[Proj] Coordinate transformation from meters to longitude/latitude

Ed McNierney ed at topozone.com
Tue Jan 23 12:24:27 EST 2007

Yuriy -

Frank's quite correct, and I think the source of the problem is some very
strange input data.

You're describing a projection centered on the Equator at 40 degrees East
longitude.  Your input point is 7 million meters east of 40 degrees East and
6 million meters north of the Equator, as measured in your projected
coordinate system.  That's a very strange place to be in a transverse
Mercator projection.

Do you have a description of where (on the Earth) you think that input point
is supposed to be?

     - Ed

Ed McNierney
President and Chief Mapmaker
Maps a la carte, Inc. / TopoZone.com
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: (978) 251-4242
Fax: (978) 251-1396
ed at topozone.com

> From: Frank Warmerdam <warmerdam at pobox.com>
> Reply-To: "PROJ.4 and general Projections Discussions"
> <proj at lists.maptools.org>
> Date: Tue, 23 Jan 2007 11:27:07 -0500
> To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
> Subject: Re: [Proj] Coordinate transformation from meters to
> longitude/latitude
> Yuriy Rusinov wrote:
>> Hello !
>> I want to transform some cartesian coordinates from its own projection
>> invproj +proj=tmerc +lat_0=0 +lon_0=38.99999952571766 +k=1.000000 +x_0=0
>> +y_0=0 +ellps=WGS84 +units=m +no_defs
>> 7.41094e+06 6.16614e+06
>> Results are
>> 135d51'17.113"W 234d40'40.974"N
>> These data exceeds latitude limits and no errors, no asterisks in
>> output. Is this error and where it can be contains ?  Any ideas ? Thanks
>> in advance.
> Yuriy,
> I see the same result.  I must confess to not understanding the math
> involved, but Transverse Mercator is a projection without a well
> defined boundary at which it becomes invalid.  Instead as you go further
> and further from the central meridian things become more distorted and
> bizarre effects accumulate.  And 7 million meters east of the central
> meridian is a *long* ways.
> I think the lesson is that you can't just throw locations several million
> meters from the central meridian into TM and expect meaningful results.
> But worse, the projection itself has no way of indicating it's reduced
> meaningfulness.
> Hopefully Gerald will give a less "black box" oriented answer.
> Best regards,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | President OSGeo, http://osgeo.org
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