# Generalisation Strategies for Building Outlines

I have a dataset of building footprints vectorised from a raster dataset. The dataset is currently just a vector representation of the raster data. I would like a polygon dataset that only has vertices at the corners of the building. I have attempted to use the Simplify command in PostGIS with limited success. Is there a better way?

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There are many algorithms dedicated to building simplification. You may have a look at this website for an overview. See also this question. You could use:

• Building outline simplification: It consist in recursively deleting too short edges of the outline and lengthening the neighbor edges. Example:

• Smallest surrounding rectangle algorithm: It consist in replacing the building geometry by the smallest surrounding rectangle. This rectangle can also be scaled to the building initial area. Example:

• Squaring algorithm: It consists in applying a light rotation to edges to make the corners perfectly squared. Example:

These transformations may be enough for your need. It can be interesting to combine them.

I am almost 100% sure none of these algorithms are implemented in PostGIS. Some are implemented in the Opencarto java library I am developing. Let me know if you want to test it!

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Thanks, it would be good to test Opencarto. – Matthew Snape Nov 24 '10 at 12:47
``````geometry ST_SimplifyPreserveTopology(geometry geomA, float tolerance);
``````

Will avoid creating derived geometries (polygons in particular) that are invalid http://www.postgis.org/documentation/manual-1.5SVN/ST_SimplifyPreserveTopology.html

should be a better output than Simplify

check your geometry first and after with `ST_IsSimple(geometry geomA);`

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Just a thought -- I wonder if something like this might do the trick:

"If we buffer by a large amount, then reverse the buffer by the same amount, we’ll get something that has a similar shape to the original, but without the crinkly bits."

http://blog.opengeo.org/2010/11/22/removing-complexities/

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that's an interesting approach and it seems relatively effective. Thanks for the pointer, it's something I'll be able to use. Your answer would be better if it contained a simple phrase containing the core idea being referenced and wasn't just a link. – matt wilkie Jan 12 '11 at 0:51

etgeo-wizards has a routine to do this. Ianko

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