[PROJ] Proj / cs2cs differences

[Roberto Toldo]

I was able to get the actual pipeline transformation used by "cs2cs" by
running:
*projinfo -s EPSG:4326 -t EPSG:5174*

Still, I'm having problems using the output proj string:












*+proj=pipeline  +step +proj=axisswap +order=2,1  +step +proj=unitconvert
+xy_in=deg +xy_out=rad  +step +proj=push +v_3  +step +proj=cart
+ellps=WGS84  +step +inv +proj=molobadekas +x=-145.907 +y=505.034
+z=685.756 +rx=-1.162        +ry=2.347 +rz=1.592 +s=6.342 +px=-3159521.31
+py=4068151.32        +pz=3748113.85 +convention=coordinate_frame  +step
+inv +proj=cart +ellps=bessel  +step +proj=pop +v_3  +step +proj=tmerc
+lat_0=38 +lon_0=127.002890277778 +k=1 +x_0=200000        +y_0=500000
+ellps=bessel  +step +proj=axisswap +order=2,1*

The error I get is "can't initialize operations that take non-angular input
coordinates". If I remove the "unitconvert" step, I can run proj bin, but I
get wrong results (even if I manually convert the input from degree to
radians). Any suggestions?

Many thanks,
Roberto


On Fri, Sep 17, 2021 at 9:35 AM Roberto Toldo <roberto.toldo at 3dflow.net>
wrote:

> Hello!
>
> I found some issues/differences from the "proj" bin and "cs2cs" bin
> utilities. I'm using the last version, directly compiled from Github.
> Probably it's something due to my inexperience with the library, so please
> bear with me :)
>
> Specifically, I was trying to convert coordinates from geodetic to
> EPSG:5174 ( Korean 1985 / Modified Central Belt).
>
> I used proj with the parameters extracted from "projinfo EPSG:5174":
> proj +proj=tmerc +lat_0=38 +lon_0=127.002890277778 +k=1 +x_0=200000
> +y_0=500000 +ellps=bessel +units=m +no_defs
>
> and then cs2cs with direct conversion parameters:
> cs2cs +init=epsg:4326 +to +init=epsg:5174
>
> However, I obtain quite different results.
>
> For example, If I use lat_0 and lon_0 as input (127.002890277778, 38), I
> obtain exactly false_easting and false_northing (200000, 500000) when using
> proj4 (as expected), while I obtain something different when using cs2cs
> (200185.04 499695.19).
> I would be expecting the same results. I tried with other coordinate
> systems (e.g., EPSG:32633 UTM zone 33N), and I noticed no differences
> between proj and cs2cs results.
> Can anybody help me understand what the reason for the differences is? Are
> the parameters used internally by cs2cs different for EPSG:5174? I also
> tried setting the accuracy, but it doesn't seem to work.
>
> Thanks,
> Roberto
>
>

-- 
Roberto Toldo
Technical Director & Co-founder | 3Dflow srl
+39 045 4500109 | roberto.toldo at 3dflow.net | http://www.3dflow.net
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