Search code examples
pythonmap-projectionsprojpyproj

Why does pyproj.Proj forward projection not seem to account for lat lon origin?

I am getting confused about how projections using pyproj.Proj are defined with respect to a point of tangency / lat lon origin.

Consider the following code:

import pyproj

p = pyproj.Proj('+proj=tmerc +lat_0=55 +lon_0=-1 +a=6378137 +b=6356752.3 +units=m +no_defs')
x, y = p(55, -1)

Now given that I specified the origin for latitude and longitude, I would expect that when specifying those coordinates I would be able to assert x == 0 and y == 0, however I actually get (7571700.820174289, -6296411.725576388).

Can anyone explain why this is the case? My knowledge of projection/coordinate systems is limited, but I did my best to understand PROJ Cartographic help and a related wikibook page.

Many thanks in advance to any who can help set me straight and put me in the right direction :-)

EDIT: Update 1

Thanks to @lusitanica and their helpful answer, I have now tried setting the scale factor to 1 and rerunning: 

x, y = pyproj.Proj('+proj=tmerc +lat_0=55 +lon_0=-1 +k_0=1 +a=6378137 +b=6356752.3 +units=m +no_defs', preserve_units=True)(55, -1)

Unfortunately this gives (7571700.820174289, -6296411.725576388) as before, so the question is what other information is needed for the projection string?

Solution

  • Turns out I was being silly! pyproj.Proj expects input in lon, lat order not lat, lon :-(

    Thus the following works: 

    import pyproj

lat0, lon0 = 55, -1 

p = pyproj.Proj(f'+proj=tmerc +lat_0={lat0} +lon_0={lon0} +a=6378137 +b=6356752.3 +units=m +no_defs')

x, y = p(lon0, lat0)

print(x, y)  # 0.0, 0.0