Mapping Center

Unfortunately there currently is no automated way in ArcGIS to evaluate contour lines and select those that are depression contour lines. There are some tools in the Spatial Analyst extension such as FLOW DIRECTION, SINK and FILL which may look useful for this purpose, but in fact are designed to find small irregularities in digital elevation models (DEMs) and fix them, and thus they don’t find larger depressions, which are typically the basis for depression contour lines.

There is a relatively simple manual method that can be used to check for and assign depression contours for a quadrangle sized area in 5-15minutes. This method depends on being able to display contour lines with different colors. Thus, depending on the contour interval a field that will contain a string value of the right-most two or three numbers of the elevation will be needed. If the contour interval is 5, 10, or 20, then just two numbers are needed; if the contour interval is 40, 50, 100, or 200, then three numbers will be needed. Follow these steps to set that field up and to identify depression contours:

1. Add a text field called Right2 (if the contour interval is 5, 10, or 20) or Right3 (if the contour interval is 40, 50, 100, or 200) that is a width of 4 to your contour lines layer. The idea is that you calculate the value of this field so it contains the right most two or three digits from each contour line's elevation.
2. Calculate that field using this statement: Right(Str([Contour]),2). Use a 3 if calculating the right3 field’s values.
3. Add another new field called DeprYN which should be a short integer field. Calculate this fields values to be 0 (zero). Later you will be manually assigning values of 1 to depression contour lines.
4. Open the Layer Properties window for the contour line layer.
5. On the symbology tab choose to show the features with Categories and the unique values many fields.
6. Use two fields, the Right2 or Right3 and the DeprYN. Initially this will not show any depression contours (having a value of 1 or greater).
7. Click the Add All Values button.
8. Apply the layer properties and close the window.
9. Next a color ramp is needed that can be used in the unique values symbology. Open the Style Manager (tools menu, styles sub menu).
   1. Expand the ESRI.Style and click on the Color Ramps folder to show its contents on the right.
   2. Scroll down to and right click on the ramp called “Spectrum-Full Bright”. Choose copy from the pop-up menu.
   3. On the left panel of the style manager expand your personal style, and click on the color ramps folder.
   4. In the right panel, right click and choose paste.
   5. Make sure the view in the right panel is for details (right-most button on the lower right of the Style Manager window. This will show the Category column.
   6. In the category column click on the text for the color ramp you just copied and delete the text. This will allow this ramp to be shown in the unique values symbology color ramp list.
10. Open the layer properties for your contour layer and set the color ramp to be the one you just added.
11. Right-click on one of the rows in the symbol list and choose properties for all symbols. In the Symbol Selector window that opens set the width to 2.0 and click OK; and click OK again to apply and close the Layer Properties.
12. Now you can see each contour line in sequence (See below for examples).
    
    
    Example of contour lines symbolized to assist in locating depression contours.
    
13. Start Editing.
14. Zoom into a scale that shows more than what the example above shows, and Identify a few contours in progression to determine which direction uphill is. In the example above, uphill is from upper left to lower right.
15. If a color is repeated to the uphill side of the main contour (as indicated by the arrows in the above example), then that is a depression contour. Select that contour and open the Attributes window and set its DeprYN field’s value to 1.
16. The <all other values> symbol will now be used to display that contour line now. Optionally, the layer symbology can now be set to include the depression contour.
17. Optionally, if contours are nested, it may be required that the most deeply nested contours by symbolized with hachures that are more closely spaced. If that is the case, the number entered for the DeprYN field can indicate the level of nesting, so for example a 2 would mean that this contour was the second contour inside a depression.

With a little practice this method can be applied fairly quickly to a large area. Though one warning is to be wary of hills inside of depressions—contour lines inside a large depression contour are not guaranteed to be depression contours. Generally speaking, contours generated from 10 meter pixel SRTM DEMs from the USGS have very few depressions and those can be found with this method in a matter of minutes. Contour lines generated from 1-3 meter pixel size LIDAR DEMs, however, will have many depressions and it will take several hours to find and tag all the contours for a city of 100,000 persons, about 15 square miles in my case.

CF