[Gdal-dev] Fwd: plate carree to latlon projection problem

[Kolberg Sjur A]

Jaap,
 
Have you got your ellipsoids right?
 
If one of your ellipsoids (commonly the one used with Plate-Carree) is a
sphere, and you ignore that in converting to geographical latlong
(presumably based on WGS84), errors up to nearly 20 km will occur
(latitude dependent). These errors are in the N-S direction, E-W is
usually OK. This effect is beyond projection, latlong/sphere to
latlong/WGS84 gives equally long offsets. Try using cs2cs and specify
ellipsoids for both input and output,
 
It is only when one of the ellipsoids is a sphere you get these large
errors, otherwise the error is much smaller.
 
See also Frank Warmerdam's answer on June 7th on gdal-dev: 
(Re: [Gdal-dev] Does GDAL handle rotated spheres?)
 
Best regards,
 
Sjur K :-)
 

Sjur Kolberg 
SINTEF Energy Research 
N - 7465 Trondheim, Norway 
Phone +47 - 73 59 72 78 
Fax +47 - 73 59 72 50 


________________________________

From: gdal-dev-bounces at lists.maptools.org
[mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of
jjknoops at netscape.net
Sent: 17. september 2007 15:37
To: gdal-dev at lists.maptools.org
Subject: [Gdal-dev] Fwd: plate carree to latlon projection problem


Hello Everybody,


I am using gdal for a couple of year now and I am very satisfied with
it. 
But now i have a problem with reprojecting from and to plate
carree(epsg:32662). 
I have data in plate carree and I need to reproject to latlon but i
there is an offset between the results and other data that already in
latlon.
I have also used Erdas Imagine 9.1 to do the reprojecting but i get a
different results then from gdal. The results from Erdas are beter then
from gdal

After testing it with a worldwide dataset. I found that the offset is
almost 0 at (0,0) to several km around Brittian. Is there a problem with
the reprojection using plate carree or do you use a different wgs84 then
Erdas?

I am using gdal from fwtools 1.3.6

Best Regards,
Jaap Knoops
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