# Identifying gaps within polygons, and excluding islands in ArcMap

I have a digitised map of Ireland, consisting of 40 features where each feature is a different polygon representing an Irish constituency.

I have created a topology to help me identify gaps and overlaps in the map. The overlaps works fine, but unfortunately, Ireland consists of more than 800 islands (ironically). This means it comes up with a lot of gap errors (typically around 870), since these island polygons are not surrounded by another polygons.

The gaps I do want to find and correct are gaps within the mainland, between polygons, and donut holes. This means I have to go through 800+ errors individually to possibly find an actual error I want to correct.

I was wondering if there was another way of doing this, so I can ignore the island polygons, and find real gaps and errors within the mainland? Trudging through nearly 900 errors to find one actual error is tough work!

• Are lakes covered by constituency polygons? – Kirk Kuykendall Nov 29 '17 at 17:18
• What license level of ArcMap do you have? With the Advanced level you could convert your polygons to lines and points (inside), then convert the lines into polygons with the points as the attributes/label features. All the gaps should now be polygons with no attributes and you can work on or using them. If you have Workstation installed you could just convert your polygon layer to a coverage and do the same. Overlaps will also become polygons. In the first approach they may have the point, in the coverage they will randomly get the attributes of one poly. – johns Nov 29 '17 at 17:53
• Split datasets in 2 using gis.stackexchange.com/questions/152554/… and proceed with mainland only. – FelixIP Nov 29 '17 at 18:12
• Perhaps convert them to single part polygons if they are multipart. – FelixIP Nov 29 '17 at 18:15
• @KirkKuykendall Yes, my mainland Ireland map is a solid collection of polygons without gaps (well that's the intention). Lakes are counted as land. – al_sweets Nov 30 '17 at 14:26

## 1 Answer

You do not say whether you have an outline polygon of mainland Ireland excluding all those islands you don't want; or whether your polygons are in separate layers or all in one layer.

I was thinking you could Select by Location, using an outline of mainland Ireland to isolate the features you are interested in and excluding the islands, then export that selection to a separate layer (i.e. so you would now have a layer that was the subset of just the polygons you were interested in that you could identify your errors in).

Info on select by location: http://desktop.arcgis.com/en/arcmap/10.3/map/working-with-layers/using-select-by-location.htm

• Unfortunately, mainland Ireland is not one polygon outline, it is made up of many smaller adjacent polygons. – al_sweets Nov 30 '17 at 14:25