DWG and projection

Georges MajerusGeorges Majerus Global Mapper UserPosts: 5
Hello all,

we get DWG drawings on a regular basis that are supposed to be georeferenced. In one particular case I know they are but when I open the DWG in Global Mapper 20 I get a message that the file does not contain projection data and that I should specify the projection system. I am then loading the Luxembourgish Grid Reference EPSG 2169 and I can load the file into GM. However if I  try to export that information for use in another tool there will be no georeference and the file won't show up where it is supposed to show up and if it shows up it's all distorted. I am puzzled to say the least and for a while now I have not found any plausible reason or solution. I read somewhere that you need an Autodesk program to be able to use georeferencing with a DWG file. Any ideas or help is highly appreciated !

Kind Regards from Luxembourg.

Georges
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Best Answer

  • sphillipssphillips Posts: 111
    Answer ✓
    Hi Georges,

    DWG and DXF files do not contain any information about the coordinate system / projection that non-Autodesk software can read, so it will usually prompt you for the projection. Essentially it can only ready the X, Y and Z values and will draw the data using these. Autodesk products such as 'AutoCAD Civil 3D' and 'AutoCAD Map 3D' can export 'Map Information' but as far as I'm aware this information can only be read by Autodesk products.

    Note: You can actually open and search a DXF file with a text editor and read this information if it exists in the file (search for 'encoding' and it will usually be on the same line).

    If the DWG/DXF has been sent with an accompanying *.prj file with exactly the same name then GM will read the projection and will not need your input. For example a folder contains two files; one called TestData.dwg and the other TestData.prj.

    If the data you have exported is in the wrong place then this will be due to using the wrong coordinate system somewhere along the way. If the data is stretched and in the wrong place then this might be a symptom of using a geographic coordinate system instead of a projected coordinate system. For example, if you setup your workspace using the default Projection; which is Geographic (Latitude/Longitude) and then importing the DWG's and specifying a projected coordinate system e.g. UTM then the data will be highly distorted. When exporting it will use the projection of the workspace and not the data.

    To avoid this make sure you setup your projection first (the projection you intend to export to) and then when importing the DWGs make sure you use the correct projection (the projection used for the incoming data).

    Cheers,
    Steve


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