Global Mapper Pro

gm assisted georeferencing

wmiswm
wmiswm Global Mapper UserTrusted User
edited June 2014 in Suggestion Box
In georeferencing a raster map, it is up to the operator to find points to be used as GCPs. The GCP is usually a printed point in whatever shape or a cross of grid lines on the printed map. It sounds so natural for the operation, the operator can choose wherever he/she feel appropriate. However, prints cannot be perfect. Every line or point will have thickness (multipixel). The operator will have to judge for every GCP where he/she should click the mouse so as to get the most reasonable location for the GCP. This sort of eyeball judging is not only inaccurate, but is also very tiring and time consuming. The operator has to zoom in several times for a suitable magnification, then in the "area" of interest, he/she need to judge where is the appropriate point, move in the mouse to the exact location, then click the mouse to specify the point. For a few GCPs this is acceptable, but for restoration of ancient maps, very often, grids are in 1km interval, you can imagine how many such operations will be needed for a relatively medium sized city.

I would suggest GM to implement a function so that the operator just has to approximately choose an area where the GCP should be, and GM will calculate where is the most appropriate point, autofill in the easting/northing if it is a grid or user fill in easting/northing if it is a point), then the operator just have to click accept and proceed to the next GCP.

Comments

  • global_mapper
    global_mapper Administrator
    edited June 2014
    If you check out the shortcut key list at http://www.globalmapper.com/helpv14/Global_Mapper_Shortcut_Key_List.pdf you can see there are different behaviors if you hold down things like Ctrl, Shift, etc. when clicking in the rectification dialog. For example, hold down Ctrl when left-clicking in the Reference Map Pane and the Zoomed Pane will automatically recenter at the clicked location based on the current control points. This requires you to have provided at least 2 control points so some position can be determined.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • wmiswm
    wmiswm Global Mapper User Trusted User
    edited June 2014
    I am not talking about those shortcuts, they are of limited help in rectification.

    What I am suggesting is pattern recognition of crossing straight lines at expected positions where gridlines cross and to locate the center of the cross, and filling in the expected grid northing/easting readings, semi or fully automatic.

    In one working day, I can at most set about 900 GCPs, which is translated to only a 30Km x 30Km area at 1Km interval, an extremely tedious and monotonous exercise. However, this is a very powerful means to correct for both paper and scanner distortions, very useful for ancient map restoration jobs.

    I understand pattern recognition is not straight forward, and do not expect this can be implemented in the very near future. But if this can be available, rectification of printed maps with any sort of printed calibrations will just take some minutes, fixing a few initial GCPs, supply the grid interval, and accept one by one the automatically determined GCPs, may be with some adjustments at some points, supply the appropriate datum parameters, and a map of over 1000 sq Km will be perfectly georeferenced and distortions restored in less then half an hour.
  • global_mapper
    global_mapper Administrator
    edited June 2014
    I will add it to the todo list, but you are right, adding pattern recognition would be a pretty big feature so there would have to be a pretty big swell of interest.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
Sign In or Register to comment.