Week 10. GIS Data structure II Nov. 7th, 2005

Outline Digital elevation models The TIN data model What is a digital elevation model (DEM). How to create DEMs. Uses of DEMs. The TIN data model What is a TIN model. How to create a TIN model.

Digital Elevation Model The term digital elevation model or DEM is frequently used to refer to any digital representation of a topographic surface Most often it is used to refer specifically to a raster or regular grid of spot heights The DEM is the simplest form of digital representation of topography and the most common A variety of DEMs are available, including coverage of much of the US from the US Geological Survey The resolution, or the distance between adjacent grid points, is a critical parameter Best resolution commonly available is 30 m, with a vertical resolution of 1 m Coverages of the entire globe, including the ocean floor, can be obtained at various resolutions

Sample DEMs A 10 Meter sample of USGS DEM data (ESRI grid format) Lexington West, Kentucky A 15 Meter sample of USGS DEM data (ESRI grid format) United States

Creation of DEMs Conversion of printed contour lines Photogrammetry Existing plates used for printing maps are scanned The resulting raster is vectorized and edited Contours are "tagged" with elevations Additional elevation data are created from the hydrography layer Shorelines provide additional contours Finally, an algorithm is used to interpolate elevations at every grid point from the contour data Photogrammetry This can be done manually or automatically: Manually, an operator looks at a pair of stereophotos through a stereoplotter and must move two dots together until they appear to be one lying just at the surface of the ground Automatically, an instrument calculates the parallax displacement of a large number of points

Use of DEMs Ddetermining attributes of terrain, such as elevation at any point, slope and aspect Interpolation needed if the point is in between two or more raster points Finding features on the terrain, such as drainage basins and watersheds, drainage networks and channels, peaks and pits and other landforms Rooks’s move direction (4 directions) Queen’s move direction (8 directions) Modeling of hydrologic functions, energy flux and forest fires Rook’s direction Starting point Queen’s direction Starting point

The TIN model The Triangulated Irregular Network (TIN) model is a significant alternative to the regular raster of a DEM, and has been adopted in numerous GISs and automated mapping and contouring packages. Irregularly spaced sample points can be adapted to the terrain, with more points in areas of rough terrain and fewer in smooth terrain In a TIN model, the sample points are connected by lines to form triangles Using triangles we ensure that each piece of the mosaic surface will fit with its neighboring pieces - the surface will be continuous - as each triangle's surface would be defined by the elevations of the three corner points Sample points A TIN model A 3D map based on a contour TIN

Creating a Tin model How to pick sample points? In many cases these must be selected from some existing, dense DEM or digitized contours Normally, a TIN of 100 points will do as well as a DEM of several hundred at representing a surface How to connect points into triangles? How to model the surface within each triangle? This is almost always resolved by using a plane surface However, if the surface is contoured, the contours will be straight and parallel within each triangle, but will kink sharply at triangle edges Marion County, Oregon Connect sample points using triangles Model triangles using a plane surface Use different colors to present heights

Summary for today Digital elevation models The TIN data model What is a digital elevation model (DEM). How to create DEMs. Uses of DEMs. The TIN data model What is a TIN model. How to create a TIN model.