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Processing Elevation DataProcessing Elevation DataProcessing Elevation DataProcessing Elevation Data
Limitations of DEMs for hydro workLimitations 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 workGetting line work
Creating the stream for our watershedCreating the stream for our watershed
24k streams for the major basin extent24k streams for the major basin extent
Clipped streams to watershed bndClipped streams to watershed bnd
Braid in stream networkBraid in stream network
Aerial photo investigating braidAerial photo investigating braid
SinksSinks
• 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 DEMFilled DEM
Flow directionFlow direction• Created from an elevation surface
• Direction values are assigned
• Flow direction grids are used in many hydro GIS functions
Flow direction map
32 64 128
1
248
16
Flow directionFlow direction• Created from an elevation surface• Direction values are assigned• Flow direction grids are used in the other flow functions
Flow direction map
32 64 128
1
248
16
Flow accumulationFlow 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 ArcGISSummary 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 areaCalculate drainage areaFlow accumulation grid = tells us the number of cells of a certain area that flow to a point
Calculate drainage areaCalculate drainage area
So, if there are 280,721 cells that flow to that location…and each cell is 10m by 10m in size (100m2)
ThenThe total drainage area is (280,721) * (100) = 28,072,100 m2
Or28,072,100 m2 * 0.00024718 = 6,938 acres
Conversion of m2 to acre
Vector versus raster streamsVector versus raster streams
Other issuesOther issues
Creating a raster stream gridCreating a raster stream grid
Result of streamgridResult of streamgrid
Thin command to thin the rasterThin command to thin the raster
Results of thinResults of thin
Changing all nonstream values to NDChanging all nonstream values to ND
ResultResult
Raising the elevation by 100mRaising the elevation by 100m
Finding the focal min values of dem for raster streamsFinding the focal min values of dem for raster streams
Con’tCon’t
Result of elevation values to streamsResult of elevation values to streams
Finds the nodata valuesFinds the nodata values
ResultResult
Actual burn Actual burn
ResultResult
SummarySummary
• 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?Questions / Comments?