Characteristics, uses, and Characteristics, uses, and sourcessources

Introduction to DEMsIntroduction to DEMsIntroduction to DEMsIntroduction to DEMs

DEMDEM

Digital Elevation Model

• Characteristics

• Uses

• Sources

DEM: Sources and ResolutionsDEM: Sources and Resolutions

• Different versions, sources and spatial resolutions– GTOPO

• Global Topographic Data, 30” (~1km)

– SRTM• Shuttle Radar Topography Mission, 90m

– ASTER• DEM derived from “Advanced Spaceborne

Thermal Emission and Reflection Radiometer,” sensor 30m

– LiDAR• Light Detection and Ranging, high resolution

DEM: CharacteristicsDEM: Characteristics

GTOPO DEM SRTM DEM

From a regional perspective, their difference in resolution is From a regional perspective, their difference in resolution is almost unnoticeable.almost unnoticeable.

DEM: CharacteristicsDEM: Characteristics

GTOPO DEM SRTM DEM

The difference becomes significant at a local level The difference becomes significant at a local level

DEM: CharacteristicsDEM: Characteristics

SRTM DEM• 90m resolution• Global reach• Few vertical errors• Not affected by

clouds(microwaves travel through the atmosphere)

• 4 versions– the most recent is the best

ASTER DEM• 30m resolution• Satellite image

scene reach• Many vertical

errors• Affected by clouds

(optical waves reflect the atmosphere)

• As many versions as satellite imagesBut elevation remains constant over time, right?But elevation remains constant over time, right?

Think: bodies of waterThink: bodies of water

Update, July 2009!See NEW

slides!

Update, July 2009!See NEW

slides!

DEM: CharacteristicsDEM: Characteristics

SRTM DEM ASTER DEMASTER DEM

Quick Lesson in Spatial ResolutionQuick Lesson in Spatial Resolution

• The difference between a spatial resolution of 100m and 10m is not a factor of 10, but a factor of 10² = 100

100m100m10m10m

100m100m

SRTM DEM: How to AccessSRTM DEM: How to Access

• http://srtm.csi.cgiar.org

• Select multiple cells

• Download cells of interest

ASTER DEM: How to AccessASTER DEM: How to Access

• Many ways, one example:– Search image and ID in Glovis– Specify DEM when ordering

(*) New ASTER GDEM(*) New ASTER GDEM

• ASTER GDEM Readme File – ASTER GDEM Version 1– The ASTER GDEM (Global Digital

Elevation Model) was developed as a joint effort by METI (Ministry of Economy, Trade, and Industry) of Japan and NASA.

– METI & NASA provided the ASTER GDEM to the Global Earth Observation System of Systems.

(*) New ASTER GDEM(*) New ASTER GDEM

(*) New ASTER GDEM(*) New ASTER GDEM

• ASTER GDEM Readme File – ASTER GDEM Version 1– “the overall accuracy of the ASTER GDEM, on a global

basis, can be taken to be approximately 20 m at 95 % confidence.” (5)

• “Some tiles have substantially better than 20 m accuracy, and some tiles have substantially worse than 20 m vertical accuracy.” (6)

– Where are there more errors? (5)• Where the number of scenes is low• Persistently cloudy areas• Where there was no alternate DEM

– Another point: bodies of water (6)• The elevation of most inland lakes is not constant, and for the

most part was not identified.

DEM: In situ sourcesDEM: In situ sources

• It is possible to generate a DEM from elevation points gathered through GPS or better yet, contour lines

DEM: UsesDEM: Uses

• A Key input for analysis and modeling– Hydrology– Soil science– Ecosystems– Geology

• Basis for applications

DEM Uses: HydrologyDEM Uses: Hydrology

• Rivers and watersheds

DEM Uses: Soil ScienceDEM Uses: Soil Science

• Pedology– The ground and landscapes influence the

origins, formations, classifications, and morphology of the soils.

• Edaphology– Soil science dealing with crop production

We will look at some uses and similar applications with hydrology and surface tools.

DEM Uses: EcosystemsDEM Uses: Ecosystems

• Elevation is essential to the identification of different ecosystems, eco-regions, life zones, climate zones, etc.

DEM Uses: Surface AnalysisDEM Uses: Surface Analysis

• Slope• Hillshade• Aspect• Viewshed• Anaglyph

DEM Uses: 3DDEM Uses: 3D

2D ASTER Image SERVIR-Viz, 3D platform

Landsat image mosaic, placed over the surface of a DEM

DEM Uses: 3DDEM Uses: 3D