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I need to represent overlapping polygons as a single polygon without changing the geometry.

July 10 2007 | 0 comments
Categories: Cartographic Effects, Data Modeling, Symbology

I need to represent overlapping polygons as a single polygon without changing the geometry.
The overlapping polygons will have a common identifier.

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Let me explain the situation a little more.

We have an archaeological sites feature class in SDE and every time our field personnel goes out and monitors the site, they create a new boundary for the site based on what they see during their visit to the site. We then load that new polygon into the feature class while maintain all of the existing polygons for the site. This creates multiple overlapping polygons of different shapes.

We then send the new polygon to our state historical office and they edit their maps to reflect what our new polygon shows. However, they never reduce the size of a polygon, they basically merge every polygon that has ever been sent to them for each site. They also require that all maps we submit to them have the same polygon shape as they have for each site.

We do not merge them, we like to keep this historical record of each visit.

I have tried using the first technique you mentioned, but I believe a fill has to be used. I have to use a hollow polygon rather than a fill.

I do not want to export the polygons and dissolve them every time I need to make a map to send to our state historical office.

Mapping Center Answer:

I’m not sure which way to interpret “without changing the geometry”.

First, literally, use the same feature class you have now:

The ArcGIS web help for Working with symbol levels has a section for doing this—it’s the example nearly at the bottom with 22 steps; follow them carefully, I’ve tried this and it works. This section in essence describes how to mimic what the old ArcInfo command DROPLINE did.

Second, you can use the Union tool based on a field. I’m going to describe this as though your data were structured this way, so if it’s not just ignore this idea. Based on a field that contains each type of polygon create layers that use definition queries to isolate (within those layers) each type of feature. Then use all of those layers as inputs to the Union tool. Then you could use the Dissolve tool with the Union_result as the input.

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I just thought of a neat little hack using masking (Advanced Drawing) that worked.

1. I used my boundaries layer and symbolized with no color for the fill and an outline width of 1.4.
2. Then I copied that layer and changed the symbol to have an outline of 0 (I renamed the copy to “mask”).
3. Then I set up masking so that my “Mask” layer masked my original layer.

I think that will do what you need and not create any extra data. Please let me know;

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