[OSRS-PROJ] cs2cs and datum shift

[Clifford J Mugnier]

Dear Roberto,

It appears that you are not considering the Geoid model that is appropriate
to your datum shift.  Try going to the www.NIMA.mil web site and
downloading EGM96 (Earth Gravity Model 1996), which will give you
transformation parameters to go from WGS84 Datum ellipsoid height to "local
mean sea level."  On a good day with clear skies, and if you went to church
last Sunday, your geoid-corrected "elevation" may match local data within a
meter or so.

Such a correspondence can occur if the original 7-parameter transformation
utilized the same geoid model when solving for those parameters.

Also, be aware that a transformation can not yield coordinates that are
more accurate than the original coordinates.  Just because you have old
coordinates of Abbazia Pisani does not mean that the original survey was of
First-Order geodetic accuracy in Latitude, Longitude, and elevation.
Furthermore, how good is the geoid model for Roma 1940 Datum?  I did not
know that one existed.  If it did not exist, how did you get the ellipsoid
height of Abbazia Pisani?

With all our computerized "gizmos" and "whatchamacallits," we continue to
be (often) faced with the conundrum that "you can't get there from here !"
PROJ4 is a very nice cartographic / occasionally geodetic utility.  It's
not a geodetic panacea.

Regards,

Cliff Mugnier

--------------------------------------

Hi to all,

  this is my first experience with cs2cs so please be patient. I'm
trying to shift coordinates between two different datums: wgs84 and
Roma40 using a 7 parameter transformation.

cs2cs -vErs +proj=latlong +datum=WGS84 \
  +to +proj=latlong +ellps=intl\
  +towgs84=-85.88,-28.85,+49.67,-1.003,-2.383,-1.808,+27.82 \
   ap

The ap file is this:

# Abbazia Pisani
45d37'01.767"N 11d51'25.843"E 77.79

My output (Linux RH 7.3) is this:

# ---- From Coordinate System ----
#Lat/long (Geodetic)
#
# +proj=latlong +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
# ---- To Coordinate System ----
#Lat/long (Geodetic)
#
# +proj=latlong +ellps=intl
# +towgs84=-85.88,-28.85,+49.67,-1.003,-2.383,-1.808,+27.82
#--- following specified but NOT used
# +ellps=WGS84
# Abbazia Pisani
45d37'01.767"N 11d51'25.843"E 77.79     45d36'59.373"N  11d51'26.646"E
77.740


The 7 parameters I have are:
Tx:  85.88
Ty:  28.85
Tz: -49.67
Rx: -1.003"
Ry: -2.383"
Rz: -1.808"
K: 27.82

The shifted lat/lon pair are quite right, the point should be at
45d36'59.372"N (error 0.001") and
-00d35'41.752" (Roma40 datum counts longitude from Monte Mario,
12d27'08.400"E =:-). Subtrating cs2cs output from Monte Mario longitude I
get 0d35'41.750" (error 0.010", strange) but the real problem is height:
77.79 is the ellipsoidal height over WGS84 and the above sea level height
of my point, Abbazia Pisani, is 32.71 m.

WHERE AM I WRONG?

I played a lot with the signs of the 7 parameters and this is the closest
result I get. Does anybody else have problems like mine with datum shifts?

Thanks for your help and for the big job you have already done.

Roberto Vidmar

--
------------------------------------

Roberto Vidmar
  ___  __  __    OCEANOGRAFIA
 /  / /__ /__    GEOFISICA
/__/ /__/ __/    SPERIMENTALE

Istituto Nazionale di Oceanografia e
di Geofisica Sperimentale
34010 Sgonico (TS) ITALY

Research Group: CARS
(CArtography and Remote Sensing)

Tel. +39 040 2140 336 direct
Tel. +39 040 2140 1  operator

 E-mail: rvidmar at inogs.it
------------------------------------


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