[Proj] Coordinate transformation from meters to longitude/latitude

[Frank Warmerdam]

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,
-- 
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