الدِم والاستقطاب

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

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/u
sgs_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/

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 Size

30m

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

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