May 15, 2008

Donuts, Whole and Holes

My favorite donut is a french crueler. I like the thought of apple fritters (often the size of the average US county). I am seldom ever glad afterwards that I ate one. I like chocolate covered cream filled donuts. For whatever reason, I am convinced I could easily eat an entire dozen Krispy Kreme donuts and still be lifting the lid of a closed box to see if there are any more. Maple bars are pretty good too. I’m not sure I understand what the hard cake donut or sprinkles have to do with anything… no accounting for taste I guess. I think the purpose of the sprinkle-donut’s existence, is the same as the black licorice gumball, it exists to mock anyone who is too late to choose something else.

GIS Polygons come in various forms. The solid polygon, like a maple bar or apple fritter, the polygon/donut with one or more holes, a polygon surrounding another polygon (filled donut), and a collection of polygons considered one polygon (One Krispy Kreme just doesn’t seem like enough). GIS Polygons can also be overlapping and non-contiguous.

In CAD, SOLIDS and HATCHES are the real polygons; closed POLYLINES are really linear boundaries, not the polygon itself, but still interpreted as polygons by ArcGIS. It is more common in CAD not to worry about closed polylines at all, and just draw the linear boundary network that can be visually inferred as polygons.

To create donuts and multipart polygons (like the Hawaiian Islands as one feature) in CAD, which are in turn interpreted as such in GIS I can create an AutoCAD block of the multiple entities like closed POLYLINE, CIRCLES, etc... ArcGIS consider all of the sub-geometries of CAD blocks to be a single feature with multiple parts. Multiple points, lines or polygon geometries can still be a single entity in a CAD block insert, which are interpreted as mulit-part features by ArcGIS. The CAD BLOCK INSERT is also considered a single point whose geometry is the insertion point of the BLOCK INSERT entity. I typically use blocks as symbols rather than as primary geometry, but this is a useful exception. Both are valid uses of blocks in CAD.

A way to generate donuts from standard concentric CAD polygons is to use the ArcGIS FEATURE TO POLYGON tool. It will cut-out the holes making both the donut and the hole a new polygon. This is the same tool that can create polygons from a set of lines. The tool will break multiple polygons into single polygons, so a combination of using COPY FEATURES and FEATURE TO POLYGON might be required to get my desired result.


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