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
24k 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 Flow direction map
Flow direction Created from an elevation surface Direction values are assigned Flow direction grids are used in the other flow functions 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 are 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 ArcGIS Elevation grid Flow direction Fill sinks Allows for additional landscape based analysis Flow accumulation Any sinks? Yes No Watersheds Stream delineations Riparian areas Stream order
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 10m by 10m in size (100m 2 ) Then The total drainage area is (280,721) * (100) = 28,072,100 m 2 Or 28,072,100 m 2 * = 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 100m
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 24K 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?