Geographical Information Systems

Geog 481/506                                                                                                  Tu Th 3:30-4:50pm
Fall 2011                                                                                                          Fillmore 170

Instructor: Ling Bian                                                                                          LabA: Tue:   5-6:20pm, W145, Chunyuan diao
Office: 120 Wilkeson                                                                                        LabB: Thur:  6:30-7:50pm, W145, Tong Sun
Office hours: Tu Th 2-3pm  or by appt.                                                             LabC: Fri:  10-11:20am, W145, Tong Sun

Digital Elevation Model

1. Digital Elevation Model (DEM)

A set of elevation measurements for locations distributed over the land surface.
 
2. Basic methods to capture and store DEM data
(1) Regular grid
        The elevation value 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.

  Advantages:
      Easy to process

  Disadvantages:
      Fixed resolution leads to redundancy or inadequacy
 

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

(3) Profiles
          elevation values along a series of parallel lines

(4) Triangulated Irregular Network (TIN)

    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.
    TIN topology
          Node table
          Edge table
          X and y coordinate table

          Z coordinate table

     Advantages:
          efficient in storage
          accurate encoding the break-point features

        Disadvantages:
          difficult to implement
 

3. Source data of DEM

    Existing contour maps
    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

4. Data availability

    USGS, state agencies, and private vendors
 
(1) 1:24,000 DEM
          Developed and distributed by USGS
          A regular grid in UTM coordinate system
    A 7.5 by 7.5 minute coverage
    Data are ordered from south to north in profiles that are ordered from west to east
    30 by 30 meter spacing along and between profiles
    The profiles do not always have the same number of elevation points
    The measuring unit for the elevation is meter in most cases
 

(2) 1:250,000 DEM

          Developed by Defense Mapping Agency and distributed by USGS
          A regular grid in geographical coordinate system
          A1 by 1 degree coverage
          The measuring unit for the elevation is meter
          Spacing along profiles is 3 arc-second
          Spacing between profiles is arc-second below 50N latitude, 6-9 arc-second otherwise

5. Applications of DEM
          volume estimation
          contour map
          3D display
          visibility
          slope, convexity, concavity, aspect
          watershed delineation/stream lines

6. Readings Chpt 3.