How to merge/cluster smaller contiguous polygons to form larger ones?

Question

Any help you can give an inexperienced college GIS user would be greatly appreciated.

I am using both ArcGIS ArcMap 9.3 and 10 on desktop (i.e. not online), and the only info I can find on License Type on college computers is 'ArcInfo'.

I have a set of forestry data that maps forestry type (species/age class etc) for a small country. With this data I am attempting to evaluate suitable habitat by identifying woodlands over a certain size (>200ha) with particular make up of tree species (mostly coniferous/broadleaf).

The problem I am having is that the shapefile has polygons made according to individual tree species. Is there a way to cluster the individual tree species, so I can attain one larger polygon for the entire woodland, and thereby have an area value for the woodland as a whole, instead of the individual parts (both due to seperate species, or division due to a fence, for example).

There are thousands of polygons in the dataset, so it is not feasible to get the sum individual ones, with the time I have available to me.

Answer 1

You should use the Dissolve tool. By default, this will create multipart features where your stands are separated by a small gap. This is what you want for your case. You can elect to dissolve everything or by a common attribute. After dissolving you will get an accurate areas calculation. Dissolve is available in all licence types.

Aggregate Polygons will not give you the result you need because it will add additional area between the individual polygons, thus effectively increasing the area of woodland. It's a great tool but I'm not convinced it is appropriate for your use-case. For the same reason Convex Hull is also inappropriate.

Answer 2

An ArcInfo or Advanced license is what you will need to use Aggregate Polygons which I think is designed for your situation. I don't think 9.3 will be as useful as 10.0 for this but 10.1/10.2 would be better.