How to dissolve “internal” polygon boundaries, or: arcinfo region coverages we miss you!

Question: We’ve been scratching our heads over how to tackle this for some time, and are no more illuminated than when we started. I hope you can help. Our task is thus: how to draw a lake with a solid outer perimeter line and no lines between internal wetlands, shoals and shallows? ‘Tis a relatively straightforward process with ArcInfo coverages and Regions, but alas, that is no longer a viable option for us. The datasets we use now are too large and the lack coverage editing ability in ArcMap crucial. Can cartographic representations solve our dilemma? Is it possible to create something like the attached without resorting to creating (and managing) duplicating duplicate geometries whose sole purpose is better looking maps?

Answer: You do not have to use Cartographic Representations to do this.  There is a Knowledge Base articleon how to symbolize polygon features so that boundary lines between like polygons are dissolved.  The solution dissolves all boundaries so for those areas you wish to symbolise with a boundary you need to modify it as follows:

For each class, edit the symbol to give it an outline style and width.  Ensure that the outline width is double the final intended width (for reasons given below)

Now you need to modify the symbol levels.  In the solution, all fills are drawn on one level and all outlines are on another level but this has the effect of masking all the outlines whatever symbols you use.

Instead, you need to set symbol levels so that the fill and outline are alternate for each class.  For example, symbol levels 1 and 2 for one class, then 3 and 4 for another and so on.

The result is to alternate the fill and outline symbol levels so that outlines can be seen – though only half the width of the outlines will ever be seen which is why it is important to double the thickness.

Note, if you use a cartographic line symbol then only one half of the width of the symbol will be visible with this workaround and that may have consequences for the line style you choose.  The solution will work best for solid or dashed lines.

Attachment: an example of the desired result

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About the author

Aileen Buckley

Dr. Aileen Buckley is a cartographer who’s been at Esri since 2003. She finds and shares best practices for mapping and analysis with ArcGIS, which leads her to publish widely and present world-wide.

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