[postgis-users] Degrees to meters

[marco vieira]

Kevin:
 You can treat "geographic" with two aprouches:
    Geographic is all coordinates projected over the earth (in any units)
    Geographic is a alias to latlon "projection" (in fact Plate Careé
projection when displayed). It is more used in some countries.
 Both treat projections, not Data (plural of Datum).You told about two
different Datum (origin+ellipsoid) in the same projection and that was the
conversion made.

If  you want your map in different Datum (4326 represents WGS84 datum in
latlong projection, then I supose you are talking about WGS-84) in a metric
projection you must:
  (A) Make a datum (ellipsoid + origin) conversion
  (B) Project your map in a cartesian projection using a metric system.

This can be done in one step but in fact it is two.
Step B depends of many factors (shape of your map,  type of distortion
desired,  position on the globe, etc).
If you want to display this points you must considere this things to choose
a projection but if you want to compare this data with another map (like
calculate distances from one point table to another) you must convert to the
same projection that another table was. To calculate distance from two
tables in latlon you can use distance_sphere function to take answer in
meters.
Maybe the best way to choose the right way is to give more information about
your objective to the list.
Regards.

2006/6/14, Kevin martins <martins506 at hotmail.com>:
>
>
>
> Yes it's just like you said, but how can I get a data in longlat in
> a  Geographic Coords  to a data in meters in another  Geographic Coords ?
>
>
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-- 
Marco Vieira
+55 21 9499-6800
e-mail: maovieira at gmail.com
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