Digital Terrain Model (DTM) 1

Transcription

1 Digital Terrain Mdel (DTM) 1 What Is A digital terrain mdel is a tpgraphic mdel f the bare earth terrain relief - that can be manipulated by cmputer prgrams. The data files cntain the spatial elevatin data f the terrain in a digital frmat which usually presented as a rectangular grid. Vegetatin, buildings and ther man-made (artificial) features are remved digitally - leaving just the underlying terrain ( n the ther hand, Digital Surface Mdel (DSM) is usually the main prduct prduced frm phtgrammetry, where it des cntain all the features mentined abve, while a filtered DSM results in a DTM). DTM mdel is mstly related as raster data type (ppsed t vectr data type), stred usually as a rectangular equal-spaced grid, with space (reslutin) f between 50 and 500 meters mstly presented in cartesian crdinate system i.e. x, y, z (there are DTMs presented in gegraphic crdinate system i.e. angular crdinates f latitude and lngitude). Fr several applicatins a higher reslutin is required (as high as 1 meter spacing). A DTM can be used t guide autmatic machinery in the cnstructin f a physical mdel r even in cmputer games, where is describes the relief map. Mdeling terrain relief via DTM is a pwerful tl in GIS (Gegraphic Infrmatin System) analysis and visualizatin. DTM can be stred in a GIS databases in several ways: 1) a set f cntur vectrs (left); 2) a rectangular grid f equal-spaced crner/pint heights (middle); r, 3) an irregularly spaced set f pints cnnected as triangles (TIN - Triangular Irregular Netwrk) (right). 1 Excerpt frm wikipedia.rg, Simn Dadsn ("DTM"), Terrainmap.cm, and Graduate Schl f Design at Harvard University

2 Figure 1 Figure 2 Figure 3 Cntur lines superimpsed n grid DTM Grid DTM terrain relief representatin TIN terrain relief representatin What Is It Gd Fr?... The DTM data sets are extremely useful fr the generatin f 3D renderings f any lcatin in the area described. 3D mdels rendered frm DTM data can be extremely useful and versatile fr a variety f applicatins. DTMs are used especially in civil engineering, gedesy & surveying, gephysics, and gegraphy. The main applicatins are:

3 1. Visualizatin f the terrain 2. Reductin (terrain crrectin) f gravity measurements (gravimetry, physical gedesy) 3. Terrain analyses in Cartgraphy and Mrphlgy 4. Rectificatin f airbrne r satellite phts 5. Extractin f terrain parameters, mdel water flw r mass mvement A DTM representatin f the Middle East 2 Where D I Buy One?... Data Surces fr DTM recnstructin can be derived frm grund survey, phtgrammetry r cartgraphic data capture. In either case, the reslutin at which data pints are sampled is imprtant in determining the usefulness f the resulting DTM, hence the derived applicatins. The mst ppular mdel cnstructin structures are the grid DTM and triangular irregular netwrk. Here is a shrt cmparisn f the tw: 2 The surce fr this picture is the USGS website, where yu can dwnlad fr free data samples

4 Grid DTM Simple strage cmpatible with satellite data gd surface analysis slwer t cmpute pssibility f redundant data pints unifrm pixel size TIN fewer pints needed fr the same accuracy reslutin adapts t terrain strage f tplgy n surface analysis initial cnstructin is time cnsuming sme peratins d nt have efficient algrithms The preferred mdel will depend mstly n: data availability; nature f surface (featureless plain ( DTM)/intricate muntains ( TIN)); applicatin techniques that will be used t analyze and manipulate mdel (cmplicity f algrithms and calculatins); scale and reslutin f the data and prblem. Aspect representatin derived frm elevatin Slpe representatin derived frm elevatin Hill shade calculated frm elevatin Visibility frm certain pint (red dt) Can I Rely On That?... The prblem with DTM data is that it des nt cntain anything ther than elevatin infrmatin. While useful fr general visualizatin purpses, it can be difficult t lcate bjects precisely n the terrain

5 mdel by referencing tpgraphic features (since these can all seem t lk alike). One way t imprve a DTM is t verlay raster data n tp r a tpgraphical map ver the terrain mdel. Nwadays, when the use f LiDAR (Light Detectin And Ranging) laser scans, a technlgy that prduces DSM pints clud, are becming mre and mre cmmn, the variety f applicatins and visualizatins are becming relevant t many prjects. Sme uncertainties and errrs can be featured in DTM data: The accuracy f a DTM depends n the accuracy f its surce data and n the mdel reslutin. Tw DTMs prduced frm the same data will nt cntain the same infrmatin if their reslutin and sampling strategies are different. Similarly a TIN r cntur map prduced frm a grid DTM will cntain uncertainties related t the design f the riginal DTM structure and its inherent errrs. Analysis and calculatins based n existing DTMs may als intrduce errr and uncertainty thrugh errrs in interplating pints r fitting surfaces. Varius parameters cmputed frm the surface f DTMs have been shwn t change with mdel reslutin. TIN Representatin f a human skull frm a lw reslutin laser scan SSX3 vide game featuring cmplex terrain relief (represented by DTM)