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I love working with quality software, and GM is definitely in that category. I don't spend money easily, but the GM product was well worth my initial investment and the 3 additional major updates since my first purchase. I love the GM product's ability to load and manipulate very large raster files. It really makes my work easy.

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Watershed Delineation in Global Mapper

You can automatically determine stream flow and delineate the watersheds for those streams based on any loaded terrain surface data. The generated line and area features can then be exported to any support vector format, such as Shapefiles.

The Generate Watershed command allows the user to perform a watershed analysis on loaded terrain data to find stream paths as well as delineate the watershed areas that drain into a given stream section. The watershed calculation uses the eight-direction pour point algorithm (D-8) to calculate the flow direction at each location, along with a bottom-up approach for determining flow direction through flat areas and a custom algorithm for automatically filling depressions in the terrain data.

When selected, the command displays the Watershed Generation Options dialog (pictured right) which allows the user to set up the watershed generation process.

The Resolution section controls the resolution at which the loaded terrain data is sampled to perform the watershed analysis. The default values should capture the full resolution of the loaded terrain data. Larger values (i.e. lower resolution) will cause the calculation to be quicker, but less detailed.

The Stream Threshold section controls how much water must flow to a particular cell before it is considered part of a “stream”. Larger values will result in only more water flow areas being delineated, while smaller values will cause more minor water flows to be marked as streams. Each stream segment (i.e. the portion between and inflow and outflow point) can optionally have the area that drains directly to that stream segment marked with a watershed area. Check the Create Watershed Areas Showing Drainage to Streams option to enable creating watershed areas.

Many terrain data sets will contain depressions in the data where flow would terminate unless allowed to fill the depression and spill into the surrounding terrain. The Depression Fill Depth section controls how deep of a depression will be filled before it is considered a basin and flow is allowed to terminate there. The depression fill depth value will automatically be filled in with some guess at a good value based on the range of loaded elevation values, but you might want to modify this, especially if you have relatively flat terrain with a lot of depressions. Note that it can take a while to fill particularly deep depressions. When trying to determine a a good value to use for the depression fill depth you should think about how high of an embankment or ‘dam’ that you might encounter that you want to allow water to fill up to the top of and pour over, or also how deep of a small pond or puddle to fill and allow spilling out of. You can also check the option to Save DEM to Global Mapper Grid File After Filling Depressions to save the depression-filled terrain to a GMG file so you can load that for future watershed operations to avoid having to fill depressions again.

The Operations at Selected Point Locations section allows you to perform additional flow and drainage network delineation based on point locations selected in the Digitizer Tool. The Create Watershed Areas Showing Drainage to Selected Point(s) option will calculate a drainage (watershed) area for each selected point features showing which portion of the loaded terrain drains to the immediate vicinity (i.e. within one sample of the point location) of the selected point. The Trace Flow from Selected Point(s) (Water Drop Analysis) option will cause a separate arrowed line to be generated starting at each selected point feature showing where a drop of water placed at that point location will flow to.

The screenshot below shows a watershed created from a collection of USGS DEMs. In it you can clearly see the stream network as well as the drainage areas for each stream segment.