[PROJ] Proj / cs2cs differences

[Roberto Toldo]

Even,

Got it! Many thanks for the help! I'll also try using the newer cct utility
to make step-by-step projections.

Cheers!

On Sat, Sep 18, 2021 at 2:22 PM Even Rouault <even.rouault at spatialys.com>
wrote:

> Roberto,
>
> the proj utility only works with a projection operation, where input
> geographic coordinates are expressed in the base CRS of the projected CRS
> (which is EPSG:4162 "Korean 1985" here), and doesn't apply any datum
> transformation. So the equivalent of your proj invokation  with cs2cs would
> be cs2cs +init=epsg:4162 +to +init=epsg:5174
>
> If you use EPSG:4326 as the input CRS, then there will be a datum shift
> from EPSG:4326 to EPSG:4162
>
> Regarding the pipeline output by projinfo -s -t, you can't use it with the
> proj utility, which is a historic one and is aimed only at understanding
> +proj={projection_id} style of PROJ strings. You should rather use the
> newer cct utility.
>
> Even
> Le 18/09/2021 à 14:13, Roberto Toldo a écrit :
>
> 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
>
> _______________________________________________
> PROJ mailing listPROJ at lists.osgeo.orghttps://lists.osgeo.org/mailman/listinfo/proj
>
> -- http://www.spatialys.com
> My software is free, but my time generally not.
>
>

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