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Here's one for those who've been around the block a few times.

ArcInfo Workstation from days of yore had an intelligent data model called Regions or Region sub coverages. With this model one could select one or more polygons and create new virtual polygons from the selected set. Furthermore one could dissolve within the set, creating a result akin to merged polygons, or have disparate polygons, possibly overlapping, that identify themselves as one unit (multipart polygons in shapefile parlance). These virtual polygons are called regions. When the regions are loaded into Arcmap (or ArcEdit/View back then) they behave like an independent shapefile or feature class in terms of querying, symbolizing, etc.

The beauty of this model and what is missing from feature classes and feature datasets in the ArcGIS world, so far as I've been able to determine, is that in regions the geometry is stored only once. Imagine, for example, the continent of North America split into polygons depicting countries, islands, states & territories, counties, municipalities, etc. down to whatever level you feel like stopping at, all stored in a single feature class. (ugh, what a mess) Yet, each level has convenient handles (country, province, whatever) that can be grabbed and used as if they were independent feature classes.

Coverages and Regions have their problems, they weren't left behind lightly, but they weren't all bad. Sooo, what is the ArcGIS answer to ArcInfo Regions?

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I spent a lot of time searching for a good graphic illustrating the region concept and came up empty. Anyone else have one? –  matt wilkie Aug 4 '10 at 22:07
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ArcInfo Workstation is still shipping with the recent ArcGIS 10 release, which is in some ways an acknowledgement that the 25 year old tool remains necessary. –  scw Aug 5 '10 at 18:39

3 Answers 3

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I'm one of those lucky ones (I think) that entered the GIS world at about the time coverages were being phased out. I've used them, and still do sometimes, but never had to deal with thm in the old pre-ArcGIS ArcInfo.

To answer your question as simply as possible, I don't believe there is anything comparable to Regions or Region sub covrages in the new geodatabase model.

Here's a link to a white paper from 2003 describing the differences between coverages and geodtabases: ArcGIS: Working With Geodatabase Topology

From what I've gleened from this paper, ESRI believed at that time that any benefits from th efficiency of the coverage model were not enough to continue supporting it. Thus the move to the more flexible geodatabase.

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+1, I got the same impression from the same document –  fmark Aug 5 '10 at 5:57
    
+1: that was my impression as well. –  Jay Cummins Aug 5 '10 at 9:46

Here's the use case for REGIONPOLYCOUNT.

Given: a polygon featureclass representing historic forest fire burn areas represented as overlapping polygons.

Find: a way to make a map symbolized by the number of times a particular region has been burned.

With a coverage you'd use REGIONPOLYCOUNT. With an ArcGIS geodatabase topology you'd need to write some complicated code.

I see no apparent purpose for esriTopologyElementType.esriTopologyFace.

This suggests to me there was a design in place to support this use case, but it was never implemented.

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thank you Kirk for that perfect example. –  matt wilkie Apr 13 '11 at 20:00

"The beauty of this model and what is missing from feature classes and feature datasets in the ArcGIS world, so far as I've been able to determine, is that in regions the geometry is stored only once."

That concept was a major driver for the entire coverage data structure. Polygons were built out of lines so that shared boundaries were not duplicated. Regions were built out of polygons as you have described. As time went on and things changed, minimizing storage became less important than minimizing retrieval time, and the geodatabase was born. So it's not as conceptually elegant, but with current technology, it works better.

The answer to "what is the ArcGIS answer to regions" is simply polygons. We gained the ability to have multipart polygons, and overlapping polygons, and polygon feature classes with void space in them, all of which were things that you could do with regions. What we lost is the ability to have nested sets of geographies that all use the same line work. The modern answer to that, just as in data warehousing, is to just set up the tools that you need to regenerate one thing from the other, and just don't worry about how much more storage you are using to accomplish it.

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I'm not worried about the storage needs, I'm concerned about my capacity to process my workload, which most emphatically does not scale with Moores Law. With coverages I didn't have to remember to make the same edits to multiple geometries with shared boundaries, or spend the time to build topology rules -- repeated for each related set -- to enable the system to do it for me. –  matt wilkie Aug 5 '10 at 21:24
    
The topology tools in ArcEditor are the real answer to that. But they do take some investment in setup and retraining. –  Jersey Andy Aug 6 '10 at 20:41

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