[Proj] Re: Problem overlaying georeferenced images in Google, Maps(projection problem!)

Jorge jorge.arevalo at gmail.com
Wed Jun 4 09:00:40 EDT 2008

> The Mercator-based coordinate system used by Google Maps can be thought of in 
> two ways. 
> 1) It uses a sphere with radius=6378137.0 m. Sent to Mercator, hopefully, the 
> code is smart enough to use the spherical equations.
> 2) It uses WGS84 with spherical Mercator equations. 
> No ellipsoid-to-sphere conversion should take place. Data that's really on WGS84 
> is assumed to be on the major auxiliary sphere (R=6378137.0m) and just projected 
> using the spherical math. This means that you can't create a custom 3 or 7 
> parameter datum transformation to convert directly to the sphere (because the 
> latitude values are converted to the sphere).
> So you might try transforming your data from ED50 (or other datum) to WSG84 
> first. Then project those coords to the Google Mercator.
> Melita
> ESRI, Inc.

Hi Melita

If I understand you, I must transform my MIF/MID files (UTM projection, ED50 datum) in a couple of MIF/MID files but with datum
WGS84 (maybe using Mapinfo) and then, create the raster file based on these new MIF/MID files

The problem is that, in the MIF/MID files, I have rectangles of 250x250m, and if I change the data from ED50 to WGS84, the
rectangles overlap, as you can see in this screenshots:


So, create a raster based on this data is difficult, because I can't sample with the freq needed (one sample each 250 meters)
without distortion. Do you think so?

Mmmm... but I think that the solution is in this way: transform the input data from ED50 datum to WGS84 datum, and then, reproject
to mercator...

Thanks a lot!

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