[Gdal-dev] Understanding GEOS projected space

[Enrico Zini]

On Mon, Nov 19, 2007 at 12:21:40PM +0100, Bas Retsios wrote:

> Note that due to the large area covered by this projection (an entire 
> earth's hemisphere), the "meters" are only correct near the nadir-point.
> As an example, a box with bounds (xMin=-3500000, xMax=3500000, 
> yMin=-3500000, yMax=3500000) would roughly include the Arfican continent 
> and a great part of the atlantic ocean.

So we're talking about a virtual kind of metre that is 1 metre long at
the nadir point but gets smaller as we move further from it.

The implementation I had (which followed directly from Eumetsat
documentation) used to work with the scaling function defined at section
"4.4.4 Scaling Function"[1],

Switching to proj I need to use ColumnDirGridStep and
LineDirGridStep[2], which I didn't see mentioned in Eumetsat's
documentation about georeferencing.  I hope that it's just the same
quantity scaled differently, and that the precision of georeferentiation
that I get in the end is the same.

Now, I need to handle some NetCDF based formats that store this
georeferentiation information in terms of CFAC and LFAC, and moving to
GDAL I'd need to rebuild ColumnDirGridStep and LineDirGridStep out of
those.

Is there a formula (it should be nothing more than a simple scaling
constant, if I get it right) that allows me to convert back and forth
from ColumnDirGridStep to CFAC (likewise for line)?


Ciao,

Enrico


[1]
4.4.4 Scaling Function

  The scaling function provides a linear relation between the
  intermediate coordinates (x,y) and the image coordinates (c,l).

  The definition is as follows:

    c = COFF + nint(x * 2^-16 * CFAC)
    l = LOFF + nint(y * 2^-16 * LFAC)

  Herein, "nint" denotes a nearest integer rounding of the real
  argument. COFF, CFAC, LOFF, LFAC are the (integer) scaling
  coefficients provided with the image navigation record. Each of the
  coefficients may have an integer value between -231 and +231-1.
[2]
  LineDirGridStep gives the grid step size in km SSP in the line
  direction. Default value is 3km for VIS and IR, and 1km for HRV. The
  on-ground grid step size of 3 km at the SSP represents an instrument
  scan step of 251.53 microrad divided by 3.
  
  ColumnDirGridStep gives the grid step size in km SSP in the column
  direction. Default value as for `LineDirGridStep'.
-- 
GPG key: 1024D/797EBFAB 2000-12-05 Enrico Zini <enrico at debian.org>
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