[GRASS-user] projection issue

[Glynn Clements]

Vincent Bain wrote:

> I believed the nadgrid was a shifting matrix applying to ellipsoid
> center (in cartesian geocentric coordinates). Does this mean
> +nadgrids=@null implicitly tells cs2cs : keep the from-ellipsoid (ie
> ignore the to-sphere), and shift it (of 0,0,0) ?

+nadgrids=@null tells it not to perform any datum transformation.

The default behaviour is to perform a matrix-based datum
transformation based upon the source and destination datums.

The nadgrids parameter overrides this. Normally, it is used to specify
a data file containing the datum-transformed positions of a set of
points; this allows for a more accurate transformation than a
matrix-based transformation.

nadgrids=@null is a "hack" which allows the datum transformation to be
suppressed entirely. This is required when the lat/lon coordinates are
based upon an ellipsoid but the cartographic transformation is based
upon a sphere, as is the case for the Google Earth projection.

Without that parameter, whatever ellipsoid was used for the
cartographic transformation would also be used for the datum
transformation.

-- 
Glynn Clements <glynn at gclements.plus.com>