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Help! Clarke 1880 IGN to WGS 84   Kevin Grootendorst Dec 04, 2002
Re: Help! Clarke 1880 IGN to WGS 84   Melita Kennedy Dec 04, 2002
Re: Help! Clarke 1880 IGN to WGS 84   Kevin Grootendorst Dec 04, 2002
Re: Help! Clarke 1880 IGN to WGS 84   Melita Kennedy Dec 04, 2002
Re: Help! Clarke 1880 IGN to WGS 84   Kevin Grootendorst Dec 06, 2002
Re: Help! Clarke 1880 IGN to WGS 84   Melita Kennedy Dec 07, 2002
Re: Help! Clarke 1880 IGN to WGS 84   Kevin Grootendorst Dec 10, 2002
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Subject Help! Clarke 1880 IGN to WGS 84 
Author Kevin Grootendorst 
Date Dec 04, 2002 
Message Does anybody know the projection parameters required when projecting to UTM 31N WGS 84 from Clarke 1880 IGN (and vice versa)using ArcGIS 8.2?
What kind of transformation method would I use to convert the data?

Does anyone know any other resources (websites) that discuss the Clarke 1880 IGN coordinate system and/or it's relationship to WGS 84?

Thanks!

Kevin 
   
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Subject Re: Help! Clarke 1880 IGN to WGS 84 
Author Melita Kennedy 
Date Dec 04, 2002 
Message Check out http://www.epsg.org and download the Access database.
Clarke 1880 IGN is a spheroid, not a datum (geographic coordinate
system). You need to figure the datum. The EPSG database may be
able to help because you can query for the country where you data's
located and see what is used there.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Help! Clarke 1880 IGN to WGS 84 
Author Kevin Grootendorst 
Date Dec 04, 2002 
Message Melita,
Well, I checked out that database but didn't seem to find any information for Benin (West Africa). You mention that Clarke 1880 IGN is not a datum. Now I'm confused. In the properties of my coordinate system file (.prj), I have the following for the GCS:

Name: GCS_Clarke_1880_IGN
Alias:
Abbreviation:
Remarks:
Angular Unit: Degree (0.017453292519943299)
Prime Meridian: Greenwich (0.000000000000000000)
Datum: D_Clarke_1880_IGN
Spheroid: Clarke_1880_IGN
Semimajor Axis: 6378249.200000000200000000
Semiminor Axis: 6356514.999904193900000000
Inverse Flattening: 293.466020000000010000


Could you explain how Clarke 1880 IGN is not a datum, even though it's listed as being the datum in properties?

Thanks for your time!

Kevin
 
   
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Subject Re: Help! Clarke 1880 IGN to WGS 84 
Author Melita Kennedy 
Date Dec 04, 2002 
Message I think your data is still a coverage (or grid) or came from a coverage
or grid. If a prj file only has (or had) a spheroid defined, but no
datum, we convert it to a spheroid-based geographic coordinate system.
So, there's no datum information, we just populate the datum/geogcs
names with the spheroid name.

The closest one I can find it Lome for Togo which does use the same
spheroid. However, I don't know of any transformations for it.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Help! Clarke 1880 IGN to WGS 84 
Author Kevin Grootendorst 
Date Dec 06, 2002 
Message Melita,
Thanks for your responses. My data is coming from a paper map created by the IGN in 1982. It appears to have UTM coordinates on it, however, the only other referencing information on the map is for the spheroid used (Clarke 1880).
Any other ideas for why data from WGS 84 is off by about 200m? 
   
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Subject Re: Help! Clarke 1880 IGN to WGS 84 
Author Melita Kennedy 
Date Dec 07, 2002 
Message Coordinates that are off about 200 meters in UTM is a
classic amount for two different datums. For instance,
that's the Y difference in much of the US for NAD27
versus WGS84.

You could try a simple spheroid-to-spheroid transformation
by assuming that the Clarke 1880 data is earth-centered
like WGS84.

Do a transformation, but set the parameters to zeroes.
This will change the spheroid. Without more information,
it's almost impossible to figure out the true datum
transformation values that should be used.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: Help! Clarke 1880 IGN to WGS 84 
Author Kevin Grootendorst 
Date Dec 10, 2002 
Message Thanks for all your help Melita.
Unfortunately, the problem still exists. But I do understand my problem a bit better now. The 1:10, 000 map used to digitize linework was created by the IGN for Cotonou, Benin in 1982. The map tells me that it uses the Clarke 1880 spheroid/ellipsoid and has UTM coordinates (it says it's zone 32N but it is actually 31N), but does not tell me the datum used. That's the problem. Until I figure out what datum was used in creating this map, the problem will remain. How I go about doing that is another question!
Thanks again for your help and suggestions.

Kevin