Geographic Information Systems Digital Elevation Models (DEM)

1. Digital Elevation Model ► ► A set of elevation measurements for locations distributed over the land surface

2. Basic Methods to Capture and Store DEM Data ► Regular grids ► Contours ► Profiles ► 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 Grids

► ► Advantages: Easy to process ► ► Disadvantages Fixed resolution leads to redundancy or inadequacy Regular Grids..

► ► A series of elevation points along individual contour lines 2. (2) Contours

► ► Elevation values along a series of parallel lines 2. (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) TINs

TINs..

X=3970 Y=3869 Z=7746 X=4562 Y=4219 Z=7906 X=4266 Y=4044 Z=7826

Slope TIN Grid

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..

► ► Advantages efficient in storage accurate encoding for the break-point features ► ► Disadvantages difficult to implement 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 DEM

► ► USGS, state agencies, and private vendors ► ► 1:24,000 DEM ► ► 1:250,000 DEM 4. 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 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..

► ► The profiles do not always have the same number of elevation points 1:24,000 DEM..

► ► Developed by Defense Mapping Agency and distributed by USGS ► ► A regular grid in geographical coordinate system ► ► A 1 0 x 1 0 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 50 0 N latitude, 6-9 arc-second otherwise 4. (2) 1:250,000 DEM

► ► A 1 0 x 1 0 coverage ► ► The spacing between and along profiles is 3 arc-second 4. (2) 1:250,000 DEM

► ► Volume estimation ► ► Contour map ► ► 3D display ► ► Visibility ► ► Slope, convexity, concavity, aspect ► ► Watershed delineation/stream lines 5. Applications of DEM

Volume Estimation

Contours

Contours and 3D

NIMA &NASA 3D

3D A. Toy, SUNY BUffalo

3D display Bowling Green Z=10 J. Yan, SUNY Buffalo

3D display J. Yan, SUNY Buffalo

Visibility Line of sight nn nn

Visibility 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.

Visibility ► 3 scenic lookouts M. Dolce, Buffalo State College

Cave modeling Fisher, Erich, D GIS archaeology in South Africa: archeologists working along the South African southern coast use multidimensional GIS applications to model Pleistocene caves and paleo-environments reconstructing the landscape CA. 420,000 to 30,000 BP. GEO:connexion, 4 (5): 40

Elevation NIMA & NASA

Slope aspect Derived from DEM

Slope angle Derived from DEM

Stream Function

Color infrared composite of the IKONOS draped over the DEM as viewed from the west side of the study area to the east from an elevation of 10,000 m, Xichang, China Xu, University of Utah, Gong, UC-Berkeley

Readings Chapter 3