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
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
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.
