[postgis-users] transformation discrepancy with postgis v. cs2cs

[Bryan Keith]

Hmmm, for my original post I did the cs2cs check on a different machine. 
When I run the cs2cs transformation on the machine that's running postgis,
then I get the same result that postgis returns.  Does this mean that
cs2cs isn't finding the grid shift files either?  Or does it mean that
it's using the wrong grid shift files?  Or?

Bryan

> It's probably a function of your installation, I get the same results
> as cs2cs on my PostGIS;
>
> postgis15=# select astext(transform(setsrid(makepoint(1234488,
> 744181), 32003), 32100));
>                   astext
> ------------------------------------------
>  POINT(366704.349230461 199132.534090009)
> (1 row)
>
>
> If the grid shift files are missing or not being found by postgis for
> some reason (which could be a host of reasons depending on how you
> compiled / installed proj) then you'd get a different result.
>
> P
>
> On Thu, Mar 25, 2010 at 8:55 AM, Bryan Keith <bryan at ideotrope.org> wrote:
>> Hello,
>>
>> I'm not sure if this is the correct list for this question, but here
>> goes.
>>  If I ought to post to another list, please let me know which one.
>>
>> I'm seeing a discrepancy between coordinate transformations in postgis
>> and
>> transformations using cs2cs with the exact same proj.4 string.  Here are
>> the proj.4 strings and transformation using postgis:
>>
>> select proj4text from spatial_ref_sys where srid = 32003;
>> proj4text
>> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>>  +proj=lcc +lat_1=46.4 +lat_2=44.86666666666667 +lat_0=44 +lon_0=-109.5
>> +x_0=609601.2192024384 +y_0=0 +ellps=clrk66 +datum=NAD27
>> +to_meter=0.3048006096012192 +no_defs
>> (1 row)
>>
>> select proj4text from spatial_ref_sys where srid = 32100;
>> proj4text
>> --------------------------------------------------------------------------------------------------------------------------
>>  +proj=lcc +lat_1=49 +lat_2=45 +lat_0=44.25 +lon_0=-109.5 +x_0=600000
>> +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs
>> (1 row)
>>
>> select astext(transform(setsrid(makepoint(1234488, 744181), 32003),
>> 32100));
>>                  astext
>> ------------------------------------------
>>  POINT(366770.791392722 199138.151643548)
>>
>> Here is the transformation using cs2cs:
>>
>> cs2cs +proj=lcc +lat_1=46.4 +lat_2=44.86666666666667 +lat_0=44
>> +lon_0=-109.5 +x_0=609601.2192024384 +y_0=0 +ellps=clrk66 +datum=NAD27
>> +to_meter=0.3048006096012192 +no_defs +to +proj=lcc +lat_1=49 +lat_2=45
>> +lat_0=44.25 +lon_0=-109.5 +x_0=600000 +y_0=0 +ellps=GRS80 +datum=NAD83
>> +units=m +no_defs
>> 1234488  744181
>> 366704.35       199132.53 0.00
>>
>> Why the discrepancy?  I'm guessing it has something to do with how datum
>> shifts are handled.  So which one is right?  That's probably going to
>> depend on something that I don't know about my input data, but maybe
>> someone will enlighten differently.  Thank you.
>>
>> Bryan
>>
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