[gdal-dev] GDAL for BNG to Lat/Long, WGS84

[Smith, Michael ERDC-USACE-NH]

Frank and Scott,

For Google Earth output (rather than Google Maps which would be in "google
mercator") the output projection should be EPSG:4326, or WGS1984 Latitude
Longitude.

Mike


-- 
Michael Smith
RSGIS Center
US Army Corps of Engineers




On 3/2/10 12:52 AM, "Frank Warmerdam" <warmerdam at pobox.com> wrote:

> scottd777 wrote:
>> Hi, and thank heaven for GDAL!
>> 
>> I was so happy to find this tool, but I am having some trouble figuring out
>> the exact command I need for an image warping application.  If there is
>> someone who doesn't mind a newbie type question, I could really use the
>> help! 
>> 
>> Essentially, I have imagery which is referenced with British National Grid.
>> I am tyring to convert / warp this so that it can be used within Google
>> Earth.  I have reference points for the upper left and lower right of the
>> image in Easting and Northing, and transformed those so that I got lat /
>> long coordinates which line up very well within Google Earth, however it
>> appears that the projection is off.
>> 
>> I am hoping to use gdalwarp to convert the TIF file (referenced as
>> mentioned), but I had some questions:
>> - For the world file that I create for gdalwarp, should I include the
>> Easting / Northing, or does this need to be lat / long?
> 
> Scott,
> 
> If the file is in the british national grid then the origin and pixel
> size should be in british national grid meters.
> 
>> - Can I just go straight to gdalwarp, or do I need to do some other
>> translation first?
> 
> If you have a TIFF with a world file you should be able to just proceed
> directly with the warp.
> 
>> - The command I am trying to use looks like this:
>> gdalwarp -s_srs "proj=utm +zone=33v +datum=OSGB36" -t_srs "proj=latlong
>> +datum=WGS84" filein.tif out.tif
>> 
>> Is this the correct command to get from a BNG source ortho image to what
>> would overlay correctly in something like Google Earth?
> 
> I'm not google earth guru, but I had assumed that it expected imagery in
> the "google mercator" projection, not in geographic lat/long as you have
> targetted.  I would instead use a command like:
> 
> gdalwarp filein.tif out.tif -s_srs EPSG:27700  \
> -t_srs "+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
> +y_0=0 
> +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs"
> 
> I don't believe british national grid is the same as UTM 33, and I don't
> believe that a "v" in the zone indicator will mean anything to PROJ.4.
> EPSG:27700 is the british national grid.
> 
> I have provided the full definition of the google mercator projection
> though shorter forms like EPSG:900913 or EPSG:3857 might work depending
> on the version of GDAL and supporting files you have.
> 
> scottd777 wrote:
>> Please?  Anyone?  I'm sure the response will be really quick...
> 
> Not such an easy answer, and I'm not sure it will be everything you
> need.  The problem with the popularity of Google Earth is that it
> has introduced a flood of new people with little GIS background all
> asking the same questions.
> 
> Best regards,