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  • Getting intersections the faster way

    calendar Mar 14, 2014 tips · 2 min read  Regina Obe · newbie geometry ST_Intersects ST_Intersection ST_CoveredBy

    Doing an ST_Intersection is much slower than relation checks such as ST_Intersects , ST_CoveredBy, and , ST_Within . In many situations you know the intersection of 2 geometries without actually computing an intersection. In these cases, you can skip the costly ST_Intersection call. Cases like this:

    1. geometry a is …

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  • pgAdmin shows no data

    calendar Oct 5, 2013 tips · 2 min read  Regina Obe · newbie geometry geography raster

    Every one or two months, someone asks on the PostGIS users list usually somewhat panicked, My geometry is missing when I look in pgAdmin III

    This is documented in the PostGIS Frequently Asked questions section of the manual I tried to use PgAdmin to view my geometry column and it is blank, what gives?

    Since this …


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  • ST_Transform and ST_SetSRID: To project or not to project?

    calendar Aug 30, 2013 tips · 2 min read  Regina Obe · newbie geometry ST_SetSRID ST_Transform

    People often get confused between the ST_Transform and ST_SetSRID functions.

    1. ST_SetSRID doesn't change the coordinates but adds meta data to state what spatial reference system the coordinate actually are. If you stamped your WGS 84 long lat data as a meter based projection. Guess what? Its still long lat. A spade …


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  • Distance proximity: Use ST_DWithin not Distance

    calendar Aug 26, 2013 tips · 1 min read  Regina Obe · newbie geometry geography ST_DWithin

    If you have a problem that involves finding the things within X distance of other things or finding what things have nothing within X distance do not use ST_Distance for filtering and also do not try to use ST_Intersects + ST_Buffer. Use ST_DWithin instead. Why?

    1. ST_DWithin can use an index and ST_Distance can not …


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  • Is it Lon / Lat or Lat / Lon?

    calendar Aug 18, 2013 tips · 1 min read  Regina Obe · newbie geometry geography ST_FlipCoordinates

    In mapping frameworks spatial coordinates are often in order of latitude and longitude. In spatial databases spatial coordinates are in x = longitude, and y = latitude.

    If you accidentally loaded data as lat,lon, you can always fix the mistake by using the ST_FlipCoordinates function introduced in PostGIS 2.0.


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