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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: WWU Created Date: 9/17/2001 8:54:52 PM Document presentation format: On-screen Show (4:3) – 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
  • IST Academic Computing
  • 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
Contour lines point up stream.
11
Digital Elevation Models

Using elevation data in raster format in a GIS
12
What is a Digital Elevation Model (DEM)?
  • Digital representation of topography
  • Model based on scale of original data
  • Commonly a raster dataset
  • Cell based data where a cell has a single
    elevation which represents the entire area
    covered by the cell

13
Why use elevation data in a GIS?
  • Easy to use
  • Importance of terrain in hydrology and
    environmental modeling
  • Visualization of landscapes

14
Creation of DEMs
  • Conversion of paper maps
  • Scanned, vectorised contour lines
  • USGS produces 10 and 30 meter DEMs
  • From original photogrammetry
  • From Space Shuttle topography mission
  • 30 meter data in US, 90 meter data elsewhere

15
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
16
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
17
Uses of DEMs
  • Determine aspects of terrain
  • Slope, aspect, spot elevations
  • Can be the source for contour lines
  • Finding terrain features
  • Watersheds,drainage networks, stream channels
  • Modeling of hydrologic functions

18
Scale in DEMs
  • Scale determines resolution (cell size)
  • Depends on source data
  • Resolution determines use of DEM and what spatial
    features are visible

19
Scale
DEM of northeast coast of US and part of Canada.
The major drainages networks are shown in blue.
20
Errors in DEMs
  • Typos occur frequently in DEMs
  • Most common variety are sinks and spires.
  • Sinks occur when a very low elevation, relative
    to surrounding cells, is entered.
  • Spires occur when a very high elevation, relative
    to surrounding cells, is entered
  • Both appear as tightly packed contours

21
A natural sink?
340 335 330 340 345
337 332 330 335 340
330 228 320 330 335
328 326 310 320 328
320 318 305 312 315
By default, this sink is removed, whether or
not it is real.
22
Correcting sinks and spires
  • Most GIS have a Fill function which looks for
    sinks and fills them or looks for spires and
    removes them
  • Sinks wreck havoc with hydrologic modeling
    functions in GIS software

23
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
24
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)
25
Hydrologic functions on DEMs
  • Modeling hydrologic function from the topographic
    form of a drainage basin
  • Determining the drainage network and associated
    drainage divides
  • Estimating slopes for understanding drainage
    patterns and processes

26
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

27
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
28
32 64 128
16 Source Cell 1
8 4 2
29
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

30
Watersheds
Once done manually
Contour lines (brown) Drainage (blue) Watershed
boundary (red)
31
Flow accumulation
  • The number of cells, or area, which contribute to
    runoff of a given cell
  • Accumulation, once it reaches a threshold
    appropriate to an region, forms a drainage
    channel
  • Accumulation is the area of a watershed that
    contributes runoff to a given cell

32
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 Errors may
occur at the edges of DEMs.
33
Flow accumulation as drainage network
Drainage network as defined by cells
above threshold value for region.
34
Visibility
What land is visible from the selected location?
35
Distributed Modeling in Hydrology using Digital
Data and Geographic Information Systems David
TarbotonUtah State UniversityCourse presented
at the University of PaduaMay 15 to 26,
2000http//www.engineering.usu.edu/cee/faculty/dt
arb/hydrogis/
  • More information available
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