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

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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.