On 8/2/2014 1:52 AM, Christian Quest wrote:
It is possible to do on the fly reprojection with postgis using
ST_Transform... but this may cause a lot of calculations.
It your queries use a lot of distance calculations, you'll want to do
them with real "spherical" distances, and not the projected one that
is not linear. In web mercator the "distance" are pixel based, and not
linear. The same "distance" will be much small at the equator compared
to iceland or south africa...
The difference between a spherical distance and a spheroid distance is
really only significant for large distances.
There are a few ways to handle distance calculations, depending on your
needs
- If dealing with a small area, use a suitable projection like UTM or a
state plane for the entire database
- Use ST_Distance_Sphere or ST_Distance_Spheroid. If doing this,
consider importing in EPSG:4326
- Convert geographies to geometries
- Make use of the fact that Mercator is conformal and web Mercator is
approximately conformal. This means that local to a point, the distance
in mercator is a scalar multiplier of the true distance
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