lat/lon to utm to lat/lon is extremely flawed, how come?

Question

I've tried the following, input: lat/lon data then I'll calculate a box around it by, let's say 50 m, so +/- 50 m on easting/northing value.

Now I reconvert it to lat/lon and with a script:

http://robotics.ai.uiuc.edu/~hyoon24/LatLongUTMconversion.py I get a result that just can't be, lon before is around 7, afterwards around 2.

zone, easting, northing = LLtoUTM(23, location.get_lat(), location.get_lon()) 

topUTM = northing + error
bottomUTM = northing - error
leftUTM = easting - error
rightUTM = easting + error
left, top = UTMtoLL(23, leftUTM, topUTM, zone)

Is the error in my code, or might be the script flawed?

So I've tried to use pyproj, just lat/lon to utm to lat/lon to see what happens

>>> p = pyproj.Proj(proj='utm', zone=32, ellps='WGS84')
>>> p
<pyproj.Proj object at 0x7ff9b8487dd0>
>>> x,y = p(47.9941214, 7.8509671)
>>> print x,y
5159550.36822 1114087.43925
>>> print p(x,y,inverse=True)
(47.971558538495991, 7.8546573140162605)

And here it's not as extremely far off as with the script from above, but it still seems strongly enough incorrect as to not be able to use it. How come? What can I do to get more exact results?

EDIT:

I ran test() and it all tests passed.

in epsg file there is no such thing. The closest I've found was this:

<32632> +proj=utm +zone=32 +ellps=WGS84 +datum=WGS84 +units=m +no_defs <>

no tmerc. Also What would I need to pass the towgs84 as parameters? The ones above?

Answer 1

I've created a small UTM conversion library for Python last week and uploaded it to the Python Package Index: http://pypi.python.org/pypi/utm

I have compared it to using pyproj and it is faster and more accurate. Given your sample data, this is the result:

>>> import utm

>>> u = utm.from_latlon(47.9941214, 7.8509671)
>>> print u
(414278, 5316285, 32, 'T')

>>> print utm.to_latlon(*u)
(47.994157948891505, 7.850963967574302)

UPDATE: Richards answer below describes the real solution for this issue.

Answer 2

The error is in your code.

First off, the PyProj issue listed in one of the other answers is real. You should check your epsg file and make sure it includes the line

<2392> +proj=tmerc +lat_0=0 +lon_0=24 +k=1.000000 +x_0=2500000 +y_0=0 +ellps=intl +towgs84=-90.7,-106.1,-119.2,4.09,0.218,-1.05,1.37 +units=m +no_defs no_defs <>

Note the towgs84 parameter.

Your problem with PyProj stems from mis-using the projection command.

If we take 47.9941214N, 7.8509671E and convert to UTM we get Zone 32, 414278 Easting, 5316286 Northing.

You perform the following PyProj operations:

p = pyproj.Proj(proj='utm', zone=32, ellps='WGS84')
>>> x,y = p(47.9941214, 7.8509671)
>>> print x,y
5159550.36822 1114087.43925
>>> print p(x,y,inverse=True)
(47.971558538495991, 7.8546573140162605)

But, if we consult the PyProj documentation, we see the following:

Calling a Proj class instance with the arguments lon, lat will convert lon/lat (in degrees) to x/y native map projection coordinates (in meters).

Let's try running the OP's PyProj operations again, but switch the order of the lon/lat arguments:

p = pyproj.Proj(proj='utm', zone=32, ellps='WGS84')
>>> x,y = p(7.8509671, 47.9941214)
>>> print x,y
414278.16731 5316285.59492
>>> print p(x,y,inverse=True)
(7.850967099999812, 47.994121399999784)

The operation inverts itself (pretty much) perfectly!

To answer the first part of your question, if you look in http://robotics.ai.uiuc.edu/~hyoon24/LatLongUTMconversion.py at the definition of UTMtoLL, you find the following:

UTMtoLL(ReferenceEllipsoid, northing, easting, zone)

Yet you use UTMtoLL(23, leftUTM, topUTM, zone) where leftUTM is an Easting and topUTM is a Northing.

Therefore, in the case of both your first script and PyProj, you've used the wrong order of arguments.

It's a good reminder to always double- (or triple-) check your work before suggesting that someone else's is wrong. That said, Python's documentation is not the greatest and PyProj's documentation in this instance is cryptic at best. A nice web-based explanation of this command and accompanied by examples of its usage would have probably prevented angst on your part.