I am now working with coordinate transformations between different CRSs, e.g. latlong to UTM.

After comparisons, I found out that the coordinate capture plugin in QGIS is quite accurate for the results.

Is there a way I can use the methods QGIS uses to do transformation in my own project?

I have checked that QGIS is using the proj4 library to do the transformation. I have implemented the proj4 transformation, but got slightly different results.

pj_svy21 = pj_init_plus("+proj=tmerc +lat_0=1.366666666666667 +lon_0=103.8333333333333 +k=1 +x_0=28001.642 +y_0=38744.572 +ellps=WGS84 +units=m +no_defs") 

pj_latlong = pj_init_plus("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs")

double test1 = 37621.665;
double test2 = 12056.915;

projResult = pj_transform(pj_svy21, pj_latlong, 1, 1, &test1, &test2, NULL);

My result (latlong):
1.1253115030392806 103.9197679941208889

QGIS results:
1.35650722603 103.690059679 (check on map more accurate)

Can anyone help me on this?

Need to include iostream and proj_api.h:

int main () 


projPJ pj_svy21, pj_latlong

pj_svy21 = pj_init_plus("+proj=tmerc +lat_0=1.366666666666667 +lon_0=103.8333333333333 +k=1 +x_0=28001.642 +y_0=38744.572 +ellps=WGS84 +units=m +no_defs")

pj_latlong = pj_init_plus("+proj=latlong +ellps=WGS84 +datum=WGS84 +no_defs")

double test1 = 12056.915;

double test2 = 37621.665;

projResult = pj_transform(pj_svy21, pj_latlong, 1, 0, &test1, &test2, NULL );

std::cout << "latlon: " << test1 * RAD_TO_DEG << ' ' << test2 * RAD_TO_DEG << std::endl;

return 0;


1 Answer 1


You may have swapped the coordinates somewhere.

If I reproject your resulting lonlat coordinates

103.919768  1.125312
103.690059679 1.35650722603

I get with cs2cs using:

cs2cs -f "%%.6f" +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +to +proj=tmerc +lat_0=1.366666666666667 +lon_0=103.8333333333333 +k=1 +x_0=28001.642 +y_0=38744.572 +ellps=WGS84 +units=m +no_defs  inlon.txt >out.txt

37621.665436    12056.969913 0.000000
12056.914916    37621.664783 0.000000

Note that QGIS usualy expects X (horizontally or E) - Y (vertically or N) orientation.

  • Hi AndreJ Thanks for the reply. I have changed the order of the north and east it seems ok but the output of pj_transform(pj_svy21, pj_latlong, 1, 1, &test1, &test2, NULL), test1 corresponds longitude while text2 corresponds latitude. Should it not be the other way around? Thanks
    – Ethan
    Oct 24, 2014 at 14:43
  • No, you requested +proj=longlat, that means longitude first. The name of your variable pj_latlong is misleading.
    – AndreJ
    Oct 25, 2014 at 7:19
  • ohh..i see..many thanks AndreJ, you solved my problems. :) Thanks.
    – Ethan
    Oct 25, 2014 at 10:17
  • Hi AndreJ sorry to ask again, I have changed the +proj=longlat to +proj=latlong but the result are still long is first. I have pasted the code above you may try it out. Thanks in advance.
    – Ethan
    Oct 25, 2014 at 11:08
  • For some strange reasons, +proj=latlong behaves the same way as +proj=longlat. Try +axis=neu, see trac.osgeo.org/proj/wiki/GenParms#Axisorientation
    – AndreJ
    Oct 25, 2014 at 11:11

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