By Charlie Frye, Esri Chief Cartographer
Not all of the new functionality for mapping and cartography was shown in the mapping section of the What’s New in 9.3 PDF that’s available on the new Resource Center. New with both the 3D and Spatial Analyst Extensions in ArcGIS 9.3 is the Contour with Barriers tool, which generates contours from a raster surface and allows you to limit the creation of the contours to either side of a barrier. This tool has several improvements over the the existing contour tool:
- Smaller output — in my tests, the output from the Contour with Barriers tool was about half the size of what the existing contour tool produced. That means faster drawing and labeling.
- Option for specifying index contour interval — with this option adds a field called Type and calculates values of 1 for intermediate contours and 2 for index contours. To run the tool you would specify, for example, a contour interval of 20 feet and an index contour interval of 100 feet. The old way of doing this was to add a new field and calculate it yourself using the VBScript Mod() function.
- A new parameter for a barriers feature class — this actually applies to a lot more than cartography, but for mapping, imagine being able to use as a barrier a line feature class representing shorelines, the edge of a glacier, or the toe of a landslide. The result be contours broken at these barriers, allowing you to decide how or whether to display those contours.
Another important use of the barriers feature class is to clip contours at the study area edges when you need to create contours over a large region. With the existing contour tool, a single contour line can noodle across very large portion of the output dataset’s extent without being broken. For a large region, there can be many of these contours and they can be extremely long, especially if the area has a lot of relief and the contours are very convoluted. This effectively defeats the dataset’s spatial index, making drawing, selection, labeling, geoprocessing, etc., very slow. Using something like U.S. Public Land Survey System lines or quad sheet boundaries (installed with ArcGIS at c:program filesArcGISreference systemsusgs24q.shp) as barriers, the resulting dataset will perform much better.
There are a couple of drawbacks to the contour with barriers tool:
- It runs in about twice the time it takes the existing contour tool to run, but that is more than mitigated by the time saved in not having to do additional geoprocessing to accomplish tasks required for cartography anyway (that is, clipping contours at barriers and attributing index contours).
- As noted in the help, this tool does not deal with large elevation raster datasets as input and small contour intervals in the output. The next to last section of the usage tips in the online help gives you strategies to work around this limitation until it is fixed. That said, I was able to produce 50 foot interval contours from USGS 30 meter NED DEM data for the State of Washington using 1:250,000 scale quadrangle sheet boundaries in only 8 hours, thought I had to write a script to do the tiling.
At this point, I can say that the Contour with Barriers tool is usually going to be a better option from the perspective of taking the time up front to run the tool and saving time later in geoprocessing and performance.