Corine land cover 2000 position offset

popejpopej Global Mapper UserPosts: 16Trusted User
edited January 2008 in Projection Questions
Hi,

I found free data for European maps, this is project Corine land cover 2000 (CLC2000):
http://dataservice.eea.europa.eu/map/clc_download/?configfile=config_clcdownload.xml&i=1

Data is in shp format. Reference system is:

Datum D_ETRS_1989
Ellipsoid GRS_1980
Semi-major axis 6378137
Axis units Degrees
Flattening ratio 3.35281068118232E-03
Projection Lambert_Azimutal_Equal_Area
False easting 4321000 meters
False northing 3210000 meters
Central median 10 degrees
Latitude of origin 52 degrees

Global Mapper 9.01 reads this data and shows projection just like above description from CLC2000. For test I'm using this area:
WEST LONGITUDE=17° 21' 26.85" E
NORTH LATITUDE=54° 58' 38.37" N
EAST LONGITUDE=19° 06' 47.10" E
SOUTH LATITUDE=54° 11' 48.38" N

Unforunately all objects on map seems to be moved by offset about 1.6 to 1.8 km, depending on position on the map. I'm comparing it with GPS map in polish format and WGS84 datum. I'm only loading 2 maps and looking at it on the screen. Do I need any additional configuration or conversion to match data on both maps?

I'm totally new to Global Mapper. I'm using trial version 9.01.

Thanks,
popej

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2008
    Popej,

    We have had this problem before with the Corine Land Cover data. There is apparently something wrong with the projection specification for that data. You need to manually correct the projection information for the data by opening the Overaly Control Center, selecting the Corine layer, pressing Options, then going to the Projection tab and adjusting the false easting and false northing values. For the tile that you are working with, a false easting of 4322900 and a false northing of 3210300 seems to make everything line up pretty well.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • popejpopej Global Mapper User Posts: 16Trusted User
    edited January 2008
    Thank you for your response. I already tried to make linear shift and got similar values. Unfortunately this is not good enough, because offset is not equal for whole area. It's about 1,6km on west and 1,8km on east.

    I'd like to make map of whole Poland. With linear shift I will probably get mismatch about 0,5-1km. This is to much for GPS.

    Is there any way to compute unlinear correction formula with errors in range 50-100m for area about 700x700km? Or maybe is possible to find true values for CLC2000 projection?

    Thanks,
    popej
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2008
    Popej,

    You might contact the Corine Land Cover producers to see if anyone else has experienced problems with the data.

    You can rectify the vector data to have it stretched to match some control points or some other loaded data. This can cause a non-linear stretch and a much better match than a single linear shift. To do this, simply open the Overlay Control Center, right-click on the land cover layer, then select the option to Reposition the layer. You can then enter control points (4 or more would be best) to get the land cover data aligned with your other data, then press OK to complete the shift.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • popejpopej Global Mapper User Posts: 16Trusted User
    edited January 2008
    Re-rectify could solve my problem, I will check this. Thank you for advice.

    popej
  • popejpopej Global Mapper User Posts: 16Trusted User
    edited January 2008
    I was shown other solution. There is a free utility ogr2ogr that can convert datum for shp files. It's a part of FWTools:
    http://fwtools.maptools.org/

    After conversion map matchs very well existing GPS data. This solves my problem, but do not explain why there is offset in Global Mapper.

    Thank you for help,
    popej
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2008
    Popej,

    What kind of conversion are you doing in that tool that makes the maps then line up? Are you reprojecting the data or something or just changing the datum?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • popejpopej Global Mapper User Posts: 16Trusted User
    edited January 2008
    Actually I don't know exactly what this tool is doing ;-)

    The command for conversion looks like this:
    ogr2ogr -t_srs "WGS84" -s_srs 100KME48N35.prj new_dir 100KME48N35.shp

    After that I get new set of files in directory new_dir. Files are dbf, prj, shp and shx. Prj is changed into WGS84, shp has diferent data and dbf, shx have small changes.

    popej
  • Hans van der MaarelHans van der Maarel Global Mapper User Posts: 14
    edited January 2008
    I've been keeping an eye on this thread... just downloaded the CORINE data for all of The Netherlands, reprojected to Dutch RD and compared it to some reference data (not in Global Mapper by the way). There are errors there, but generally in the 50-75 meter range, something to be expected given the different capturing techniques.

    So does that mean I just got lucky with the quality of CORINE over here?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2008
    I'm beginning to suspect that there may be something amiss with the Lambert Azimuthal Equal Area projection in Global Mapper. I will try switching this to use a different library (PROJ.4 vs GCTPC) and see if I can get the CORINE data to match up.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2008
    I updated the Lambert Azimuthal Equal Area projection to use PROJ4 instead of the USGS' GCTPC library and now everything lines up perfectly. Apparently the GCTPC libraries implementation of that projection had some hard coded stuff that caused it to only work properly with some spheroids.

    I have placed a new build at http://www.globalmapper.com/global_mapper9.zip with the change for you to try. Simply download that file and extract the contents into your existing v9.xx installation folder to give it a try.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • popejpopej Global Mapper User Posts: 16Trusted User
    edited January 2008
    Yes, now all match correctly.

    Thanks,
    popej
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