Is point cloud export to EPSG:2956 correct?

[nickponline]

Hi there,

I wanted to export a point cloud in EPSG:2956. To confirm the results I also exported the point cloud in EPSG:4326 and converted it to EPSG:2956 using Proj4 unfortunately the results are different. If there any reason for this - how do I know which is correct. Here is a sample script should the different for one point:

import pyproj

p1 = pyproj.Proj("+init=EPSG:4326")
p2 = pyproj.Proj("+init=EPSG:2956")

# a point from photoscan exported point cloud in in EPSG:4326
epsg4326_point = (-110.28868093, 64.46331476, 391.28830036)
# a point from photoscan exported point cloud in EPSG:4326
epsg2956_point = (534215.31186246, 7148835.02251115, 391.45030254)

print "Source point", epsg4326_point
print ""
print "Target point", epsg2956_point
print 'Manually converted result (different): ', pyproj.transform(p1, p2, *epsg4326_point)

[Alexey Pasumansky]

Hello nickponline,

The reason of the difference is in fact that these coordinate systems are on different Datums and transformation between datums are approximate. So it looks like PhotoScan and Proj4 uses different approximation for this transformation.