DEM’s, Watershed and Stream Network Delineation

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DEMs, Watershed and Stream Network Delineation

• DEM Data Sources
• Study Area in West Austin with a USGS 30m DEM
  from a 124,000 scale map
• Eight direction pour point model (flow direction
  and flow accumulation grids)
• Stream network definition
• Watershed delineation

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DEM Data Sources

• Shuttle Radar Topography Mission (SRTM)
• 30m DEMs from 124,000 scale map
• 1 National Elevation Dataset
• 3" (100m) DEMs from 1250,000 scale maps
• 30" DEM of the earth (GTOPO30)

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Santa Barbara, California
http//srtm.usgs.gov/srtmimagegallery/index.html
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San Andreas Fault, California
http//srtm.usgs.gov/srtmimagegallery/index.html
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Salt Lake City, Utah
http//srtm.usgs.gov/srtmimagegallery/index.html
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Mt Kilimanjaro, Tanzania
http//srtm.usgs.gov/srtmimagegallery/index.html
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Shuttle Radar Topography Mission (SRTM)

• 1 arc-second elevation data for the United
  States, 3 arc-second data for the globe
• Produced by radar measurements from a Shuttle
  mission, Feb 11-22, 2000
• http//srtm.usgs.gov/data/obtainingdata.html

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Interferometry used by SRTM
In interferometry, two images are taken from
different vantage points of the same area. The
slight difference in the two images allows
scientists to determine the height of the
surface.
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SRTM Coverage 56S to 60N
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30m DEMs

• Best resolution standardized data source
  available for the US
• Coverage of the country is incomplete
• Data by 7.5 map sheets in UTM projection
• Link for US http//edcwww.cr.usgs.gov/Webglis/glis
  bin/guide.pl/glis/hyper/guide/usgs_dem
• Link for Texas http//www.tnris.state.tx.us/Digita
  lData/DEMs/dems.htm

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National Elevation Dataset

• Seamless 1 DEM for the US in 1 x 1 blocks
• Compiled by synthesizing the 30m DEMs from
  124,000 scale maps
• Link to website http//edcnts12.cr.usgs.gov/ned/

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Seamless Data Distribution
http//seamless.usgs.gov/
http//edcnts14.cr.usgs.gov/Website/store/viewer.h
tm
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Coverage of 30m, 1 and 3" DEMs
1 and 3 DEMs
1º
30m DEM
7.5
7.5
1º
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3 DEMs

• Derived by US Defence Mapping Agency, available
  from USGS for the whole US
• Data in geographic coordinates by 1250,000 map
  sheet names (1ºx 1º) cells in (1ºx 2º) maps
• Needs to be projected to planar coordinates
• Link http//edcwww.cr.usgs.gov/doc/edchome/ndcdb/n
  dcdb.html

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30" DEM GTOPO30

• Produced by USGS from 3" grids and Digital Chart
  of the World topography
• Coverage complete for the earth
• Projected cell size is 1km
• Data must be projected before use
• Link http//edcwww.cr.usgs.gov/landdaac/gtopo30/gt
  opo30.html

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GTOPO30
Source http//edcwww.cr.usgs.gov/landdaac/gtopo30
/gtopo30.html
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Drainage in North America
Source http//edcwww.cr.usgs.gov/landdaac/gtopo30
/hydro/namerica.html
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Drainage Basins of North America
Source http//edcwww.cr.usgs.gov/landdaac/gtopo30
/hydro/na_basins.html
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DEMs, Watershed and Stream Network Delineation

• DEM Data Sources
• Study Area in West Austin with a USGS 30m DEM
  from a 124,000 scale map
• Eight direction pour point model (flow direction
  and flow accumulation grids)
• Stream network definition
• Watershed delineation

