QCT problems

acpacp Global Mapper UserPosts: 2
edited May 2009 in Technical Support
I'm evaluating Global Mapper with a view to using it to work with some British Ordnance Survey QCT files, and I have encountered a couple of problems.

Firstly, it loads the maps with Geographic (Latitude/Longitude) projection, though the original image is intended for use with British Grid projection. This means the image is transformed and distorted, and I always have to convert the projection to British Grid after loading the file. I don't know if this is a bug, or whether the QCT file contains an incorrect projection code. If it's the latter, is there any way of forcing Global Mapper to use British Grid by default when loading these files, or could I edit the QCT files to set the correct projection?

Secondly, once converted to British Grid projection there is a border round the map. I don't think this border exists in the original QCT image, and it may be an artefact resulting from the re-projection process I suppose. But what makes it even more tricky is that the automatic cropping tool doesn't work to remove this border correctly - it crops large chunks of the map as well. However, I wouldn't be too worried about this if the border was avoided in the first place by loading with the correct projection.

Can you shed any light please?

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited May 2009
    QCT files provide their position via a list of coordinate values inside the file. If your file is automatically loaded using a Geographic (lat/lon) projection, this means that all of the coordinates listed inside the QCT file are lat/lon values. There is not any internal specification of what projection the data should be displayed in, so Global Mapper just displays the data by default in the native projection of the coordinates in the file.

    You can of course change the display projection on the Projection tab of the Configuration dialog. If you set that to British Grid prior to loading your QCT files they should automatically display in that projection.

    I would not expect any extra border to be added to the image just from reprojecting it, unless you are exporting it to a new file when reprojecting it in which case a border may be added to fill the reprojected result to a rectangle. Can you provide a screenshot of the border you are talking about?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • acpacp Global Mapper User Posts: 2
    edited May 2009
    OK, thanks for explaining how the default projection is determined. I'm new to Global Mapper and didn't realise that setting the projection in the configuration would then affect all subsequent map loads - that solves the problem.

    As for the border round the image, I can't now reproduce it. I may be going mad, but I'm sure I saw this recently, though it was in an older version. I've since downloaded the current version, which I see was built only 3 days ago, and it doesn't seem to happen any more. So let's consider that one solved as well.

    Thanks for your help.
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