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?