Converting paper (scanned) contour topo maps into DEM

Hunter GeophysicsHunter Geophysics Global Mapper UserPosts: 75Trusted User
edited September 2013 in Elevation Data
Hi Mike,

I have a friend who has scanned a paper topo contour map of an area dating from the 1860s. Unfortunately, that is the only record of the site's topography and the area nowadays has been changed an awful lot by town development (so I can't go out with a GPS and collect new data).

Do you know if it is possible to convert this old contour map into XYZ points to create a DEM? Our ultimate goal is to perform View Shed analysis on the map.

It would be ideal if we could import the map (in some image format, probably a TIF), allow us to georeference it, and then for GM to automatically detect the contour lines and then allow us to define the elevations of each line (which GM would then apply throughout the image). Or, if automatic detection of contour lines is difficult, perhaps allow us to manually draw lines over the contour lines?

I realize this is a big ask and it may not happen until GM14 or later, if you do decide to implement it.

Thanks in advance. :)

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited June 2012
    David,

    You can certainly use the Digitizer Tool to trace the contour lines to create new lines and then assign elevation attributes to them with the height of the contours. Once you have the contour line layer simply right-click on it in the Control Center and select the option to generate an elevation grid from the data. You can then do a view shed of that.

    Typically though modern data like perhaps ASTER GDEM would be much better than an old survey from the 1860s. How does the data for your area available on the File->Download Online Imagery/Data menu command look?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • Hunter GeophysicsHunter Geophysics Global Mapper User Posts: 75Trusted User
    edited September 2013
    David,

    You can certainly use the Digitizer Tool to trace the contour lines to create new lines and then assign elevation attributes to them with the height of the contours. Once you have the contour line layer simply right-click on it in the Control Center and select the option to generate an elevation grid from the data. You can then do a view shed of that.

    Typically though modern data like perhaps ASTER GDEM would be much better than an old survey from the 1860s. How does the data for your area available on the File->Download Online Imagery/Data menu command look?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com

    Hi Mike,

    Thanks for your reply and sorry for my really delayed response - I didn't receive an email notification about your reply so I forgot all about this forum post...

    Regarding your idea of creating lines over the contour lines on the image: isn't the contour line the point where it changes value? So half-way between two contour lines is more likely to be where that specific elevation would be? So it would be better to make a bunch of points between each contour line with an average elevation of the two contour lines assigned to each point (and then create a DEM out of those points)?

    The problem with getting current data as you suggested is that the landscape has changed entirely due to construction over the last 150 years. The topography is completely flat now but was previously quited hilly.

    This is now more of a hypothetical situation as the project has been and gone. But I would still be interested in your thoughts. :)

    Thanks again,
    David
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2013
    David,

    The contour line should be where the elevation passes through that height, so along the contour the height should be exactly the contour height. In between the contour lines there could be any variation between the bracketing contour lines, so it's best not to assume too much about the values between the contours, only what range they are in. So if you digitize the contour lines themselves with elevations, then grid those you will get one interpretation of the surface assuming a pretty smooth transition between contour lines.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • Hunter GeophysicsHunter Geophysics Global Mapper User Posts: 75Trusted User
    edited September 2013
    Ah, I get ya. Thanks! :)
  • RMWRMW Posts: 1
    Is there a better/smarter/more efficient way of doing this now in 2018?
  • CarrickConCarrickCon Posts: 7

    G'day RMW.

    Not sure of the details of original request, but I'd suggest manually rectifying scanned image (jpg, pdf, whatever) of old contour map using GM "Image Rectifier" tool and then "vectorising" the contour lines based on line colour using the "Create Area Features from Equal Values" under the Layer menu. A good example of this is done during GM 2-day training course.

    Cheers,

    /al   

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