[postgis-users] Comparing two Building Footprint Tables

David Siegel david.siegel at artcom.de
Sat Aug 23 03:51:44 PDT 2014

Hi Clifford

just a quick saturday morning thought: One approach is to use a hash function like SHA256. To get started I’d try something like this:

1. For each footprint get a text representation of the geometry
2. Compute and store the SHA256[1] hash of the text representation
3. Find those hash values that are unique to the new footprint table

Note that you can influence the result by changing the text representation. Using a sorted list of just the coordinates for example ignores any changes in topology, &c. To speed things up just add an index on the hash column.



[1] http://www.postgresql.org/docs/8.3/static/pgcrypto.html

On 23.08.2014, at 00:49, Clifford Snow <clifford at snowandsnow.us> wrote:

> The city of Seattle published a new shapefile with the latest building footprints. The previous file was from 2009. I'd like to find which outlines have changed. While I can see the differences using QGIS, what I need is a list of changed or new building footprints. I'm struggling where to start. There are about 300K footprints. The city did not retain any unique ids from the previous file to the new. 
> Any suggestions would be appreciated.
> Thanks,
> Clifford
> -- 
> @osm_seattle
> osm_seattle.snowandsnow.us
> OpenStreetMap: Maps with a human touch
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