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I have a shapefile defined by the source as: "The layer is in geographical (lat-long) coordinates (GCS_Clarke_1866; Datum: D_Clarke_1866; Prime Meridian: 0)."

opening the shapefile in QGIS 1.8 the proj4 format is:

+proj=longlat +ellps=clrk66 +no_defs

Why is there no datum defined and how can I define it? Is it necessary to define the Prime Meridian?

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2 Answers 2

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The provider either doesn't know or doesn't wish to include a complete coordinate reference system definition in the metadata. The provider has given you the ellipsoid (spheroid) and prime meridian only. Unless you can find out somewhere else, there's no way to know what the GeoCRS and datum should be.

Usually, Clarke 1866 means a NAD 1927 datum. However, my suspicion is that this data originated out of ArcInfo workstation or ArcView 3.x where the default coordinate system is a Clarke 1866-based GeoCRS IF you didn't define it otherwise. It could actually be anything.

I would try using NAD 1927 instead and see if the data lines up well with your other data or some reference data. If it seems like it's offset, try whatever is the usual GeoCRS for the data's area of interest.

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thanks for the insight. Could you explain to me exactly how I would specify a NAD 1927 datum within the proj4 string? –  XNSTT Apr 30 '13 at 8:11
    
searching through EPSG geodetic parameter registry to find the the GeoCRS for the area of interest (East Africa) it gives me the option to search for Geographic CRS; 2D or 3D; and Geocentric CRS. What do you mean by GeoCRS here? –  XNSTT Apr 30 '13 at 11:42
    
For this case, look at the 2D geographic CRS. For East Africa, probably Arc 1950 or Arc 1960. Neither use Clarke 1866! –  mkennedy Apr 30 '13 at 14:10
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The datum is not part of the original Shapefile .prj definition, and ESRI do not incorporate any datum information into their .prj files. They handle datum definitions seperately, but do not save them in the shapefile.

QGIS does it, but creates also a .qpj file which feeds the need of datum shift definitions.

EDIT:

If the projection is in degrees, a prime meridian at 0° (Greenwich) should be assumed as default.

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Esri does incorporate datum (and geographic CRS) information into shapefile prj definitions IF it's provided. I think you mean we handle geographic/datum transformations separately. A "datum" does not inherently include a particular transformation. –  mkennedy Apr 29 '13 at 15:57
    
@Andre Joost, how do i set the prime meridian as 0° within the proj4 string? –  XNSTT Apr 30 '13 at 8:18
    
@XNSTT, that should be the default. Only if the prime meridian was not zero, would you need to set it. –  mkennedy Apr 30 '13 at 14:09
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