Geography 360Principles of Cartography

May 19, 2006

Outlines: Digital Elevation Model

1. How is DEM different from map display?• DEM is data structure, not map

2. What is DEM?• Digital representation of elevation• Data structure for representation stored in the

computer

3. Three commonly used DEM• Contour• TIN• Grid (most of time considered equivalent to DEM)

Phenomenon, data, map

• Distinguish these

Phenomenon Spatial data structure

Spatial dimension

Map display

#1 Population Point Polygon

0D2D

Dot map, Proportional symbol map, Choropleth map, Dasymetric map

#2 Bus route LineGraph

1D Flow map

#3 Land use Polygon 2D Categorical map

#4 Elevation GridTIN, Contour

3D Contour, shaded relief map, fishnet map

Refer back to SDTS spatial data concepts

DEM

• Data representation is different from map display• How is elevation (or the third dimension) represented in

database?• DEM is a rubric term that refers to data structure used to

represent elevation in the computer• There are three widely used DEM: (1) contour (2) TIN (3)

grid • DEM, narrowly defined, refers to gridded form for

representing elevation • DEM is data representation, not map display• Understanding DEM is important because it serves as

the source of map display (you will create surface map from DEM)

DEM

• Data structure for elevation

• Representing pyramid…

Source: NCGIA

Point dataContour

Triangulated Irregular Network (TIN)

Grid

Earth’s topography in different data structure

Source: Bolstad 2005

Source: Nyerges

How is DEM generated?

• 1) From paper map (most common)• Vectorization of contour lines printed on the map (it can

be seen as scattered points which has varying elevation values)

• If necessary, hydrogrphy features (e.g. river) are added to enhance the quality of DEM

• At grid points, elevation values are interpolated from measured points

• 2) From photogrammetry• Extraction of elevation from photographs either manually

(using stereoplotter) or automatically (using instruments)

TIN

• TIN = Triangulated Irregular Network

TIN

How is DEM used?

• DEM is useful in (1) data storage (2) map display (3) further analysis (e.g. deriving slope, aspect, viewshed, computing least cost path, and much more)

• The use of DEM not limited to representing elevation - you can apply the concept of DEM to any continuous surface (statistical surface such as demographic structure, geological profile, and so on)

Where do I obtain DEM?

• EROS Data Center

• You may have to convert DEM to the format compatible with software (the conversion capability is available in most commercial GIS package)

• You can create your own DEM or surface map by interpolating point data provided that the phenomenon is continuous