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Conversions from NGVD 29 to NAVD 88.

TraceyTracey Global Mapper UserPosts: 1
edited September 2011 in Elevation Data
Hi,

There seems to be some confusion where I work regarding the reliability of elevations when converting from NAD 29 to NAD 83. My understanding is that they are running through muiltiple conversions, turning rasters into shapefiles, then DXFs in order to create a series of points that can then be run through CORPSCON in order to compensate for the improved elevation when going from NAD 27 (which uses NGVD 29) to NAD 83 (which uses NAVD 88).

Furtunately, my new employer agreed to purchase Global Mapper for my use, as I grew very fond of the program at my last job. My understanding is that when I reconfigure the projection in Global Mapper, going from NAD 27 to NAD 83 that these elevation corrections are made automatically, without the need for all of the excess processing through various other programs.

Am I correct?
Thanks

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited June 2008
    That is actually not correct. Global Mapper does not support any automatic vertical datum conversions. It will of course convert the XY coordinates correct between horiziontal datums (i.e. NAD27 and NAD83). For vertical datums there is not any support for automatic conversion of elevation values.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • DC KelleyDC Kelley Global Mapper User Posts: 77Trusted User
    edited September 2011
    Mike: I know this post is a bit old but is the statement "Global Mapper does not support any automatic vertical datum conversions" still true?

    If I import a USGS elevation data plots from the on-line sources that GM supports and then I look in the small insert frame at the bottom of the window for the value at a point, am I not seeing these values expressed wrt to WGS84 ellipse (when expressed in WGS84 otherwise) ?
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited September 2011
    That is correct, no vertical datum conversion is available yet. Any elevation values will be expressed in whatever vertical reference system they are provided in, which is independent of the horizontal projection system.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
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