[Gdal-dev] Affine Transformation and Calibration Equation for Maptech/BSB format

[Frank Warmerdam]

On 9/27/07, marcello ferrero <marcello_ferrero at yahoo.com> wrote:
> I believe the parameters for a more precise transformation are already in
> the BSB header in the WBX,WPY and PWX, PWY fields,
> but I'm not sure what are the correct data to use.
> For example if we consider the chart file 11013_1.KAP     (STRAITS OF
> FLORIDA AND APPROACHES)
> the following values are in the header.
>
> WPX/3,74057.63283,914.1573321,-4.457688988e-006,0.9314526304,4.199727384e-007
>     8.909433511e-007,0.003888412047,4.049230257e-009,4.656999088e-009
>     -7.344637416e-009
> WPY/3,24849.77398,-0.0001719938458,-841.3888082,-2.236403169e-006
>     -4.23802978e-007,0.5645817982,-9.615822208e-009,-2.118680351e-009
>     1.85245272e-009,-0.05788953807
> PWX/3,-85.82846924,0.001190193579,1.242062009e-015,1.177436646e-010
>     4.828087799e-020,-4.158931771e-019,-7.813843316e-015,-1.938704561e-024
>     -3.205324874e-024,3.339741723e-023
> PWY/3,28.48799091,4.631711681e-016,-0.001052175742,-7.983285528e-020
>     -7.030373462e-020,-5.245662991e-009,4.239300639e-024,1.848231681e-024
>
> What are the parameters between them to take into account?
> Also, what kind of trasformation is this one? Is it a sort of calibration
> equations that uses also the ref points and errors contained in the same BSB
> file?

Marcello,

I do not know for sure, but I presume these parameters are for
at least a 2nd order polynomial.  I have made only a very
minor effort to deduce these details but was not successful.

Internally GDAL can compute 2nd order polynomial warp
equations from the reference points and this is what I
generally encourage people to use (via gdalwarp) to
convert the images to a nice "north up" image.  But, in
fact, I presume the files are actually normally in a mercator
projection and that we would be even better off to work out
the details of that and then use proper reprojection to get
to other coordinate systems.

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