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What is the difference between GCS_Clarke_1...   Tracy Timmins Oct 11, 2003
Re: What is the difference between GCS_Clar...   Melita Kennedy Oct 15, 2003
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Subject What is the difference between GCS_Clarke_1880_Arc and GCS_Arc_1950? Where is the origin of these datums? 
Author Tracy Timmins 
Date Oct 11, 2003 
Message I need to transform data on Tanzania which is based on the Arc 1960 datum, but the Clarke 1880 Modified spheroid to WGS 84. GCS_ARc_1960 under the projection Utility, specifies Clarke 1880 RGS as the spheroid which is NOT what I want. The parameters of Clarke 1880 Arc are very similar to those of Clarke 1880 Modified (which is not listed). But I still need to select a datum that is placed in the correct position. Could anyone share some insight on which datum is where, which one is more suitable, or what is the current Tanzanian datum, that would be great. 
   
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Subject Re: What is the difference between GCS_Clarke_1880_Arc and GCS_Arc_1950? Where is the origin of these da 
Author Melita Kennedy 
Date Oct 15, 2003 
Message We pulled the information for Arc 1960 from the EPSG database
(http://www.epsg.org) and from NIMA (www.nima.mil, look for
the G&G group). They both list the Clarke 1880 RGS spheroid.
If you want to define a custom spheroid, you'll have to modify
an existing geographic coordinate system (I would suggest
changing the name) or create a new one from scratch.

You'll then also have to define a custom geographic (datum)
transformation to convert to WGS84. EPSG lists these values
for Arc 1960 to WGS84 for Tanzania.

dX = -175
dY = -23
dZ = -303

Use the geocentric translation or molodensky transformation
method.

Melita 
  Melita Kennedy
ESRI Product Specialist