[OpenLayers-Users] GMT raster (*.tiff + *.pgw) in epsg4326 shifts few kms (4326-900913-3857 problem?)

[Gery .]

Hi,

This seems to be an old problem with an already known solution (ie. "...stop using 900913 and start using 3857..."), but not sure whether this problem is about software versions, the raster itself or just projections.

The thing is that I did a quite big raster (TIFF image + its PGW projection text file based on lon/lat values) with the software GMT (Generic Mapping Tools). It covers a big area (range: -83ºW, -75ºW, -17ºS,-5ºS). Its center is around -79ºS and -11ºS. To produce it, GMT requires both the lon/lat ranges and the projection. For the former I used the previous lon/lat range and for the latter I used a Mercator projection and a arbitrary width (in GMT language: -JM10i). So, the output raster was beautifully produced (as usual), and I loaded into my HTML with OL (of course) and served by Mapserver (as WMS). It overlays google, bing and esri base maps (so in MAP file I added the corresponding EPSG:4326 and 900913 as usual, and the latter was added to the epsg file in the /usr/share/proj directory, plus the usual "projection" and "displayProjection" calls in OL).

So, it loaded normally as the other ones served by Mapserver (I have some more TIFF like this one, ie. GMT raster, but much more small ones, ie. less area). However, I noticed that around -80ºS and -8ºS, it should line up with other WMS layers (eg. seismic lines) but it didn't (it has an offset of ~3 km to the north in regard to other WMS layers), so I started changing things (900913 by 3857) in my MAP file and the HTML but nothing, the problem didn't go away. Interestingly, I did some tests and merged all the TIFFs and got only a unique TIFF (ie. new range: -83 W,-70 W,-19 S,-5 S), and this one produced an offset of ~10 km (instead of 3 km at the same location). Also, I loaded all the WMS layers and used the unique TIFF as base layer (because all the WMS layers served by Mapserver are in epsg4326), but the erroneous offset is still there.

So, I was wondering if this problem could be caused by the OL 2.11, or Mapserver 5.6.7 and Proj 4.8.0 (these two are RPMs from ELGIS for RHEL5) I'm using. Another possibility is that the -JM10i used in GMT to generate the TIFF was the problem, or it is because the TIFF area (in lon/lat) covers two UTM zones (UTM zone 17 South and UTM zone 18 South).

I think the "EPSG:900913 or EPSG:3857" thing is not the problem. After testing both in Mapserver and OL, I checked in Firebug and saw that the request (SRS=EPSG) were the same for both cases, ie. when 900913 was used it appeared SRS=EPSG=900913, and when 3857 was used: SRS=EPSG=3857. I also read that using either one of them it should work all in the same way (even though this may imply the other way: http://alastaira.wordpress.com/2011/01/23/the-google-maps-bing-maps-spherical-mercator-projection/). In my case I prefer using 900913 because google v3 uses it as far as I know.

I hope someone could enlighten me here and provide me a solution,

Thanks in advance,

Gery

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