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Study Region in West Austin
Hog Pen Ck
4 km
4 km
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Topographic MapAustin West, 124,000
Scalehttp//www.tnrcc.state.tx.us/gis/raster.html
20 ft contour
100 ft contour
Stream Center Line
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Flow in Direction of Steepest Descent
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Watershed Delineation by Hand Digitizing
Watershed divide
Outlet
Drainage direction
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200 Meter Mesh (Universal Transverse Mercator
Coordinates)
1km
1km
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100 Meter Mesh (UTM Coordinates)
1km
100m
1km
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30 Meter MeshStandard for 124,000 Scale Maps
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Austin West 30 Meter DEMElevations in meters
ftp//ftp.tnris.state.tx.us/tnris/demA.html
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USGS 30 Meter DEM for This Area
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30 Meter DEM Cell Boundaries
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DEM Lattice Points
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DEM Cell Stores Elevation at Lattice Point
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DEM Elevations
720
720
Contours
740
720
700
680
680
700
720
740
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DEM Elevations
Contours
700
680
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DEMs, Watershed and Stream Network Delineation

• DEM Data Sources
• Study Area in West Austin with a USGS 30m DEM
  from a 124,000 scale map
• Eight direction pour point model (flow direction
  and flow accumulation grids)
• Stream network definition
• Watershed delineation

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Digital Elevation Model
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Cell Definition
cell size
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(cell value)
cell
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Eight Direction Pour Point Model
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Direction of Steepest Descent
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1
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56
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58
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Slope
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Flow Direction Grid
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Austin West 30 Meter DEMElevations in meters
ftp//ftp.tnris.state.tx.us/tnris/demA.html
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Flow Direction Grid
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Grid Network
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DEMs, Watershed and Stream Network Delineation

• DEM Data Sources
• Study Area in West Austin with a USGS 30m DEM
  from a 124,000 scale map
• Eight direction pour point model (flow direction
  and flow accumulation grids)
• Stream network definition
• Watershed delineation

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Flow Accumulation Grid
0
0
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3
2
2
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2
0
2
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11
0
1
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11
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1
15
0
0
1
0
15
1
0
2
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5
2
5
0
1
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Link to Grid calculator
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Flow Accumulation gt 5 Cell Threshold
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Stream Network for 5 cell Threshold Drainage Area
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Streams with 200 cell Threshold(gt18 hectares or
13.5 acres drainage area)
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Burning In the Streams
? Take a mapped stream network and a DEM ? Make a
grid of the streams ? Raise the off-stream DEM
cells by an arbitrary elevation increment ?
Produces "burned in" DEM streams mapped streams


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Filling in the Pits

• DEM creation results in artificial pits in the
  landscape
• A pit is a set of one or more cells which has no
  downstream cells around it
• Unless these pits are filled they become sinks
  and isolate portions of the watershed
• Pit filling is first thing done with a DEM

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DEMs, Watershed and Stream Network Delineation

• DEM Data Sources
• Study Area in West Austin with a USGS 30m DEM
  from a 124,000 scale map
• Eight direction pour point model (flow direction
  and flow accumulation grids)
• Stream network definition
• Watershed delineation

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Watershed Outlet
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Watershed Draining to This Outlet
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Watershed and Drainage Paths Delineated from 30m
DEM
Automated method is more consistent than hand
delineation
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Stream Segments
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Stream Segments in a Cell Network
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5
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1000 Cell Threshold Exceeded at Stream Junction
989
510
1504 (gt1000)
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Subwatersheds for Stream Segments
Same Cell Value
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Vectorized Streams Linked Using Grid Code to Cell
Equivalents
Vector Streams
Grid Streams
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Delineated Subwatersheds and Stream Networks
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Summary Concepts

• Topographic maps are the traditional way of
  representing land surface terrain and streams
• Watersheds can be hand-delineated from these maps
• DEMs of equivalent accuracy are now available
  for most map series in the US

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Summary Concepts (2)

• DEM cell elevation is at the cell center
• eight direction pour point model leads to flow
  direction and flow accumulation grids
• stream network is defined as cells whose flow
  accumulation exceeds a threshold
• watershed outlet is the cell with highest flow
  accumulation

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Summary Concepts (3)

• Streams can be divided into links and
  subwatersheds defined for them
• Resulting grids can be vectorized
• Standardized DEM data sources 30m, 1 3, 30
• Coverage from local scale to the whole earth