[postgis-users] maps google problem with SRID

[lisek lichu]

 Hello Simon,
>
> I will try to answer your second question:
>
> The result PostGIS calculates is perfectly right. This is, because 900913
> is a Mercator projection. One characteristic of this projection is, that it
> is not equidistant. The scale is only valid along the equator. Along the
> meridians, the map gets distorted towards the poles. PostGIS's
> transform-function will output the coordinate, but this has nothing to do
> with the distance from the equator on the underlying spheroid. The valid
> coordinates for the Mercator projection range from -PI*r to PI*r for
> longitudes and from negative infinite to positive infinite for latitudes,
> i.e. the y-coordinate for the north pole in a Mercator projection is
> positive infinite. Google only uses a coordinate range of approximately
> +/-20000000 in both directions (North-South and Est-West). This corresponds
> to 180°W to 180°E, but only about 85°S to 85°N on the sphere. If you would
> like to read a bit more, I suggest, that you search the Internet for
> "Mercator projection". Wikipedia would probably be a good start.
>

Thanks for that :) I was thinking that it is like that


>
> Regarding your forth question:
>
> select ST_length_spheroid(GeomFromText('LINESTRING(0 45,90
> 45)',4326),'SPHEROID["WGS 84",6378137,298.257223563]');
> i get 6690232.93269468 so it is smaller as should be
>
> The distance is correct. Why do you think it is to small?
>

Haha maybe it is beacuse of my English :) it was not a question I write that
this calculation is correct :P

when i wrote: "so it is smaller as should be" I meant "So it is smaller so
it is correct"

:)

thanks
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20081127/ea90e478/attachment.html>