[Gdal-dev] Problem creating GeoTiff across International Date Line

[Ed McNierney]

Todd -

I don't see anything about GeoTIFFs in there <g>.  You're asking about
pj_inv, pj_fwd, and proj, and you'll just have better luck on the proj4
list.  I'd suggest posting to that list, and I'm replying to it, too.

What is the central meridian of your Mercator projection?

	- Ed 

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: gdal-dev-bounces at lists.maptools.org
[mailto:gdal-dev-bounces at lists.maptools.org] On Behalf Of Todd Jellett
Sent: Thursday, October 20, 2005 12:29 PM
To: gdal-dev at lists.maptools.org
Subject: [Gdal-dev] Problem creating GeoTiff across International Date
Line

Hi all,

I have a question which is related to geotiff images, the international
date line and proj4.
It may ultimately belong on the proj4 list but since it relates to
GeoTiff I'm posting here.

Is it the intention that pj_inv can be used to convert a projected coord
(say mercator) to lat/long then use pj_fwd to convert back to mercator
units and get the same x, y mercator value?

With the current implemetation of proj4 "adjlon" being used in pj_inv.c
to make sure that the returned longitude is always within the range of
+/- 180 degrees. While this is fine most of the time, it causes problems
when working around the international date line.

eg. I have a mercator coordinate for an image corner. note: the image
spans the date line.
1) x,y = (23370182, 22807431)

2) results from  pj_inv
     Lat, Long = (86.81, -150.06)

3) results from pj_fwd
     x,y = (-16705270,  22807431)

The difference occurs because the orgininal longitude (before adjlon) of

+209.04 was
"corrected" to be within the range of -180. to +180 before being
returned by pj_inv.
The end result is an X coordinate which is 40 million meters east of
where it originally was.
Strictly speaking a longitude of -150.06 is the same location on the
earth as +209.04 but when you project this point to x, y you do not get
the same Y coordinate.

Thanks
Todd



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