[OSM-dev] OSM data import using osm2pgsql... what's next.

[Paul Norman]

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