# [Proj] Help with a custom projection that is Transverse Mercator with Affine post-process

Hermann Peifer peifer at gmx.eu
Tue Jan 29 08:54:24 EST 2013

```About the meaning of the 6 parameters for the affine transformation, a
random Internet search gave me this definition:

-------------
The six values A, B, C, D, E, F have to satisfy this formula:

AX + BY + C = X'
DX + EY + F = Y'

testdrive.mapinfo.com/techsupp/miprod.nsf/kbase_by_product/0B57EA4620B5EBF285256A010082A16A
-------------

Hermann

On 2013-01-29 7:02, Mikael Rittri wrote:
> Hello James.
>
> Just a partial answer, I am afraid:
>
> I don’t think you can add an affine post-processing to a Proj.4 CRS
> definition.
>
> For your example, the interpretation of the MapInfo CoordSys clause
> should be
>
> “CoordSys Earth Projection”, // Just a name (probably the wrong one in
> this case)
>
> 8,                  // The projection type, in this case Transverse Mercator
>
> 33,                // The geodetic datum, in this case GRS80 (treat it
> like WGS84)
>
> 7,                  // length unit, in this case meter
>
> 117,               // central longitude
>
> 0,                  // central latitude
>
> 0.9996,          // scale factor
>
> 500000,         // false easting
>
> 10000000,      // false northing
>
> 7,                  // length unit for affine transformation, also meters,
>
> 0.890953, -0.455062, 2903977.24, 0.455063, 0.890954, -6919253.68,
> // affine parameters A, B, C, D, E, F
>
> -100000, -100000, 100000, 100000               // bounds
>
> I think the basic CRS in this example can be expressed in Proj.4 as
>
> +init=epsg:28350
>
> which gives GDA94 / MGA zone 50, but I think you must implement the
> affine transformation yourself.
>
> I got the information from
> http://www.sgsi.com/dnld/MapInfoPro_UserGuide_Current.pdf
>
> but I didn’t understand the meaning of the affine parameters.
>
> And I think the 8 ought to have been 3008, maybe…
>
> Best regards,
>
> Mikael Rittri
>
> Carmenta
>
> Sweden
>
> http://www.carmenta.com
>
>
>
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>

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