of Vector - Raster Generaton The process of vector - raster generaton can be dvded nto the followng parts: - readng of database - generaton of lnes elements and borderlnes (lne generaton algorthm) - organzng of surface codes generaton of surface elements (surface generaton algorthm) wrtng of raster database Readng the data means to read element descrbng parts, element coordnates and elmnatng redundant nformaton. Wth the help of the layer and the type (pont, lne polygon or text) of the element we can defne the character of lnes and surface codes. These data get to the memory of the computer nto blocks occuped by the program. Wth the help of the lne generaton algorthm we convert the lne (defned by ts two fnal coordnates) nto raster elements. Several algorthms have been worked out for ths purpose, we present only one possble soluton. Startng from the frst pont of the lne the raster elements follow each other both horzontally and vertcally (dx and dy) dependng on the ste"epness of the lne and the last raster element s n the fnal pont. (Fg. 1). The lne can defne an object or a borderlne, t s the last bt whch dfferentates (f 0: object else borderlne). In the case of a pont

4 ,! 138 T. Q.HUNG I I I t I 1/1 I /1 : V pil II PUlIl.Y dy lew 1 dy -r YI I ~t t t,! I. t 1 I 1.,o~t : dxnew dx Fg. 1. element there s only one raster element, because the startng and the fnal ponts are dentcal, so dx and dy are 0 [ACKLAND, 1981J. Durng the vector - raster converson the lne algorthm s very mportant because not generatng precsely t can cause a change n the sze of the gven terrtory (n case of a borderlne) or naccuracy of the data (n case of a lne). As after the generaton of a lne the borderlnes of the surface are not dfferentated only the rank of the surface code helps us to decde \vhch borderlne belongs to a certan surface. Ths way the subordnaton of dfferent types of surface s necessary. The arrangement of surface codes s necessary because the surface generaton algorthm s happenng from the hgher rank code to the 10\ver ones. The value of the raster elenent, whch s on the borderlne, s always dentcal wth the hgher rank surface raster value (e. g.: f a forest and a lake n that forest have a common border then borderlne belongs to the forest because the forest s on a hgher level than the lake). Here we have to be careful: f the dstance between the borderlnes belongng to a surface s smaller than the raster resoluton, t s advsable to order more surface codes to t, otherwse the programme consders t as a closed surface. The surface generaton algorthm starts from a pont nsde a surface and examnng a certan drecton t orders the sutable value to the other ponts. Ths process contnues untl reachng the borderlnes of the surface. The basc prncple of possble surface generaton algorthm can be seen n Fg. 2. The program frst gves raster value to the element of the row contanng the surface code, n the meantme t examnes the upper and lower

5 VECTOR - RASTER CONVERSION 139 Fg. 2. rows. If durng the examnaton t fnds the borderlne the program gves the surface value to the element of the borderlne and orders a new surface code to the examned row. Every new surface code gets a code ndex on the base of whch the fllng of the other rows s takng place. As you can see t n the fgure the orgnal code s marked wth BK. The fllng of the row s takng place n both drectons (rght and left). The examnaton of the rows up and down starts from Xleft and contnues to Xrght. Arrows show the place of the examnaton ponts. BKl, BK2, etc. are new surface codes created durng the examnaton [PAULIDIS, 1979]. Besde the database the raster fle can contan the number of the map shfts, the number of rows, columns and references that show where and what knd of further nformaton can be got. These data make the access to the nformaton for processng easer. 4. Applyng Vector - Raster Generaton Algorthm n Order to Create a Hungary Landcover Database Wth the growng demand and techncal level the GISsystem plays a more and more mportant role n the feld of telecommuncaton. Wthout takng the surface of the earth nto consderaton the expanson of the telecommuncaton network s not possble. In the plannng of these networks t s very mportant to examne the reflecton of waves nfluenced by the relef of the surface and by the natural and artfcal objects stuated on the terrtory. We can get nformaton about terran and objects on the surface from

6 140 T. Q.HUNG the DTM and the landcover database. The Hungary landcover database was realzed by Photogrametry Dep!lrtment and Geomatk Ltd. wth the Frequency Management Insttute of Hungary cooperaton. The natural and the artfcal terran object's heght and character determne the reflecton of land surface. In ths project 14 dfferent coverng types were defned. The determnaton of the dfferent elements was made by topographcal maps, aeral photos and feld control. The dfferent coverng types were: Pont elements: - chmney, - antenna, - tower, etc. Lne elements: - rver, - road, - ralway, - aeral electrc lne (hgh voltage), - brdges/overpasses. Surface elements: - meadow, low vegetaton, - water surface, - bushy dstrct, medum hgh vegetaton (max. 4 m), - low forest (max. 15 m), - hgh forest (greater than 15 m), - low buldng, vllages (max. 8 m) - medum buldng, small town (max. 15 m), - hgh buldng, ctes (greater than 15 m). The creaton of the database was made on a Mcrostaton graphcal system wth PC-s and Intergraph workstatons. The database conssts of two parts: the vector and the raster databases. The vector database has been put nto the computer wth the dgtalzaton of 1 : 50,000 topographcal maps. The elements of objects are represented wth dfferent colours and thckness on dfferent layers so by swtchng the layers on and off there s a possblty of clear representaton. Fg. 3 shows a vector database. The raster database was grouped accordng to the 1 : 100,000 scale EOTR map wth a resoluton 50 x 50 and 200 x 200 m. The length of a 1 : 100,000 EOTR map shft s 48,000 m, the wdth s 32,000 m so a row of a raster fle contans 960 or 480; a column contans 640 or 420 elements dependng on the resoluton of 50 or 200 m.

7 VECTOR - RASTER CONVERSION 141 Fg. 3. The tme of collverson s determned by the number of rows and columns of raster fle and by the effcency of the computer. Because of the lmts of PC-s the converson took place on workstaton (UNIX operaton system). Fg. 4 shows a raster database wth a resoluton of 50 m. Fg. 4.

8 142 T. Q.HUNG References ACKLAND, B. D. - WESTE, N. H., (1981): The Edge Flag Algorthm a Fll Method for Raster Scan Dsplay. I.E.E.E. Tra7lsactons 071 Computer, Vol. 30, p. 41. PIWOWAR, J. M. - LEDREW, E. F. - DUDYCHAX, D. J., (1990): Integraton of Spat on Data n Vector and Raster Formats n Geographcal Informaton System Envronment. I7lternatonal Journal of CeographcalInformato71 System. PAULIDIS, T. (1979): Fllng Algorthms for Raster Graphcs. Computer Crap,hcs a7ld Imager Processng, Vol. 10, p. 126.

9 INDEX KADAR, 1. - PAPP, Eo: Subrange Data Type Appled to Defnton of Such Vrtual Coordnate Systems where the Domans Are Constraned by Geometrc Boundares of Each Object YUN, HoSo: Geod Computaton by Analytcal and Dscrete Spectrum VOLGYESI, Lo: Test Interpolaton of Deflecton of the Vertcal n Hungary Based on Gravty Gradents DETREKOI, A.: Data Qualty n GIS Systems ALHUSAIN, 00 - MINDA, Jo: Referental Classfcaton - a New Intellgence Based Classfcaton Algorthm WINKLER, Go: Development of Photonterpretaton for Envronmental Use n Hungary FEKETE, Ko: Geometrc Informaton for Doctors KNYIHAR, Ao: Dgtal Image Processng n Educaton BARSI, A: Thematc Mappng of the Navasha-regon (Kenya) from LANDSAT Images HUNG, To Qo: Vector - Raster Converson

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