MIT Libraries



Working with Hydrology Tools in Arcgis

Daniel Sheehan

Office of Educational Innovation & Technology

Anne Graham

MIT Libraries

IAP 2009 - January 27, 2009

In this part of the exercise, you will use the DEM you already processed. Use your input raster rather than the name listed in the figures.

All of the hydrology tools we will use are found in the Toolbox. Navigate to Spatial Analyst Tools then Hydrology to find these.

The ArcGIS Hydrology Tools assumes no sinks, either natural or created by errors in the Digital Elevation Model (DEM), therefore you need to run the DEM through the FILL routine. This removes what the software calls imperfections. If natural sinks do occur in your data, you will need to operate on the areas around them in separate runs. Starting the Fill tool opens this form:

[pic]

Select your projected DEM, define an output surface raster called “filled”, which should be in your F:\ drive folder (in the Usertemp folder), and click OK.

Next, you need to find the flow directions for each of the cells in the DEM. The model assumes a single cell output, which means that all water that falls in a cell or flows into a cell exits that cell to a single neighboring cell, following a path with the largest slope. When you start the Flow Direction tool, you will see this form:

[pic]

Change the input surface raster to your new “filled” DEM. The output flow direction should called “flowdir” and should be stored in your F:\ drive folder and click OK. The output raster has a very small set of values: 1, 2, 4, 8, 16, 32, 64, and 128, which represent the 8 cells that the water may flow into. This graphic from the presentation should help you understand the image:

|32 |64 |128 |

|16 |Source |1 |

| |Cell | |

|8 |4 |2 |

Another way of looking at it is this: cells of equal value have similar aspects. All 32 cells are NW facing, all 1 cells are east facing. Your result may look like this:

[pic]

Next, you need to calculate the flow accumulation raster. This raster represents the number of uphill cells for every cell in the DEM. Therefore, cells in stream beds should have very high values and slope cells should have low values. You should see this form when you open the flow accumulation tool:

[pic]

Change the input raster to “flowdir” and the output raster to “flowacc”. The latter should be stored in your F:\ drive folder. Click on OK. Your result should look like this:

[pic]

This is a zoomed in image of a small part of the entire DEM. The stream cells are prominent since then have a very large number of cells contributing to them while the slopes have comparatively few contributing cells.

You can create a vector copy of this. First, you need to select only those cells that reach a threshold value for the area you are working in. The threshold can be determined empirically by finding a streambed and finding the flow accumulation value for it and repeating this exercise. The hydrologists in the Parson’s Lab have determined that the threshold for the New England is approximately 400 cells for a 30 meter DEM. The example in the document is in the SW United States and that threshold is used. In the SW US however, it is far too large and many streams are missing. To start to make your vector stream layer, first use the Raster Calculator (under the Spatial Analyst toolbar). You should see this form:

[pic]

You need to run the command: [flowacc] > 400. Spaces are important. It is best to use the interface to type the command. Your result will likely have the prosaic name of “calculation”. You can change this use the General tab on the layer’s Property form if you wish. To convert to a shapefile, or vector layer, use the Spatial Analyst Convert tool, Raster to Feature (meaning vector). You should see this form:

[pic]

Be sure to use the correct input raster and change the Output Geometry Type to Polyline.

Once you have the new shapefile layer, you can export this layer to KML format, the data type used in Google Earth. Use the convert to KML too in the Conversion section of the Toolbox. This is the result from my example:

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[pic]

con([drainage],1,10

flowlength([flow_dir], [weightgrid])

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