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)

Santa Barbara, California

San Andreas Fault, California

Salt Lake City, Utah

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 , /obtainingdata.html

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.

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 sgs_demLink Link for Texas

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

Seamless Data Distribution

7.5´ 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

Cell Size 30m100m

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

GTOPO30 Source:

Drainage in North America Source:

Drainage Basins of North America Source: 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

Topographic Map Austin West, 1:24,000 Scale 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

100 Meter Mesh (UTM Coordinates) 100m 1km

30 Meter Mesh Standard for 1:24,000 Scale Maps

Austin West 30 Meter DEM Elevations 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

DEM Elevations Contours

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

Digital Elevation Model

cell size cell (cell value) Cell Definition

Eight Direction Pour Point Model

Slope: Direction of Steepest Descent

Flow Direction Grid

Austin West 30 Meter DEM Elevations in meters ftp://ftp.tnris.state.tx.us/tnris/demA.html

Flow Direction Grid

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

Flow Accumulation Grid LinkLink to Grid calculator

Flow Accumulation > 5 Cell Threshold

Stream Network for 5 cell Threshold Drainage Area

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

Stream Segments

Stream Segments in a Cell Network

(>1000) 1000 Cell Threshold Exceeded at Stream Junction

Same Cell Value Subwatersheds for Stream Segments

Vectorized Streams Linked Using Grid Code to Cell Equivalents Vector Streams Grid Streams

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