The goal is to show just the perimeter for all the polygons in the dataset

We were asked, not long ago:

"I’ve got a dataset with polygons that represent the extent of field study areas at different dates; these study areas have grown over time and I want to show a single polygon that represents each site’s current study area, while still allowing me to Identify anywhere within the area, be able to see other features inside of the study areas, and I cannot be saddled with a geoprocessing experience every time I need to make my status maps for any of the study areas; How can I do this?"

In this user’s particular case, the study areas were archeological sites; however, they could be extents for wildfires, floods, hazards or other phenomenon that spread over space and time that need to be mapped to show the total area of effect or impact. A definition query could not be used on the dates for study area changes because different sites were changing at different times. Here is a complete depiction of some example data, showing the extent for each year, all the extents superimposed, and finally just perimeter for these extents:

Here the three site extents are shown, then superimposed, and finally just as a perimeter

We’ve heard of similar challenges in the past – this is the kind of thing that ArcInfo users used the DROPLINE command to address. There is a way to use Symbol Level Drawing to mimic the effect of DROPLINE, and it is described in the online help topic: Working with symbol levels. The problem, in this case, with the symbol levels method is that it requires a fill symbol that uses a fill color, so you can’t see any features behind the polygons once they are drawn. Transparency won’t help this case because there could be several overlapping features, each one increasing the amount of opacity

If you want to use Geoprocessing to achieve this result you can use the Union tool and then the Dissolve tool to derive the geometry that will meet your needs. The trick is to NOT specify a dissolve field.

However, to solve this problem, which is slightly more complex, we did the following:

  1. We symbolized the boundaries so the fill symbol was set to No Color and the outline width of 1.4.
  2. Then we copied that layer and pasted it back into our data frame, unchecked its visibility checkbox, and changed the symbol to have an outline of 0. We also renamed this copy of our boundaries layer to "Boundary mask".
  3. Then, using the Advanced Drawing options, we set up variable depth masking so that our "Boundary mask" layer masked our original boundaries layer

The effect is that the masking polygons (in the "Boundary mask" layer) mask out any of the internal boundary outlines, leaving only the largest extent, and in the case of the archeology sites, the most recent site extents for each site. This works in this case because the solution can be stored as either a group layer or as a map document and will be ready to use anytime it’s needed, with no additional work.

CF