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

[jjknoops at netscape.net]

Hi Sjur,

Thanks for your replay. I think you are right when saying it's caused by ellipsoid. So I have used this command to reproject

call gdalwarp -s_srs "+proj=eqc +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +ellps=sphere +a=6378137.0 +es=0.0 +units=m" -t_srs "epsg:4326"

After comparing this result with previous results then I would say it's worse. The offset between the result from gdalwarp and erdas has increased. My college has also compared the results with envi and found that it's comparable to the erdas results. 


Best Regards,
Jaap Knoops


 


 

-----Original Message-----
From: Kolberg Sjur A <Sjur.A.Kolberg at sintef.no>
To: gdal-dev at lists.maptools.org
Sent: Mon, 17 Sep 2007 6:39 pm
Subject: RE: [Gdal-dev] Fwd: plate carree to latlon  projection problem














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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