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Can I make polygon fill symbols follow a flow path?

August 02 2009 | 0 comments
Categories: ArcGIS Methods, Cartographic Effects, Cartographic Representations, Symbology


I have a polygon feature class holding landslide data. The polygons are filled with marker symbols indicating movement. I'd like the arrows to point downhill, to indicate the flow direction. Rather than rotating all symbols as one block, is it possible to have them follow a path?


Mapping Center Answer:

That's not so easy as a fill symbol--you could convert the fill symbols of your landslide layer to representations and then individually rotate the arrow markers within those fill symbols (that might get really tedious).

If you've got an elevation dataset you could do the following:

1.  Use the Aspect tool in either the Spatial or 3D Analyst extensions.  That will produce an output raster with values that represent downhill azimuths. (This assumes that gravity is in play in the sense that flow movement is being derived from an after the fact elevation dataset where downhill movement is assumed to be the path of least resistance.)

2.  The idea here is to produce a point dataset from the Aspect raster.  But, depending on your map scale you will likely need to reduce the resolution of the Aspect raster before using the Raster to Point tool.  To do that use the Aggregate tool in the Spatial Anaylst.

3.  Once you have an acceptable outcome from the Raster to Point tool for your map scale (density of points is not too dense or too sparse), then I would add a Short Integer field to the point dataset to have a value of 0 for outside the landslide areas, or 1 for inside the landslide areas.  First calculate all values to 0, and then use Select by Location to select the points inside your landslide areas and calculate that field's value to 1.  That will allow you optionally include (via definition query) or symbolize differently the points outside the landslide areas.

This will result in a more regularly spaced look, like the bottommost of your scans.

Another option, particularly, if you want a more hand rendered look (like the uppermost of your attached scans), you could hand-digitize flow lines and then use cartographic representations on those.  We explain how to set up that kind of symbology on the Mapping Center blog when talking about symbolizing one-way streets.  You could control the spacing and offsets of each flowline's symbol, creating a similarly expressive effect as shown in your attachments.


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