My study extent in central Iran straddles two UTM zones (39 and 40):

enter image description here

I would like to create a user-defined CRS in Transverse Mercator projection with central meridian inside this area. Through Custom CRS>> Add new CRS, I try to add the following proj4 string:

+proj=tmerc +lat_0=Latitude of natural origin 
          +lon_0=Longitude of natural origin
          +k=Scale factor at natural origin 
          +x_0=False Easting
          +y_0=False Northing

How can I calculate these parameters from my study area? I use QGIS 2.6.1.

1 Answer 1


Set lon_0 to the middle of your study area.

k can be set to 0.9996 (as UTM has).

lat_0, x_0 and y_0 have no effect on the quality of the projection. They are usually set to the equator and/or the inverse of the lower left point so that all coordinates are positive.

  • Thanks @AndreJ. So I modified the string accordingly: +proj=tmerc +lat_0=33.141751 +lon_0=55.249785 +k= 0.9996 Does it make sense to you? To work in transverse Mercator projection system, shall I now reproject all my layers with this new CRS?
    – Hmmm
    Aug 27, 2015 at 21:31
  • Yes, the projection looks ok. Go ahead.
    – AndreJ
    Aug 28, 2015 at 3:51

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