By Charlie Frye, Esri Chief Cartographer
Dash-dot line symbols for polygon outlines should be displayed in a way that consistently shows the pattern of dashes and dots and clearly shows the shape of the polygon. In ArcGIS, a multi-layer line symbol is required. One layer is a cartographic line symbol with a dash pattern. The second is a marker line symbol that shows a marker (the dot) with a complementary pattern to the dashes.
We often see maps where people have used such a symbol for polygon outlines, but instead of the result in at the left below, we more typically see something like the image at the right.
There are several problems here. For instance the patterns for the symbol do not match up where one polygon meets another, and the drawing order causes one polygon’s fill to draw on top of another polygon’s dots.
The most useful thing to know about this is that in ArcGIS assigning a line symbol as a polygon’s outline will not work to solve this problem. The reason is that when two polygons share an edge there’s no way to predict where that edge starts and ends relative to the start/end node of each of the polygons.
In order to create the solution for this problem, the polygons should not be drawn with an outline symbol. Do that by setting the outline to a simple line symbol with a line width of zero. In place of the outlines a line feature layer will be used. That layer will be created using the Polygon to line tool. Specifically, the of polygon to line tool collapses the topology of the polygons such that no coincident lines will be produced, i.e., the edge where two polygons meet will be represented with just one line.
Once you have a line layer that represents your outlines, you’ll need to store that line feature class in a geodatabase, because you will need to use some of the new (at 9.2) cartographic representations symbology to get the markers to align. In the image below to the right, an example is shown using symbolized lines, note that the dash length is often not consistent, and that dots often cover corners, obscuring them. Initially using cartographic representation symbology will also have the same problem, though not to nearly the same extent. By using the control points geometric effect that can be fixed.
To set up the representation symbology follow these steps:
- Symbolize the line using the same symbol you are currently using as the outline for your polygons.
- Right-click on that line layer and choose Convert Symbology to Representation; take the default options and that will add a new layer with your line data to your map that uses representation symbology.
- Turn off the visibility or remove the original layer.
- Some changes to the representation symbology are needed. I included a couple of screenshots to show how I set up a line symbol layer and the point symbol layer (for cartographic represenations each of the tabs in the symbology are for symbol layers). Note the relationship between the step property for the marker and the numbers in the pattern property for the line. I usually change my step property to be the sum of my pattern property.
- Set up the Control Points; to do this click the little “+” button in the upper right. This will open the Geometric Effects window, where you will click on the Add Control Points item in the list. Then click OK.
The result, in this case works well because it consistently portrays the dash-dot pattern, and now all significant details of the polygon shapes are clearly defined.