# [Proj] proj.4 and Blue Marble Geographics Geographic Calculator question

Frank Warmerdam warmerdam at pobox.com
Mon Jun 21 10:34:51 EDT 2004

```Konstantin Tokar wrote:
> How to execute transformation from latlon to Gauss-Kruger
> GK-PULKOVO, " Gauss-Kruger (Pulkovo 1942) Coordinate Systems ", PULKOVO
> 1942, TM
> GK-12, " GK Zone 12 ", (Pulkovo 1942), METERS, TM, 69, 0, 1, 12500000,
> 0, " ", 28412
> (so she is called in Blue Marble Geographics Geographic Calculator)
> Geographic Calculator RETURNS coordinates in meters:
>
>      lon          lat
> latlon      45          51
> meters      10826898.08      5931828.02
>
>
> proj.exe +proj=tmerc +lat_0=0 +lon_0=69 +k=1.000000 +x_0=12500000 +y_0=0
> +ellps=krass +units=m +no_defs
>
> Returns
>
> lon          lat
> 45          51
> 10826825.65      5931859.27
>
> That is not that. What it is necessary to correct?

Konstantin,

You are reprojecting a poin at longitude 45, which is 24 degrees away from
the central meridian of the transverse mercator projection in question.  I
don't think it is appropriate to use transverse mercator this far from
the central meridian and the error accumulation may vary substantially in
different packages.

Try comparing a point much closer to the central meridian and see if it
agrees with Blue Marble.

eg.

68 51
12429801.89     5652661.14

In general, if you use different packages to reproject transverse mercator data
a long ways away from the central meridian you are likely to get substantially
error, and differing errors in each package.

PS. This mailing list is currently in transition.  It should be properly
populated in a day or two.  In the meantime only two or three people are
currently on the list.

Best regards,
--
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent

```