NRES 401 August 28, 2007
Watershed delineation and characterization using ArcGIS with Spatial Analyst Tool
The computers in the ACES Computing Center (basement of ACES Library) have ArcGIS (version 9.1) installed in the desktop folder labeled “statistics and econometrics”.
The digital elevation model (DEM) for the homework assignment was obtained from the National Elevation Dataset website
It is relatively easy to download this data if you use Firefox as a web browser. But the USGS system periodically experiences some outages, so I am making the DEM files available in a zipped folder here http://courses.nres.uiuc.edu/nres401/NED_philosec15.zip
Download this file and unzip the contents (several files and a subfolder containing additional files) into your working folder for this exercise. The DEM is contained in the file and subfolder named “NED_26994145”. The file and folder labeled “rastert_flow” contain a raster file that contains a point indicating of the outlet of the watershed to be delineated (coordinates 88°10'49.668"W 40°0'46.738"N).
If you are using the machines in the ACES Computer Facility, open the statistics and econometrics folder and then open the ArcGIS9.1 folder. You will see icons for ArcCatalog and ArcMap, along with some others. The program ArcCatalog is designed to help manage ArcGIS data files. Run ArcCatalog, and click on “File” and then “Connect to” which will open a window browser. Browse to the folder that contains the unzipped DEM (not the subdirectory) and connect to this folder. This can help you locate the files in ArcGIS.
Double click on the ArcMAP to start it. A small menu of initial options opens up, offering options to open an empty map, a template or an existing map. Cancel this by clicking on the red x in the upper right hand corner of the menu. To do this exersize in ArcGIS the extension called Spatial Analyst Tools needs to be activated. Click on “Tools” at the top of the screen, and this will open a menu; click on “extensions”. This will open a window with available extensions and the box next to “Spatial Analyst” needs to have a check mark in it to run the tools that analyze watershed surfaces. After checking the box, you can close this window.
To load the DEM, click on “File” and “Add data”. This will open up a menu that will allow you to browse to the folder containing the DEM, and when you do so, the file name should appear in the browser window. (If you have trouble finding the folder, go to the browse to the Catalog icon, and the folder you connected to in ArcCatalog should appear as a connected folder). Click on the file name to highlight it, then the “Add” button to load the file. The DEM image should appear on the right side of the screen.
If you click on the red toolbox icon at the top of the screen, it will open a panel with a series of other toolboxes that contain the tools needed to analyze the DEM.
Projecting the DEM in UTM Coordinates
The raw DEM provided by NED is oriented to a coordinate system that reflects the curvature of the Earth’s surface. In order to analyze the surface on a horizontal plane, these files must be projected into a 2 dimensional coordinate system. I recommend projecting to 1983 NAD Zone 16N. Zone 16 N covers much of Illinois and Indiana. Another possible projection is CGS NAD 1983 Eastern Illinois FIPS UTM, but when I tried this projection the contouring tool did not work in ArcGIS 9.1.
To project the DEM, find the toolbox labeled, Data Management Tools, and click on the “+” next to it to open up more tool boxes. Within this new list, open the box labeled “Projections and Transformations” and then open the toolbox named “Raster” and then finally double click on the tool named either “Project Raster” (in ArcGIS9.1) or “Project” (in ArcGis9.2). This will open a window for you to enter the input file and specify the projection. The input raster is the raw DEM file, and its name should appear in the input pull down menu. An output raster location and name will be automatically generated, but you can modify this if you wish. The output coordinate system needs to be selected by clicking on the icon next to the “output coordinate system” box. Clicking on this icon will open a selection menu. When you click on the “select” button, it will open up a menu with folders for projected coordinate systems and geographic coordinate systems. Click on “projected coordinate systems”, then click on UTM, then click on NAD 1983, then select “NAD 1983 Zone 16N” and then click on the “add” button and this will load the coordinate system into the selection menu. Click OK to this selection and then click OK on the project tool window, which will begin the process of producing a new projection, which will take a few seconds. When the process is complete, a small window will appear announcing its completion, and giving an error message if there were any errors. If there were no errors, the projected raster will have been created, and displayed on the screen, and it will resemble the original raster. Make note of the name of the projected DEM and close ArcMap. Restart ArcMap (yes, that’s right, restart the program you just quit), and load up the projected DEM file, not the original DEM.
The properties of the project DEM can be seen by right clicking on the file name, as it appears in the column on the left of the screen. This opens up a menu, and the last choice on the menu is “properties”. When you click on properties, a window opens with seven different tabs. In the “source” tab, under Raster, you can note that the cell size is 27.057 x 27.057 (the units are meters). If you scroll down in this window, you can also see the spatial coordinate system.
