Geographic Information Geographic Information SystemsSystems

Digital Elevation Models (DEM)Digital Elevation Models (DEM)

1. Digital Elevation 1. Digital Elevation ModelModel

► A set of elevation measurements for locations distributed over the land surface                                  

2. Basic Methods to 2. Basic Methods to Capture and Store DEM Capture and Store DEM

DataData► Regular gridsRegular grids► ContoursContours► ProfilesProfiles► Triangulated irregular network (TIN) Triangulated irregular network (TIN)                              

► The elevation values are stored as a matrix of regularly spaced ground positions ► Each data point represents the elevation of the grid cell in which it is located

2. (1) Regular Grids2. (1) Regular Grids

► Advantages:   Easy to process

► Disadvantages   Fixed resolution leads to redundancy or inadequacy

Regular Grids ..Regular Grids ..

► A series of elevation points along individual contour lines

2. (2) Contours2. (2) Contours

► Elevation values along a series of parallel lines

2. (3) Profiles2. (3) Profiles

► Triangulated Irregular Network► It is a network of triangular facets►  For each vertex, the x, y, and z value are recorded►  The nodes and edges follow the important terrain features such as ridges, stream lines, high points, passes, and so on.   

2. (4) TINs2. (4) TINs

TINs ..TINs ..

TINs ..TINs ..

X=3970Y=3869Z=7746

X=4562Y=4219Z=7906

X=4266Y=4044Z=7826

SlopeSlopeTIN

Grid

TINs ..TINs ..

► TIN topology  X and y coordinate table: node ID, x and y  Z coordinate table: node ID, z value

Node table: triangle ID, node IDs  Edge table: triangle ID, edge IDs

TINs ..TINs ..

TINs ..TINs ..

► Advantages   efficient in storage    accurate encoding for the break-point features

► Disadvantages    difficult to implement

TINs ..TINs ..

► Existing contour map► Stereoscopic aerial photography► Stereoscopic satellite images► Ortho-photos and Ortho-images - Aerial photo or image that has been corrected for all motion, attitude, and viewing perspective as well as relief displacement

3. Source Data of DEM3. Source Data of DEM

► USGS, state agencies, and private vendors

► 1:24,000 DEM► 1:250,000 DEM

4. Data Availability4. Data Availability

► Developed and distributed by USGS► A regular grid in UTM coordinate system►  A 7.5 by 7.5 minute coverage

4. (1) 1:24,000 DEM4. (1) 1:24,000 DEM

► Data are ordered from south to north in profiles that are ordered from west to east

► A 30 by 30 meter spacing along and between profiles (spatial resolution)

► The profiles do not always have the same number of elevation points

► The measuring unit for the elevation is meter in most cases

1:24,000 DEM ..1:24,000 DEM ..

► The profiles do not always have the same number of elevation points

1:24,000 DEM ..1:24,000 DEM ..

► Developed by Defense Mapping Agency and distributed by USGS► A regular grid in geographical coordinate system► A 10 x 10 coverage► The measuring unit for the elevation is meter► The spacing along profiles is 3 arc-second (“spatial resolution”)► The spacing between profiles is arc-second below 500N latitude, 6-9 arc-second otherwise

4. (2) 1:250,000 DEM4. (2) 1:250,000 DEM

► A 10 x 10 coverage► The spacing between and along profiles is 3 arc-second

4. (2) 1:250,000 DEM4. (2) 1:250,000 DEM

► Volume estimation► Contour map► 3D display► Visibility► Slope, convexity, concavity, aspect► Watershed delineation/stream lines

5. Applications of DEM5. Applications of DEM

Volume Volume EstimatioEstimatio

nn

ContoursContours

Contours and 3DContours and 3D

NIMA &NASANIMA &NASA                                 

3D3D

3D3D

A. Toy, SUNY BUffalo

3D display3D display

Bowling Green

Z=10

J. Yan, SUNY BuffaloJ. Yan, SUNY Buffalo

3D display3D display

J. Yan, SUNY BuffaloJ. Yan, SUNY Buffalo

VisibilityVisibilityLine of sight

VisibilityVisibility

The above maps show the results of analyses around Saddleback (left) and Old Blue (right) mountains. On the maps, green areas are visible from the location cited (shown in yellow), while gray areas are obstructed from view. The Appalachian Trail is shown in red.http://www.northgeo.com/services/carto/projects/LURCViewshed.htm

VisibilityVisibility►3 scenic 3 scenic lookoutslookouts

M. Dolce, Buffalo State CollegeM. Dolce, Buffalo State College

Cave modelingCave modeling

ElevationElevation

                                 

NIMA & NASANIMA & NASA

Slope Slope aspectaspect

Derived Derived from DEMfrom DEM                                  

Slope Slope angleangle

Derived Derived from DEMfrom DEM                                  

Stream FunctionStream Function

Color infrared composite of the IKONOS Color infrared composite of the IKONOS draped over the DEM as viewed from the west draped over the DEM as viewed from the west

side of the study area to the east from an side of the study area to the east from an elevation of 10,000 m, Xichang, Chinaelevation of 10,000 m, Xichang, China

Xu, University of Utah, Gong, UC-BerkeleyXu, University of Utah, Gong, UC-Berkeley

ReadingsReadings

Chapter 3