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

[Bryan Keith]

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