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For a raster dataset, trying my.raster@crs or proj4string(my.raster) in R gives the PROJ.4 string "+proj=longlat +ellps=clrk66 +no_defs". I downloaded the raster from SEDAC here - the World link, unprojected (geographic). However, on the metadata link - Spatial Reference it is declared that the ellipsoid and datum are WGS84. However, the PROJ.4 string indicates the ellipsoid clrk66 and no information about the datum. Moreover, in the help file of spTransform-methods {rgdal}, Roger Bivand mentions that:

In general +datum= is to be prefered to +ellps=, because the datum always fixes the ellipsoid, but the ellipsoid never fixes the datum.

So, given this situation what datum can I assume? Can I assume a WGS84 both for datum and ellipsoid as in the online metadata? I also asked for feedback on the SEDAC's page here, but maybe some of you can help me understand this issue. I also saw a combination of +ellps=clrk66 +datum=NAD27 (epsg:4267) presented here or here, so maybe should I assume datum=NAD27?

  • EPSG 4008 doesn't specify datum. If online metadata says that is in WGS 84, you can try adding world's boundaries in WGS 84 and NAD 27 and judge yourself (but I think there will not be significative differences) – aldo_tapia Aug 9 '17 at 14:27
  • Hi @aldo_tapia! This lack of datum is really confusing in EPSG 4008. I am curious how can I even make a datum transformation when there is no datum? For example, the tool Project from ArcMap will not let me transform without a datum transformation. – Valentin Aug 9 '17 at 15:42
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    Yes, but you can follow online metada and set CRS to EPSG 4326. For global scale, is weird to see data in NAD 27, is more common to see it in WGS 84 – aldo_tapia Aug 9 '17 at 15:59
  • Entries like EPSG::4008 were added for use when people know the ellipsoid, but not the datum. Older maps sometimes would have the ellipsoid but not other information. Sometimes you can guess the datum; sometimes there may have been 2 different datums using same ellipsoid in the area. – mkennedy Aug 9 '17 at 20:53
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    If you use 4008, you're not going to be able to use any geographic/datum transformations to convert to WGS84 or anything else. The data has a 1km resolution. Just set it to WGS84 (4326). The offsets between older datums and newer ones are anywhere from tens of meters to a few hundred m. All are less than the data resolution. – mkennedy Aug 11 '17 at 18:39

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