Take the 2-minute tour ×
Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It's 100% free, no registration required.

I have created a geotiff file through gdal_translate:

gdal_translate -a_ullr 547812.2384341886 6834734.336439242 547967.5011478929 6834622.35081697 -a_srs EPSG:900913 pdz3.jpg pdz.tif

When I load this tif file in qgis, I see my map on the right location. Also, when I ask the gdalinfo from this file I get the following information:

Driver: GTiff/GeoTIFF
Files: pdz.tif
Size is 1024, 724
Coordinate System is:
PROJCS["Google Maps Global Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]
Origin = (547812.238434188649990,6834734.336439241655171)
Pixel Size = (0.151623743851815,-0.154676273856209)
Metadata:
  AREA_OR_POINT=Area
Image Structure Metadata:
  INTERLEAVE=PIXEL
Corner Coordinates:
Upper Left  (  547812.238, 6834734.336) (  4d55'15.89"E, 52d22'37.58"N)
Lower Left  (  547812.238, 6834622.351) (  4d55'15.89"E, 52d22'35.36"N)
Upper Right (  547967.501, 6834734.336) (  4d55'20.91"E, 52d22'37.58"N)
Lower Right (  547967.501, 6834622.351) (  4d55'20.91"E, 52d22'35.36"N)
Center      (  547889.870, 6834678.344) (  4d55'18.40"E, 52d22'36.47"N)
Band 1 Block=1024x2 Type=Byte, ColorInterp=Red
Band 2 Block=1024x2 Type=Byte, ColorInterp=Green
Band 3 Block=1024x2 Type=Byte, ColorInterp=Blue

I tested the 4d55'15.89"E, 52d22'37.58"N values in google map, and they also point to the proper location.

However, when I want this geotiff tiled - through tilecache - the map is placed about 20km south of the point I want it to appear (and appears in qgis). When I georeference the map in qgis self, and send it to tilecache I get the map on the right location. Calling gdalinfo on the qgis referenced tif I get roughly the same information (a couple of meters difference, due to mismatch clicks ;-))

The tilecache configuration file of the map is

[cache]
type=GoogleDisk
base=/home/gworx/data/tilecache

[jvv-chan-5-pdz]
type=GDAL
file=/home/gworx/data/layer-files/1320850804986-pdz.tif
bbox=547812.238434,6834734.33644,547967.501148,6834622.35082
spherical_mercator=true
srs=EPSG:900913
metaTile=true
extension=jpeg
mime_type=image/jpeg
tms_type=google

The bbox values are in line with the values I've set when calling gdal_translate. I am pretty much clueless at this moment on how to get this to work. If anyone can point me in the right direction on what I could be doing wrong, I would greatly appreciate it!

share|improve this question
    
Speculation, thus comment, not answer. The raster's coordinate system isn't truly 900913 aka 3857--nothing to trigger using the spherical equations. When I check the UL in 3857 versus the defn given, it differs in Y by approximately 34 km. Could the raster be reprojected when it's cached? –  mkennedy Nov 9 '11 at 20:48

2 Answers 2

I suggest to use EPSG:3857 as CRS for the raster during the tiling process.

EPSG:900913 is not included in current GDAL and QGIS anymore, thus the special spherical mercator treatment is not applied. The proj definition for EPSG:3857 is

+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

which is a bit contradictional to

SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]]

typically causing the offset of 20 to 30 km to the North.

share|improve this answer

20 km wrong in north/south direction is very probable to be caused by mixing spherical and ellipsoid representations of Earth.

Google Maps and Open Street Maps (OSM) use a tweaked Mercator (which seem to be called "Web Mercator") that is faster to calculate. The tile-software behind OSM likely also uses the spherical faster model, while your input data is probably using an ellipsoid earth representation.

The problem can possibly be fixed by adding a parameter semi_minor = 6378137 and re-projecting all data to a single common coordinate system.

For a long discussion, see (somewhat heavy to digest) http://alastaira.wordpress.com/2011/01/23/the-google-maps-bing-maps-spherical-mercator-projection/

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.