DEM’s, Watershed and Stream Network Delineation DEM Data Sources Study Area in West Austin with a...

DEM’s, Watershed and Stream Network Delineation

• DEM Data Sources • Study Area in West Austin with a USGS

30m DEM from a 1:24,000 scale map• Eight direction pour point model (flow

direction and flow accumulation grids)• Stream network definition• Watershed delineation

DEM Data Sources

• Shuttle Radar Topography Mission (SRTM)

• 30m DEMs from 1:24,000 scale map

• 1” National Elevation Dataset

• 3" (100m) DEMs from 1:250,000 scale maps

• 30" DEM of the earth (GTOPO30)

http://srtm.usgs.gov/srtmimagegallery/index.html

Santa Barbara, California

http://srtm.usgs.gov/srtmimagegallery/index.html

San Andreas Fault, California

http://srtm.usgs.gov/srtmimagegallery/index.html

Salt Lake City, Utah

http://srtm.usgs.gov/srtmimagegallery/index.html

Mt Kilimanjaro, Tanzania

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

Interferometry used by SRTMIn 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.

SRTM Coverage 56°S to 60°N

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/glisbin/guide.pl/glis/hyper/guide/usgs_dem

• Link for Texas http://www.tnris.state.tx.us/DigitalData/DEMs/dems.htm

National Elevation Dataset

• Seamless 1” DEM for the US in 1° x 1° blocks

• Compiled by synthesizing the 30m DEM’s from 1:24,000 scale maps

• Link to website http://edcnts12.cr.usgs.gov/ned/

Seamless Data Distribution

http://edcnts14.cr.usgs.gov/Website/store/viewer.htm

http://seamless.usgs.gov/

7.5´

7.5´

1º

1º

1” and 3” DEMs

30m DEM

Coverage of 30m, 1” and 3" DEMs

3” DEMs

• Derived by US Defence Mapping Agency, available from USGS for the whole US

• Data in geographic coordinates by 1;250,000 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/ndcdb.html

Cell Size30m 100m

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/gtopo30.html

GTOPO30

Source: http://edcwww.cr.usgs.gov/landdaac/gtopo30/gtopo30.html

Drainage in North America

Source: http://edcwww.cr.usgs.gov/landdaac/gtopo30/hydro/namerica.html

Drainage Basins of North America

Source: http://edcwww.cr.usgs.gov/landdaac/gtopo30/hydro/na_basins.html

DEM’s, Watershed and Stream Network Delineation

• DEM Data Sources • Study Area in West Austin with a USGS

30m DEM from a 1:24,000 scale map• Eight direction pour point model (flow

direction and flow accumulation grids)• Stream network definition• Watershed delineation

Study Region in West Austin

Hog Pen Ck

4 km

4 km

Topographic MapAustin West, 1:24,000 Scale

http://www.tnrcc.state.tx.us/gis/raster.html

20 ft contour

100 ft contour

Stream Center Line

Flow in Direction of Steepest Descent

Watershed Delineation by Hand Digitizing

Watershed divide

Drainage direction Outlet

200 Meter Mesh (Universal Transverse Mercator Coordinates)

1km

1km

100 Meter Mesh (UTM Coordinates)

100m

100m

1km

1km

30 Meter MeshStandard for 1:24,000 Scale Maps

Austin West 30 Meter DEMElevations in meters

ftp://ftp.tnris.state.tx.us/tnris/demA.html

USGS 30 Meter DEM for This Area

30 Meter DEM Cell Boundaries

DEM Lattice Points

DEM Cell Stores Elevation at Lattice Point

DEM Elevations

Contours

720

700

680

740

680700720740

720 720

DEM Elevations Contours

700

680

DEM’s, Watershed and Stream Network Delineation

• DEM Data Sources • Study Area in West Austin with a USGS

30m DEM from a 1:24,000 scale map• Eight direction pour point model (flow

direction and flow accumulation grids)• Stream network definition• Watershed delineation

67 56 49 46 50

12 11 12

53 44 37 38 48

58 55 22 31 24

61 47 21 16 19

3453

Digital Elevation Model

67 56 49 46 50

12 11 12

53 44 37 38 48

58 55 22 31 24

61 47 21 16 19

3453

50

cell size

cell

(cell value)

Cell Definition

32

16

8

64

4

128

1

2

Eight Direction Pour Point Model

67 56 49

53 44 37

58 55 22

1

67 56 49

53 44 37

58 55 22

1

26.162

4467

14

1

5367

Slope:

Direction of Steepest Descent

2 2 4 4 8

1 2 16

1 2 4 8 4

128 1 2 4 8

2 1 4 4 4

11

Flow Direction Grid

Austin West 30 Meter DEMElevations in meters

ftp://ftp.tnris.state.tx.us/tnris/demA.html

Flow Direction Grid

32

16

8

64

4

128

1

2

Grid Network

DEM’s, Watershed and Stream Network Delineation

• DEM Data Sources • Study Area in West Austin with a USGS

30m DEM from a 1:24,000 scale map• Eight direction pour point model (flow

direction and flow accumulation grids)• Stream network definition• Watershed delineation

0 0 000

0

0

0

0

0

0

0

0

03 2 2

11 1

1 152 5

24 1

0 0 00 0

0

0

0

0

0

0

0

0

0

3 2 2

11 1

1

12

15

245

Flow Accumulation Grid

Link to Grid calculator

0 0 00 0

0

0

0

0

0

0

0

0

0

3 2 2

11 1

1

12

15

245

Flow Accumulation > 5 Cell Threshold

Stream Network for 5 cell Threshold Drainage Area

0 0 000

0

0

0

0

0

0

0

0

03 2 2

111

115

2 5

241

Streams with 200 cell Threshold(>18 hectares or 13.5 acres drainage area)

+ =

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

“Burning In” the Streams

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

DEM’s, Watershed and Stream Network Delineation

• DEM Data Sources • Study Area in West Austin with a USGS

30m DEM from a 1:24,000 scale map• Eight direction pour point model (flow

direction and flow accumulation grids)• Stream network definition• Watershed delineation

Watershed Outlet

Watershed Draining to This Outlet

Watershed and Drainage Paths Delineated from 30m DEM

Automated method is more consistent than hand delineation

0 0 000

0

0

0

0

0

0

0

0

03 2 2

111

115

2 5

241

Stream Segments

55

11

1

3

22

334 4 4

4 45

5

666

Stream Segments in a Cell Network

9895101504 (>1000)

1000 Cell Threshold Exceeded at Stream Junction

Same Cell Value

Subwatersheds for Stream Segments

Vectorized Streams Linked Using Grid Code to Cell Equivalents

VectorStreams

GridStreams

Delineated Subwatersheds and Stream Networks

Summary Concepts

• Topographic maps are the traditional way of representing land surface terrain and streams

• Watersheds can be hand-delineated from these maps

• DEM’s of equivalent accuracy are now available for most map series in the US

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

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