The Create 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 below) 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. If you would like to discard any short stream segments that don't have any other streams flowing into them (i.e. short little stub streams feeding into a main trunk), check the option to Discard Stream Starts Less than Some Length. This will remove any short little stream stubs that are below the specified distance threshold (in meters).
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 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 Locations
section allows you to perform additional flow and drainage network delineation
based on line and point features selected in the Digitizer
Tool. The Create Watershed Areas
Showing Drainage to Selected Line(s) option will calculate a drainage
(watershed) area for each selected line feature showing which portion
of the loaded terrain drains to the immediate vicinity (within the specified
flow threshold) of the selected line. This is useful to see what drains
to something like a road feature.
The Create Watershed Areas Showing Drainage
to Selected Point(s) option will calculate a drainage (watershed)
area for each selected point feature showing which portion of the loaded
terrain drains to the immediate vicinity (i.e. within the specified flow
threshold 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
Trace Flow from Selected Line(s) option will cause an area feature
to be generated showing all areas that a particular line feature will
drain to. This is useful for determining something like where a leak from
a pipeline might leak to.
Flow Direction Point Creation creates
a point feature (example from 3D viewer below) with the direction (FLOW_ANGLE) and
magnitude (FLOW_ACCUM -the count of cells flowing to a given cell) attributes at each point, then enabling the layer point style
for the quiver plot. The display of the quiver plot is not visible until
zoomed in far enough for decluttering purposes.
The example 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.