Week 10. GIS Data structure II

Nov. 7th, 2005

OutlineOutline

Digital elevation models 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 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 heig

hts

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 va

rious resolutions

Sample DEMsSample 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 DEMsCreation of DEMs

Conversion of printed contour lines 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 DEMsUse 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 modelThe 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 modelCreating 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

Model triangles using a plane surfaceConnect sample points using triangles Use different colors to present heights

Marion County, Oregon

Summary for today

Digital elevation models 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.