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