[Proj] WGS84 to Sphere Inconsistency Between Proj Versions

[Faron Anslow]

Frank Warmerdam wrote:
>
> Faron,
>
> In PROJ 4.6.0 and later the cs2cs command (and pj_transform()) will
> no longer attempt to apply any datum shift or change of ellipsoid
> transformation unless both the source and destination coordinate system
> have meaningful datum definitions provided (either +towgs84 or 
> +nadgrids).
>
Hi Frank,

This makes sense to me and explains the difference between versions. I 
would guess that the version of proj that matlab uses and that the mex 
file links against when "-l proj" is given is an older version of proj.4.

The question then is if it is more correct to not have the datum shift 
applied (and get the values I get from cs2cs as specified earlier) or to 
have the datum shift applied and get the value with the very negative 
elevation and different northing and eastings?

>> I suspect the problem is that documented in the FAQ, which requires 
>> the addition of +nadgrids=@null to the proj command.  However, when I 
>> add this to the cs2cs call, the results don't change.  When I add it 
>> to the mex call, the mex file crashes (not set up to handle the @null 
>> I guess).  Our question is which result is the correct one?  We 
>> suspect that a big change in elevation should be expected when 
>> switching from the WGS84 ellipsoid to the sphere.  cs2cs from proj 
>> 4.4 installed on another machine gives results that agree with our 
>> mex file.
>>
>> Even stranger is that when we compile the mex file with the line:
>>
>> mex Work/Code/Matlab_tools/cs2cs_proj4.c /usr/lib/libproj.a
>>
>> instead of:
>>
>> mex -l proj /home/fanslow/Work/Code/Matlab_tools/cs2cs_proj4.c
>
> I can only conclude that when you link against the static library
> you get one version, while the other link command is using a shared
> library and you end up picking up a different version of libproj.so.
>
> It isn't easy to deduce your build configuration, but basically I
> believe this comes down to the announced change of behavior between
> PROJ 4.5 and PROJ 4.6.
>
> Best regards,

I agree as above.  Using "-l proj" links to the matlab mapping toolbox 
library, which probably is older than the library I compiled a few days ago.

I just ran this:

cs2cs +proj=latlong +datum=WGS84 +to +proj=lcc +lat_1=50 +lat_2=50 
+lat_0=50 +lon_0=-107 +a=6371200.0000000000 +es=0.0 +f=0.0 +towgs84=0,0,0 -r

on:
70.933 -8.667

and got:
2873633.37    4593659.18 -12148.43

which is my original matlab answer and that from the old proj4.4.  Now, 
the question is if forcing the datum shift with +towgs=0,0,0  is 
appropriate? Doesn't +towgs48 conflict with/override the spherical 
projection definition?


Thanks,
Faron



-- 
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Department of Earth and Ocean Sciences
The University of British Columbia
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