Adrian Custer a écrit :
In a seven parameter transform, from CRS1 to CRS2, there are three
'shift' parameters in x,y,z, three 'rotate' parameters, and one
'scaling' parameter.
Doesn't this implicitly assume that all three axes in each CRS are using
the same unit? (NB obviously the two CRS's can have different units from
each other.)
I can see how scaling solves the difference between the units in the two
CRS's but it seems that if CRS1 has all its axes in one unit but CRS2
has x,y in one unit and z in another then the transform has too few
parameters. I'll grant it's probably a rare issue and therefore academic
from that point of view but I wondered anyhow.
The seven parameter transform is always applied on a geocentric CRS: a cartesian CS with the origin
at Earth's center and (x,y,z) axis in the same units (usually meters). It is never applied on a
geographic or projected CRS.
The seven parameter transform is used for datum shift. It is not used for unit conversions. Unit
conversions are performed by an affine transform instead. Geotools provides a helper static method
for that:
AbstractCS.swapAndScaleAxis
which is used quite intensively in coordinate transformations by Geotools.
Martin.
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