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From Topographic Maps to Digital Elevation Models

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Title: Digital Elevation Models Author: Daniel Sheenan Last modified by: Daniel Sheehan Created Date: 9/17/2001 8:54:52 PM Document presentation format – PowerPoint PPT presentation

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Title: From Topographic Maps to Digital Elevation Models


1
From Topographic Maps to Digital Elevation Models
  • Daniel Sheehan
  • DUE Office of Educational Innovation Technology
  • Anne Graham
  • MIT Libraries

2
Which Way Does the Water Flow?
3
A topographic map shows the relief features or
surface configuration of an area.
4
A hill is represented by lines of equal elevation
above mean sea level. Contours never cross.
5
Elevation values are printed in several places
along these lines.
6
Contours that are very close together represent
steep slopes.
7
Widely spaced contours or an absence of contours
means that the ground slope is relatively level.
8
The elevation difference between adjacent contour
lines, called the contour interval, is selected
to best show the general shape of the terrain. A
map of a relatively flat area may have a contour
interval of 10 feet or less.
9
Maps in mountainous areas may have contour
intervals of 100 feet or more.
10
A city can be overlain on a topographic map.
11
A bench mark is a surveyed elevation point.
12
Contour lines point up stream.
13
United States Geological Survey Topographic Map
Symbols Explained
  • http//erg.usgs.gov/isb/pubs/booklets/symbols/

14
(No Transcript)
15
Digital Elevation Models

Using elevation data in raster format in a GIS
16
What is a Digital Elevation Model (DEM)?
  • Digital representation of topography
  • Cell based with a single elevation representing
    the entire area of the cell

17
Basic storage of data
340 335 330 340 345
337 332 330 335 340
330 328 320 330 335
328 326 310 320 328
320 318 305 312 315
DEM as matrix of elevations with a uniform cell
size
18
Adding geography to data
Xmax, Ymax
340 335 330 340 345
337 332 330 335 340
330 328 320 330 335
328 326 310 320 328
320 318 305 312 315
Cell index number x cell size defines position
relative to Xmin, Ymin and Xmax, Ymax and
infers An exact location
Xmin, Ymin XY are in projected units
19
Uses of DEMs
  • Determine characteristics of terrain
  • Slope, aspect
  • Watersheds
  • drainage networks, stream channels

20
Scale in DEMs
  • Scale determines resolution (cell size)
  • Depends on source data
  • Resolution determines use of DEM and what spatial
    features are visible

21
Estimating slopes in a DEM
  • Slopes are calculated locally using a
    neighborhood function, based on a moving 33
    window
  • Distances are different in horizontal and
    vertical directions vs diagonal
  • Only steepest slopes are used

1.41 1 1.41
1 0 1
1.41 1 1.41
cell size
22
Slopes
340 335 330
337 332 330
330 328 320
(elevations)
8/42.47 3/30 2/42.47
5/30 0 -2/30
-2/42.47 -2/30 -12/42.47
(difference/distance)
23
Flow Direction
  • Useful for finding drainage networks and drainage
    divides
  • Direction is determined by the elevation of
    surrounding cells
  • Water can flow only into one cell
  • Water is assumed to flow into one other cell,
    unless there is a sink
  • GIS model assumes no sinks

24
Flow direction in a DEM
340 335 330 340 345
337 332 325 335 340
330 328 320 330 335
328 326 310 320 328
320 318 305 312 315
Flow directions for individual cells
25
32 64 128
16 Source Cell 1
8 4 2
26
Finding watersheds
  • Begin at a source cell of a flow direction
    database, derived from a DEM (not from the DEM
    itself
  • Find all cells that flow into the source cell
  • Find all cells that flow into those cells.
  • Repeat
  • All of these cells comprises the watershed
  • The resulting watershed is generalized, based on
    the cell size of the DEM

27
Watersheds
Once done manually
Contour lines (brown) Drainage (blue) Watershed
boundary (red)
28
Flow accumulation
  • The number of cells, or area, which contribute to
    runoff of a given cell
  • The accumulation function determines the area of
    a watershed that contributes runoff to any given
    cell

29
Flow accumulation in a DEM
0 0 0 0 0
0 1 3 1 0
0 1 8 1 0
0 1 13 1 0
0 2 24 2 0
Flow accumulation for individual cells
30
Flow direction in a DEM
340 335 330 340 345
337 332 325 335 340
330 328 320 330 335
328 326 310 320 328
320 318 305 312 315
Flow directions for individual cells
31
Flow accumulation as drainage network
Drainage network as defined by cells
above threshold value for region.
32
Things to do before the exercise In Windows,
create a new folder under your username on the
F\Usertemp folder. In Arcgis Desktop, click on
tools then Extension. Check the box for Spatial
Analyst and close the window. Again click on
tools and then Customize. Again, check the box
for the Spatial Analyst toolbox and close the
window.
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