[Proj] Re: Albers reprojection anomoly?

Frank Warmerdam fwarmerdam at gmail.com
Wed Jun 29 14:13:56 EDT 2005

On 6/29/05, Frank Warmerdam <fwarmerdam at gmail.com> wrote:
> Gerald, or others,
> In bug report:
>   http://bugzilla.remotesensing.org/show_bug.cgi?id=877
> a user reports that albers reprojection seems to not match what
> he expects.  Based on his test datasets, I get:
> input (+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=23 +lon_0=-96
>           +x_0=0 +y_0=0 +ellps=clrk66 +units=m +no_defs):
>   581715E, 1070148N
> output (+proj=utm +zone=16 +datum=NAD27):
>   239958E, 3602624N
> output (expected for UTM 16 NAD27):
>   239958E, 3602624N


It turns out the problem was related to datums.  If the input was marked
as datum=NAD27 then the correct results are received.  The problem
was that internally the pj_transform() function was converting from 
clark1966 to WGS84 based on a pure geocentric conversion and then
back to NAD27 based on the NAD83 to 27 datum shift tables resulting
in a big offset. 

Not a projection problem at all, per-se. 

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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