[postgis-users] Postgis topology help

pcreso at pcreso.com pcreso at pcreso.com
Sun Apr 1 00:26:27 PDT 2012

I'm after some pointers on how I might use topology in this case:

We regularly undertake random stratified two phase trawl surveys for fisheries stock assessments. 

This requires the survey area to be divided into arbitrary strata, with sample sites randomly defined for each strata. Generally these strata are defined from:

1. depth contours
2. arbitrary lines between contours (generally following a lat or long)
3. areas of land or other excluded areas which are excluded from any strata overlapping them.

My current Postgis model defines strata outer boundaries as polygons, holes as polygons, & instantiates strata as a set of one or more outer boundaries less any holes.

This has the usual issues of slivers & overlaps between strata as common boundaries are stored twice, & as not always identical.

If I used topologies, I could store the constituent lines (contours, etc) and assemble the outer boundaries from these, to give properly normalised polygons.

I have seen the docs & Strk's presentation, but these to not (to me anyway) list the steps to follow to do something like this, & the Postgis functions to carry out those steps.

I figure the steps should be:

load contour linestrings into a topology table
- these will never intersect/overlap by definition

load other dividing lines - with extensions to ensure they do NOT need to snap to make nodes, as this can move then off the defining lat/lon, so they always overlap the polygons

generate the outer strata boundaries & export to geometries
- add polygon attribute data based on the constituent linestrings 

remove any overlapping holes to generate the actual strata

Any suggestions as to how to go about this?


   Brent Wood
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