[OSRS-PROJ] cs2cs and datum shift

Clifford J Mugnier cjmce at lsu.edu
Fri Aug 23 12:46:20 EDT 2002


For the inverse, you have to change the sign of the components for the
translations AND for the rotations.  The scale factor is one minus the
reciprocal.  Once you do that, then you should get sub-millimeter precision
in duplicating what you started with.

Also, note that there are two different conventions for the sense of the
rotations.  The correct way is that used by NIMA as defining the WGS84
Datum (Counter-clockwise rotations as in vector mechanics and physics).
The ITRF people in Paris confused the math and they use clockwise
rotations.  What Firenze uses depends on whether they use American GPS
software or French/Swiss software ...

So when you diddle with the parameters you were sold; you SHOULD have been
provided at least ONE test point to validate the math model you choose to
use (PROJ4), versus what the results should be.  The parameters are
intended to be used from Latitude, Longitude, ellipsoid heights to
Latitude, Longitude, ellipsoid heights.  The relation with elevation is
dependant on geoid models (at both ends of the transform).

Make sense?  I hope so; if not, continue asking questions.


Clifford J. Mugnier (cjmce at LSU.edu)
Chief of Geodesy
Department of Civil Engineering
Baton Rouge, LA  70803
Voice and Facsimile:  (225) 578-8536
Pager:              1-(888) 365-5180

Dear Clifford,

  thanks for your answer but now I'm even more confused. I got the 7
parameters from a sheet issued and SOLD by IGM (istituto Geografico
Militare) where they tell me that Abbazia Pisani is located at
phi, lambda (geographic, datum Roma40) = Norhing, Easting (Gauss-Boaga)
= phi, lambda (geographic, WGS84) = Norhing, Easting (UTM-WGS84).
They say also that Abbazia Pisani is located at 32.71 m above sea level
= 77.79 ellps. height.
The 7 parameters come from the same sheet
AFAIK the 7 parameters transform should bring my point from one datum to
another, it shouldn't know anythink about geoid.

But I made another test:

cs2cs -rs +proj=latlong +datum=WGS84\
  +to +proj=latlong +ellps=intl \
  +towgs84=-85.88,-28.85,+49.67,-1.003,-2.383,-1.808,-27.82 \
  +proj=latlong +datum=WGS84 aa | \
cs2cs -rIs +proj=latlong +datum=WGS84\
  +to +proj=latlong +ellps=intl \
  +towgs84=-85.88,-28.85,+49.67,-1.003,-2.383,-1.808,-27.82 \
  +proj=latlong +datum=WGS84

where aa contains my damned Abbazia Pisani coordinates in WGS84:

45d37'01.767"N 11d51'25.843"E 77.79

...and to my surprise the output is:

45d37'1.805"N   11d51'25.842"E -276.471

I thought the output should be pretty the same of the input or... am I
wrong again?



Roberto Vidmar
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