Basic proj4 conversion UTM grid -> longlat and back

Question

I'm a GIS novice, and am struggling to understand how to drive proj4 to accomplish what seems like a simple thing. I've googled for a couple of days, but everything I find seems to presuppose quite advanced knowledge, and leaves many things unsaid. Stack Overflow couldn't clear it up for me, either..

I have a conversion between UTM easting/northing and lat/long to do in both directions. Once I get this working, I'll add some different projections/datums.

I'm working on my recently acquired knowledge/assumptions:

1. This conversion will require a datum translation, because WGS84 (standard datum for UTM) has it's prime meridian approximately 100m east of Greenwich (where longitude is zero).
2. I don't need to worry about ellipsoids, because the lat/long are geographic coordinates, rather than projected ones.
3. I can either start the conversions using one string (as used in the command-line instantiation), or can pre-assemble a source and destination object, and call pj_fwd(data, ref) directly.

Another issue is that if I call the code directly, while I've found the projUV datatype - I can't find the corresponding UTM version.

Could someone please get me started with the format of the string(s) of either method required?

Perhaps using conversion from zone 56 499451mE 6960921mS to lat/long as an example.

Answer 1

Personally I'd use the Python bindings for the Proj4 library - http://pypi.python.org/pypi/pyproj/1.8.6

Further details at http://pyproj.googlecode.com/svn/trunk/README.html

You can pass your projection parameters directly to the projection object as follows:

PROJ_32756 = """
+proj=utm +zone=56 +south +ellps=WGS84 +datum=WGS84 +units=m +no_defs
"""
p1 = pyproj.Proj(PROJ_32756)

A full list of these strings can be found at http://spatialreference.org/ For zone 56 see http://spatialreference.org/ref/epsg/32756/proj4/

Many of these projections are already built into the library so you can just pass in the code. Below is an example of reprojecting some geojson from one projection to another. You should be able to adapt to your needs.

import pyproj
import simplejson

p1 = pyproj.Proj(init='epsg:3857')
p2 =pyproj.Proj(init='epsg:29902')

json='{"type":"Polygon","coordinates":[[[-972279.15280781,6907135.0149664],[-995516.00940234,6867999.2564914],[-946596.31130859,6844762.3998969],[-964941.19809375,6892459.1055383],[-972279.15280781,6907135.0149664]]]}'

pydata = simplejson.loads(json)
print str(pydata)
new_coords = []
for p in pydata['coordinates'][0]:
    x2, y2 = pyproj.transform(p1,p2,p[0],p[1])
    new_coords.append([x2, y2])

new_json = simplejson.dumps(dict(type="Polygon", coordinates=[new_coords], srid="29902"))
print str(new_json)

Answer 2

You may want to take a look at the route-me project, they seem to have some working examples of wrapping Proj.4 on the iOS platform in Objective-C. The actual transformation looks to happen in Mapview/Map/RMProjection.m.