[postgis-users] Degrees to meters

[David William Bitner]

You are still trying to convert from a sphere to a sphere... on a sphere,
you are always going to have angular coordinates like lat, lon.  As we've
said before, you need to find a projected coordinate system -- that is one
where the sphere has been converted to a flat surface.  The three concepts
which you really need to gain a basic understanding of before continuing on
are datum, spheroid/ellipsoid/geooid, and projection.  Without understanding
these in a basic way, you will continue to be lost.

http://en.wikipedia.org/wiki/Map_projection
http://en.wikipedia.org/wiki/Datum

Somebody previously sent some links to some other resources as well.

We really would like to help you, but you are really getting into an area
where you really should (perhaps I'm too much of a geographer) understand
what is going on.

Please don't be disheartened.  Stepping back for a little bit here to do
some extra homework will really help you as you do work in the geography
world.

David



On 6/18/06, Kevin martins <martins506 at hotmail.com> wrote:
>
> 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.
> > 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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-- 
************************************
David William Bitner
bitner at gyttja.org
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