[OSRS-PROJ] PROJ and ellipsoid shifts

[Jean-Paul Berroir]

Hello all

being unfamiliar with cartographic projections, I hope I won't bother
you with too naive questions.

I have to process together data of different nature and sources:
    * satellite (landsat ETM) data, geocorrected (level 1G) , delivered
in UTM projection and unspecified ellipsoid -in which case I assume
WGS84
    * data given in latitude/longitude for WGS84
    * data from ground based stations, delivered in the local
co-ordinate system, in my case (former East Germany):
                             Projection: Gauss Krueger in 6° stripes (or
Transverse Mercator)
                                 projection parameters: central meridian
15E false northing 3,500,000m
                              Ellipsoid: Krassowsky with Pulkovo Datum
(dX=28, dY=-130 dZ=-95)

then come my questions:
-up to version 4.3, proj was restricted to projections only, within the
same ellipsoid/datum system. It seems that the newer version -and
program cs2cs- is able to do the job. Am I right, or is cs2cs also
restricted to one ellipsoid at a time?

-to check my results I was provided with some checked remarkable points,
which co-ordinates are known in lat/long and in the local system.
I was never able to find these results with proj/cs2cs
Example:
"ground truth": geographical, WGS84: 52.4172632N 13.6254409E becomes
y=3406605m x=5810804m on GK/Krassowsky etc

The command
cs2cs  +ellps=WGS84 +proj=latlong +to +ellps=krass +proj=tmerc +lon_0=15
+x_0=3500000
gives me approximately the same figures, yet slighty different:
y=3406478.97   x=5810764.06 (difference is 40m in x and 126m in y)

-although this difference might be sufficiently small for my application
I would be glad to know if it is normal or much too large, in comparison
to cartography standards
-the difference in x and y looks very much the same as the Pulkovo
datum. I'm not sure of the meaning of this datum: is it a shift from
Krassovsky ellipsoid or from WGS84? In the first case, how can I tell it
to cs2cs, and in the second case, why the same cs2cs command with datum
specified as +towgs84=28,-130,-95
gives me absurd results (e.g. x=3500000.00  y=10002137.50)

-last but not least, wanting to check my command before bothering you,
I've asked for more info with option -v, and look at what is said about
ellipsoid:
(with above cs2cs command, option -v added)
# ---- From Coordinate System ----
#Lat/long (Geodetic)
#
# +ellps=WGS84 +proj=latlong
# ---- To Coordinate System ----
#Transverse Mercator
#       Cyl, Sph&Ell
# +ellps=krass +proj=tmerc +lon_0=15 +x_0=3500000
#--- following specified but NOT used
# +ellps=WGS84

how do I have to understand the last comment?

Many thanks for your help

Jean-Paul Berroir

PS: I'm using the proj package version 4.4.4 on a lunix i686 machine

--
----------------------
AIR project - INRIA - BP 105 - F78153 Le Chesnay Cedex
Tel: +33 1 3963 5703 Fax: +33 1 3963 5880
jean-paul.berroir at inria.fr


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