lucasmation

Automatically Identify isolated small areas (polygons)

Discussion created by lucasmation on Mar 26, 2013
Latest reply on Mar 28, 2013 by mdenil
I´m working on a small area shape-fille (Setores Censitários for Brazil). Using ArcMap 9.2.
I need to identify which small areas (lines in the dbf) correspond to isolated poligons, i.e., polygons that are islands off the coast.   
There are also some small areas which are "multipart" (made from disjoint polygons) and one of the parts is off the coast. I also need to identify those.

See attached image.
Any ideas on how I can solve this?
The file has 320,000 small areas so I need an automated process.

A solutions that I imagined was:
1) dissolve all small areas borders, with the "single part" option. This creates a file with 539 lines (one for continental Brazil and the other 538 for islands of the coast).
the problem is that by doing this I loose the details on the small area code (and I would like to retain tha information for the islands).
(obs: trying to retain that information with dissolve' s {field {statistic type} wont work because, the statistics will be for the group formed by the dissolve_field, and not withing single parts of that.
2) in the dissolved shape-file, add a field  "is_island" indicating 1 for the islands and 0 for the continental part.
3) From the original small area file, do a "spacial join" with the dissolved file, pulling information from the "is_island" field.

I think this could work in principle. but step 3 crashes my ArcMap.

best regards
Lucas

PS: the files I´m using can be downloaded from the Brazilian statistical office here:
http://downloads.ibge.gov.br/downloads_geociencias.htm > malhas digitais > censo_2010 > setores_censitarios > choose "ma.zip" for example (=state of Maranhão)

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