Processing Elevation Data Limitations of DEMs for hydro

Processing Elevation Data

Limitations of DEMs for hydro work • • Dates Static, does not evolve Matching to linear line work due to scale Processing errors Interpretation errors Lack of ground truth Artificial versus real peaks and sinks

Getting line work

Creating the stream for our watershed

24 k streams for the major basin extent

Clipped streams to watershed bnd

Braid in stream network

Aerial photo investigating braid

Sinks • Depressions in the DEM where water gets trapped • A sink prohibits calculating future flow direction grid values. • A sink occurs when all neighboring cells are higher than the processing cell. • Sometimes they are natural features!

Filled DEM

Flow direction • Created from an elevation surface • Direction values are assigned • Flow direction grids are used in many hydro GIS functions 32 64 16 8 128 1 4 2 Flow direction map

Flow direction • Created from an elevation surface • Direction values are assigned • Flow direction grids are used in the other flow functions 32 64 16 8 128 1 4 2 Flow direction map

Flow accumulation • The accumulated flow is based upon the number of cells flowing into each cell in the output grid. The current processing cell is not considered in this accumulation. • Output cells with a high flow accumulation areas of concentrated flow and may be used to identify stream channels. • Output cells with a flow accumulation of zero are local topographic highs and may be used to identify ridges

Summary of spatial tools available within Arc. GIS Elevation grid Any sinks? Fill sinks Yes No Flow direction Flow accumulation Allows for additional landscape based analysis Stream delineations Watersheds Stream order Riparian areas

Calculate drainage area Flow accumulation grid = tells us the number of cells of a certain area that flow to a point

Calculate drainage area So, if there are 280, 721 cells that flow to that location… and each cell is 10 m by 10 m in size (100 m 2) Then The total drainage area is (280, 721) * (100) = 28, 072, 100 m 2 Or 28, 072, 100 m 2 * 0. 00024718 = 6, 938 acres Conversion of m 2 to acre

Vector versus raster streams

Other issues

Creating a raster stream grid

Result of streamgrid

Thin command to thin the raster

Results of thin

Changing all nonstream values to ND

Result

Raising the elevation by 100 m

Finding the focal min values of dem for raster streams

Con’t

Result of elevation values to streams

Finds the nodata values

Result

Actual burn

Result

Summary • • Used 24 K lines from NHD for reference Clipped streams to watershed boundary fill, flow direction, flow accumulation Drainage area calculation Raster streams, thinned Raised off stream elevation values Assigned true elevations to stream cells Combined the two for a hydrological “corrected” elevation surface

Questions / Comments?