Global Mapper v25.0

Importing Maps With NAD

Iceman
Iceman Global Mapper User
edited November 2012 in Technical Support
Hello Mike:

I am importing early 1920's maps that have a nice NAD lat\lon grid that I plan to use for rectification.
I read that NAD was established in 1913 & was the precursor to NAD27.
I cannot find this datum in GM.
Any advice on how to proceed will be appreciated.
Thank you.

Comments

  • global_mapper
    global_mapper Administrator
    edited November 2012
    I'm not aware of any good references on that datum, if you use NAD27 is it pretty close? I think that original NAD is based on the Clarke 1866 ellipsoid, so you might just create a new custom datum that is just the Clarke 1866 ellipsoid with no transformation, but I would first use NAD27 as I suspect it is pretty close.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • Iceman
    Iceman Global Mapper User
    edited November 2012
    In comparing a 1924 NAD map to a 1968 NAD27 map, the few road intersections I have appear to be about 100' - 200' off.
    Of course the rural roads could have changed by that much in 40 years.
    An internet search returned a 2003 USGS paper & a 1995 Corps of Engineers paper that discuss incorporating old maps based on NAD with newer data.
    They refer to a 1936 book that sits on a NOAA library shelf in Maryland.
    Apparently this book is helpful since it was reprinted in 1985 & 1999.
    Unfortunately it is only available to NOAA employees or NOAA contractors.
    I also downloaded a historical USGS GeoPDF central Texas topo map based on NAD to see what they used.
    As discussed in an earlier thread in April, the USGS is using that Canadian NAD27 datum.
    Thank you for you help.
  • global_mapper
    global_mapper Administrator
    edited November 2012
    If you can setup some known points in the old NAD datum and WGS84 you can define a new custom datum based on a text file listing the lat/lon in old NAD then the lat/lon in WGS84. Of course if your old maps are inaccurate it might be hard to find some good common points. I would suggest perhaps road intersections.

    Thanks,

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