Incorrect false northing in pyproj?

Question

I am using version Rel. 5.2.0, September 15th, 2018 on OSX but get the same answers on other platforms with other versions.

prompt> echo 3 0 | proj +proj=utm  +zone=1  +ellips=wgs84

500000.00   19995929.89

Seems correct for the easting as the center of the zone should be 500km. The northing decreases to about 10km for a value of 90 degrees latitude as shown below.

prompt> echo 3 90 | proj +proj=utm  +zone=1  +ellips=wgs84 

500000.00   9997964.94

However, when I give a sightly negative latitude I get

prompt> echo 3 -.0001 | proj +proj=utm  +zone=1  +ellips=wgs84 

500000.00   -19995918.83

Which means that (3,0) and (3,-.0001) have very, very different coordinates in the northern hemisphere utm zone 1. I don't think this is correct -- hopefully, I am simply wrong and have made an error somewhere.

Answer 1

The fundamental problem is that the UTM zone is so far off the correct one that the projections don't behave as expected. The utm zone should be 31. It is odd that the central meridian is correct even in this case, but it is. Using the correct zone gives

prompt> echo 3 0 | proj +proj=utm +zone=31 +ellips=wgs84

500000.00 0.00

prompt> echo 3 -.0001 | proj +proj=utm +zone=31 +ellips=wgs84

500000.00 -11.05

which are much more reasonable values. So - some of the coordinates can be correct even if your utm zone is very wrong. Hopefully, this helps someone who made the same mistake.