Dr Mike J Smith
Kingston University

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Creating Polygons

Sat, 14 Apr 2007

I was sent some polygons (coastline) of a group of islands recently that Ken was using to produce a map for a joint project we have. The detail of the coastline was very good so we wanted to use this dataset however it was very odd!! On close inspection each island had an exterior buffer to (I think!) 1km. It was these buffers that were the polygons. Clearly we needed to get rid of the buffers and retain the islands as polygons (not least because we wanted to use to polygons to clip some other datasets we had).

So how to go about doing this? Well the first step was to convert the polygons to polylines, then go around in detail (and select)and delete all lines that were related to the buffers. This would then leave just the coastline. This was a fairly pain staking process as the dataset is quite large. Once complete, all we needed to do was rebuild the polyline layer as a polygon layer (doing a CLEAN and BUILD). And, it didn't work.... At this point I asked Ken to work with the dataset (because I'm on leave!). He spent ALOT hours looking at the islands in detail and found a couple of spurious lines related to the buffer, but nothing else. There were, however, several very small gaps in the polylines (10-20m in size). This will stop the layer being created as polygons because there are, well, gaps! This required some further editing and merging of lines and conversion to polygon which... didn't work. Thankfully Ken has got some hair, so was able to pull some out and then mull over it a bit further.

And the conclusion that he came to was that the standard ArcGIS tools wouldn't work in our situation, possibly because the algorithms they employ are quite strict. A trawl of the internet brought him back to, you've guessed it, one of the BIG THREE add-on tool sets (mentioned earlier), in this case ET GeoTools. In particular the clean polyline and polyline to polygon tools.

Strange how most ArcGIS problems seem to end up with solutions not from ESRI but one of the BIG THREE!!!

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