Under the “symbology” tab, you can alter the appearance of the map, by changing the color scheme, or by changing it from a “stretched” presentation (where variation appears to be continuous) to a “classified” system where elevations would be presented in a finite set of categories.
Adding Contour Lines
For orientation, it may help to add contour lines, but it is not necessary to do so for the analysis. Contour lines can be added by clicking on the red toolbox to open the toolbox panel, and going to ArcToolbox, Spatial Analyst Tools, Surface, and Contour. When you double click on Contour took, it will ask you to specify the input raster (which is the projected DEM… the file should show up in the input pull down menu), and for a contour interval, in meters. I recommend a 2 meter contour interval, which will produce contours similar to the 5 foot interval in the topographic map.
The Hydrology Toolbox
Most of the rest of the analysis can be done using the tools found in the Hydrology tool box, within Spatial Analyst Tools. Any depressions, or “sinks,” in the topography can cause problems for the tools that are useful for identifying watershed area and flow length. These tools follow the downward slopes, and when a depression is encountered, they assume that is the end of the flowpath. So, to enable the tools to follow flow paths to the watershed outlet, the sinks have to be “filled” which is done with the “Fill” tool, which creates a new DEM with the depressions filled. Double click on the “fill tool” and enter the projected DEM file as the input raster. This operation will take a few sections and will produce a message window when completed, as well as a filled DEM that will look a lot like the original DEM.
The next step is to generate a flow direction raster from the filled DEM, using the “flow direction” tool. The filled DEM file created in the previous step should be specified as the input.
When you have a flow direction raster for the filled DEM, you can then apply the “flow accumulation” tool, using the flow direction raster as input. The resulting raster displays the number of cells that are draining to any point on the landscape. The highest number of cells will look like stream channels, and the image should resemble (or can be modified to resemble) the channels that appear in the topographic map. You can use the identification tool (the letter i in the circle) to click on any point of this layer and obtain the number of cells draining to this point. By multiplying the number of cells by the area of each cell, you can obtain the area draining to any point in the watershed. The outlet or pour point of the assigned watershed is located at the following coordinates: 88°10'49.668"W 40°0'46.738"N. If you load the raster file “rastert_flow”, which indicates the watershed outlet, a single point should appear in the south west portion of the field, located in an area with relatively high drainage area and near the elevation contour of 210 meters (690 feet). You may have to zoom in on the south west portion of the map to see the point. But when “rastert_flow” is the top visible layer, it blocks the information tool from getting information from the flow accumulation layer. You can convert “rastert_flow” to an individual point feature, by going to the “conversion tools” tool box and clicking on the “from raster” tool box and double clicking on the “raster to point” tool. Enter “raster_flow” as the input raster, and the tool will create a new layer which is a single point feature. You can then turn off “raster_flow” and will see the single point on the map, and be able to access information on the layers below it.
You can also obtain the maximum flow length in the watershed by using the “flow length” tool and choosing the “upstream” option. Again, the input to this tool is the flow direction raster of the filled DEM. The greatest flow lengths will appear in the stream channels, and you can determine the lengths of any point by using the identification tool and clicking on points in the watershed. In this case the output is in meters.
The “Watershed” tool will identify the boundary of the watershed draining to the pour point. This tool requires both the flow direction raster of the filled DEM, and the pour point as the point feature file that you created from “rastert_flow” as described in a previous paragraph. The output is a new layer with the watershed boundary. You can determine the area a couple of different ways. If you right click on the name of the file, and go to the attributes table, one of the columns is labeled “count”, which is the number of cells in the watershed. If you multiply the area of each cell by the number of cells in the watershed, the result will be the area. In ArcGIS9.2, you can also use the measure tool, which allows you to measure distance and area. Click on the icon that looks like ruler with a question mark over it, and a window opens that provides options for measuring length or area. (In ArcGIS9.1 this tool measures only length, not area). Click on the icon that looks like a wedge to measure area, and trace around the outside of the watershed, clicking periodically to match the traced feature, and double click when you are done. You can change the units of measure by clicking on the triangle icon.
Identifying depressions or “sinks”
This is not necessary for the assignment, but as a matter of interest, if you apply the slope direction tool to the projected DEM without first using the “sink” tool to fill the sinks, you can then use the “sink” tool, to identify the sinks or depressions in the landscape. These areas where could potentially be wetlands.