ellipsoid height to orthometric height

turriperturriper Global Mapper UserPosts: 9
edited June 2013 in Elevation Data
Hi all

I have some gps points supplied as an XYZ .csv file

  • XY is in Cartesian British Grid format
  • Z is given as an ellipsoid height

I want to convert the Z values to orthometric height for use with Ordnance Survey maps (Ordnance Datum Newlyn). Currently, the difference between the mean sea-level based Ordnance Survey heights and the GPS ellipsoid heights is around 50m.

There is no reference to what ellipsoid was used but I've asked the company. This dataset is a few years old which is partly why I don't know much about it. I suspect heights would be based on the ellipsoid used by ETRS89 or WGS84.

Can this be done with global mapper?

I've just started reading properly about spatial reference systems today, so my apologies if I'm not understanding the principles correctly. I have the feeling that I'm unable to see the forest for the trees. :)

Thanks

Comments

  • turriperturriper Global Mapper User Posts: 9
    edited June 2013
    I've just spotted this thread with a similar question to mine.

    I shall try a simple offset. I think this will be ok because the points cover a relatively small area.

    It would be good to know if global mapper supports such conversions though, just in case anything's change din the past year.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited June 2013
    There isn't currently a way to do an automatic conversion between different height/geoid models, but for small areas if you know the approximate shift then usually a single offset is enough. In the near future there should be a version of Global Mapper with height model transformations available if you also have the Geographic Calculator product from Blue Marble.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • tjhbtjhb Global Mapper User Posts: 452Trusted User
    edited June 2013
    You can make your own conversion of heights in terms of the WGS84 ellipsoid to heights in terms of the EGM2008 geoid, by loading a conversion grid into GM and using its heights to apply an offset to the supplied Z values of the points.

    EGM2008 is a good global approximation of sea level, produced by the United States National Geospatial Intelligence Agency from satellite measurements of gravity. For you to judge whether it is fine enough for your purposes, but given that the target is to match Ordnance Survey heights (not survey grade), my guess is that it would be more than fine enough.

    You can download a grid from this page: NGA: EGM2008 - WGS 84 Version
    Try the small-endian 1x1 degree grid. You can apply bilinear interpolation to it in GM to get finer scale.

    (Getting the data into GM might be the trickiest part. I haven't tried for a while.)
  • turriperturriper Global Mapper User Posts: 9
    edited June 2013
    Thanks Mike. It's only a small area and not survey work so I will try a simple offset.

    Thanks very much for the help tjhb. I'm downloading these data at the moment. I sometimes wish my Uni course had more rigorous mapping modules sometimes. This pdf from Ordnance Survey has been quite useful.
  • AmitKumarAmitKumar Posts: 2
    Hi Mike

    Has this feature been implemented? I need to convert few DEMs from EGM2008 to WGS84 ellipsoid.

    Also, how do we use vertical files on this page for height model transformation?
    http://www.bluemarblegeo.com/products/vertical-files.php

    Amit

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