# [postgis-users] Degrees to meters

Kevin martins martins506 at hotmail.com
Sun Jun 18 20:30:38 PDT 2006

```Hi folks,

Well ,a few days ago i had a problem with coordinates projeted on maps.Well, I still have that problem:

I have a point (-7.48081E,39.821566N), in WGS-84, and I want it to be in lisbon datum with the units in meters.
I have created a new datum-> "+proj=tmerc +lat_0=39.6666667 +lon_0=-8.131905 +k=1.000000 +x_0=0  +y_0=0 +a=6378388 +b=6356911.9 +units=m +no_defs ".
Note that point of reference of lisbon is =-8.131905 E,39.6666667 N and a and b refers to the ellipsoid of Hayford.
Well I think that this should work but the problem is that when i do the convertion the coordinates of the point is:X->56542.734,Y->17497.474;
but it should be something like:X->55838.227,Y->17741.34;

What am I doing wrong? Could you help me pplllllllllllleeeeeeeeese?
Cheers
Kevin

-----------------------------------------------
> Date: Wed, 14 Jun 2006 16:45:35 -0300
> From: maovieira at gmail.com
> To: postgis-users at postgis.refractions.net
> Subject: Re: Re: Re: [postgis-users] Degrees to meters
>
> 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.
> 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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