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?
Answer
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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