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I have a list of proj4 calls. Some of them have one format and some have another, I've been told they are equivalent. Is that so? If so, which one should I use?

for example, for ETRS89 / UTM zone 20N:

+proj=utm +zone=20 +ellps=GRS80 +towgs84=0000000 +units=m +no_defs
+proj=utm +zone=20 +ellps=WGS84 +datum=WGS84 +units=m +no_defs
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1  
Welcome! Please take the tour. I'm unsure what you are asking...the two lines of code you posted are for two different projections, One World Geodetic 1984 and one Geodetic Reference 1980. You may want to look at: reddit.com/r/Surveying/comments/3k2hca/… It's difficult to answer the question based on the data given--using one or the other may make no difference or it may make a slight difference depending upon your data and expected end result. – MaryBeth Jan 5 at 14:15
up vote 3 down vote accepted

They are close, but not identical:

Geodetic Reference System 1980
----------
Semi-Major Axis: 6378137 meters
Inverse Flattening: 298.257222101 unity

World Geodetic System 1984
----------
Semi-Major Axis: 6378137 meters
Inverse Flattening: 298.257223563 unity 

And in your proj4 string, there is no shift defined with: +towgs84=0000000

To answer the question, ETRS89 uses the GRS80 ellipsoid so the correct string would be the first one: +proj=utm +zone=20 +ellps=GRS80 +towgs84=0000000 +units=m +no_defs

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No. One uses GRS 80 Ellipsod and the other uses WGS 1984 Ellipsoid.

Semi-major axis Inverse of flattening

WGS84 6378137 298.257223563

GRS80 6378137 298.257222101

There is small difference between the flattening of the two ellipsoids but before fretting too much about it, let's calculate how different are the minor semi-axis of the two ellipsoids: About 0.1mm (What is the difference between WGS84 and NAD83?).

So I would say they are the same for almost all practical purposes even if not quite equal.

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Thanks! Both answers were very good and claryfing. I've marked the other answer because it points out which one is more correct given the information I had. – quimnuss Jan 5 at 16:35
    
I agree. The answer goes a little further in helping in your specific case. – If you do not know- just GIS Jan 9 at 20:17

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