# [Proj] Custom projection wildly inaccurate

Thy, Kristian Kristian.Thy at atkinsglobal.com
Mon Nov 20 04:13:00 EST 2006

```From: Oscar van Vlijmen
> Another problem is that System 34 is not coupled to any
> projection. I have heard of a transverse mercator approximation,
> but an orthographic?

Now that you mention it, this seems extremely strange. But I just went
with what was in the official .prj files. You wouldn't happen to have
any information about the transverse mercator approximation?

> The Danish use a 11th (13th?) degree polynomial to convert System 34
> coordinates to UTM. I haven't seen this polynomial roaming on
> the internet, so I cannot comment on the accuracy of approximations.

I had them in my notes from a geodesy course in uni, but they have
unfortunately been mislaid. At any rate, I am not sure of the copyright
implications of publishing these polynomials from the tech note I got
them in.

However, using the coordinate transformer thingy on valdemar.kms.dk,
I get the following results for the top left corner of a TIFF orthophoto
I have:

System 34 Sjælland (Zealand): (-86714.42, 162554.98)
Transformed to lon/lat (WGS84): (12d20'42.97906"E, 55d52'40.50251"N)

(The negative X is due to the system being west-oriented, but for ease
of use a "normal" east-oriented X-axis is used with all X coords then
being negative. These coordinates are confirmed using KMS's KMSTrans
program, a desktop application for conversion of coordinates.)

Using cs2cs with the orthographic projection inferred from the .prj file:

C:\>cs2cs -E +proj=ortho +lat_0=55.29235 +lon_0=12.105661 +x_0=-100000.0 +y_0=100000.0 +to +proj=latlong +datum=WGS84 cs.txt
-86714.420 162554.980   12d19'5.829"E   56d1'56.147"N 14670.382

Clearly, that projection doesn't fit the bill. But when I pull the same
TIFF into ArcMap and examine the spatial reference for it, this is what
I see:

The orthophoto at bottom right is the original in System 34, and also the
one for which the SRS is shown in the dialog. At top right you see the
same orthophoto reprojected using GDAL with what I thought were the
parameters shown in the .prj file.

Any help in unravelling this mystery is much appreciated. I suspect that
the problem lies in my interpretation of the ArcMap .prj file.

best regards,
Kristian Thy

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