[Proj] Datum-Shift WGS84 to Arc1960 not working propberly

OvV_HN ovv at hetnet.nl
Sun Aug 9 07:14:52 EST 2009

----- Original Message ----- 
From: "Micha Neubauer" <Micha.Neubauer at yahoo.de>
To: <proj at lists.maptools.org>
Sent: Sunday, August 09, 2009 12:19 AM
Subject: [Proj] Datum-Shift WGS84 to Arc1960 not working propberly

> Dear List,
> I am in Tanzania and I want to transform some location data which I was 
> given
> in WGS84 to the locally used Arc1960 Datum.
> So I do:
> $ cs2cs +proj=utm +zone=37 +south +ellps=WGS84 +datum=WGS84 +units=m 
> +no_defs
> +to +proj=utm +zone=37 +south +ellps=clrk80 +units=m +no_defs
> But it is giving me a different result than my colleague gets using the 
> garmin
> software and I think his results are correct and mine aren't
> For the UTM WGS84 Point:
> 332368 9613672
> Using the above command I get for UTM / Arc1960:
> 332365.02   9613707.55 0.00
> And Garmin (which is probably correct) gives for UTM / Arc1960:
> 332274 9613972

Most importantly: add datum transformation parameters in a towgs84 string.
How? Please read the manual.
I merely analyzed the procedure and the parameters.
In detail, the procedure I followed was:

* Convert wgs84 coordinates x,y to lat,lon for UTM zone 37 South.
* Molodensky transform to Arc 1960 Tanzania datum.
The transformation parameters can be found on the NGA website:
Note that the NGA values are from local datum to wgs84, so negate the values 
for the opposite direction.
The negated NGA values are:
dx = 175; dy = 23; dz = 303;
* Convert transformed lat,lon to UTM x,y on Clarke 1880 ellipsoid, zone 37 
This gives:
x = 332276.33; y = 9613976.60; (meters)
I used the Abridged Molodensky transform, a procedure likely to be used in a 
GPS device.
Using PROJ software can give a difference of a couple of centimeters.

Some manuals of GPS related products list a datum transform parameter set 
dx = 160; dy = 8; dz = 300;
This gives:
x = 332273.56; y = 9613972.32;

The Garmin values you mentioned: 332274 9613972

Oscar van Vlijmen

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