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I am currently working with spatial data in R using sp When I used WGS84-referenced coordinates, I used to specify it by setting the proj4string to:

+proj=utm +zone=18 +datum=WGS84 +units=m +no_defs +ellps=WGS84"

But I don't really understand how this works internally. Now I want to use data that is referenced to ETRS89 datum. How should I set my proj4string? Should I use a reprojection?

  • Are you sure that your new data is in utm zone 18 and ETRS89? That'd be a very dirty thing to have done, given that ETRS89 is a decidedly Europe-focused (Eurasian plate) georeference and zone 18 is in the Americas. – Mikkel Lydholm Rasmussen Aug 5 at 14:07
  • @MikkelLydholmRasmussen You are right! I didn't know what the "zone" part stood for. The example is a projstring that was given to me from a previous work. I'd like to know what's a valid proj4string for my ETRS89 (Spanish) data – David Aug 5 at 14:12
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Finding the proper proj4-string can be a bit funny.
Such strings provide information on both the ellipsoid and how coordinates on the surface are arranged. Deciphering your example gives us the following two core elements:

+proj=utm +zone=18 +datum=WGS84 +units=m +no_defs +ellps=WGS84 

projection is UTM zone 18 with a WGS84 datum
ellipsoid is WGS84

When you want to use ETRS89 (EPSG entry) instead of WGS84 (EPSG entry), one would think that it was a simple matter of changing those parameters, with the key bit of knowledge being that it is not referred to as ETRS89, but GRS80:

+proj=utm +zone=18 +datum=GRS80 +units=m +no_defs +ellps=GRS80

However, it is unfortunately not that simple, because not all systems are defined everywhere. The ETRS89 system is designed for data on the Eurasian plate, and as such it is only properly defined for data in that region.

For data in Spain (mentioned in comments), the string would likely be:

+proj=utm +zone=29 +ellps=GRS80 +units=m +no_defs 

or

+proj=utm +zone=30 +ellps=GRS80 +units=m +no_defs 

Note that the datum is not defined here, as it is not actually required.
See wiki for additional details on the relationship between ETRS89, GRS80 and WGS84.

If you wish to use the data in the same system, one of the datasets should be reprojected to match the other one, just to be safe. Most systems should deal with the difference seamlessly internally, but no need to run the risk of encountering strange errors due to inconsistent use of georeferences.

Finally, most datasets contain these strings, so hard-coding the string is usually not recommended.

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If you are working with old wgs84 coordinates you can substitute etrs89 with wgs84.

Etrs89 is wgs84 attached to the european continental plate. Wgs84 observations in 1989.0 are identical to Etrs89. Starting from 1989 there is a shift of approx 2.5 cm per year.

So if your details are less than a meter, the difference between etrs89 and wgs84 is academic.

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