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Triangle marker symbol along polygon outline

January 04 2010 | 0 comments
Categories: Cartographic Representations, Symbology

I need to make a symbology where a polygon-featureclass has a triangle-shaped marker along the outline. Using cartographic representations significantly improved the map, but I still have a problem. When the outline makes a sharp bend parts of the triangles are drawn a little bit outside of the polygon (picture attached). Is there a way to avoid this protrusions?

Mapping Center Answer:

You can only control the placement of the triangle relative to sharp angles to a certain degree if you are creating a polygon outline that has a repeating pattern. To create the repeating pattern for the outline, use the ‘Along outline’ marker placement option.   You can then add control points to the representation geometry to control where the triangle pattern breaks along the outline.  You can do this manually with the Insert Control Point tool in an ArcMap Edit session or with the Set Representation Control Point By Angle tool in the ArcToolbox under Cartography Tools, Symbolization Refinement. In both cases, you will want to have a reference scale set for your map before you add the control points.

However, control points will not remedy the problem in all cases, as polygon angles, marker size, marker direction and marker pattern may interfere with each other. If the markers point in towards the center of the polygon, the markers may over lap and/or run over the polygon boundary if the polygon angles are too small relative to the marker size and pattern. 

In the example below, the right image shows the result of adding a control point to the corners of a parallelogram. Notice how the pattern adjusts appropriately at the larger angle of the two angles, but the markers overlap the polygon boundary and interfere with each other at the smaller angle.

Triangle symnbol along polygon outline - Figure 1

One way to work around this type of symbol interference is to approach the task from a different angle.  You can symbolize the polygon and its boundaries with two different layers, a polygon layer and a line layer.  The polygon layer would simply be your original polygon layer, with only the fill layer symbolized.  The line layer would be created from your original polygon layer so exceptions can be made to the polygon boundary symbol rule.

To create a line layer, convert the polygon feature class to lines by using the Feature To Line tool in the ArcToolbox under Data Management Tools, Features.  Use your original polygon feature class as the Input Features.  After the tool runs, symbolize this new feature class to achieve the desired outline affect using cartographic representations.  Then create a new rule that will be used to symbolize the polygon boundary symbol exceptions.  Use the Split tool to break the lines where the angles are too sharp to accommodate for the pattern and assign it the new Rule ID.

Modifying the polygon boundary pattern might be a good option.  The example below shows the same parallelogram with the two layers symbology option, the line and the polygon.  The polygon color in the left image has been modified to emphasize the symbology for the polygon boundary.  The line pattern has been modified from the original line boundary symbology to better accommodate the awkward angles of the parallelogram through the use of spacing between markers.

Triangle symnbol along polygon outline - Figure 2

For more information on working with control points, please refer to ArcGIS Desktop Help

Set Representation Control Point By Angle Tool: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?id=1505&pid=1496&topicname=Set_Representation_Control_Point_By_Angle_(Cartography)

Control point tool: http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?id=416&pid=401&topicname=Working_with_representation_control_points

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