GPS to make topo

kyletownsendkyletownsend Global Mapper UserPosts: 2
edited September 2009 in GPS Features
I want to capture GPS data to create topo maps with global mapper. I want to make high resolution small scale maps. I need the most accurate GPS for this, especially for elevation data. Ideally, I would like to be able to achieve 1 foot vertical resolution relative to a local reference point. I was thinking that one of the Garmin GPS that has a built in barometric altimeter would be the best choice. But I don't know if the GPS outputs baro-corrected elevation data to the .GPX file or if the .GPX file even captures elevation data.

Anybody know about this?

Which GPS would you suggest?


  • Roger EdrinnRoger Edrinn Global Mapper User Posts: 721Trusted User
    edited June 2009
    Which GPS would you suggest?
    The best any Garmin will do is + - 20 feet elevation.

    Think Trimble and think $1000 where the first digit is not a one.

    Have fun!!! ;)
  • kyletownsendkyletownsend Global Mapper User Posts: 2
    edited June 2009

    Thanks for the info.

    I believe garmin's manuals specify 10' accuracy on the baro equipped models. I have seen some indication that they actually achieve 1' relative precision as long as they are baro-corrected every 15 minutes. I have a precision calibration source from an old airport that I can use to do the periodic calibrations in the field.
  • HowardHoward Global Mapper User Posts: 10
    edited September 2009
    Ideally, I would like to be able to achieve 1 foot vertical resolution relative to a local reference point.

    I have a Garmin Vista HCx, its baro calibration goes to 2 decimal places in millibars. Each 1 millibar is about 28 feet in height, so 1 foot is about 0.036mb, implying that it will do what you want, but I doubt it.

    A fragment of a gpx file from this GPS has
    <trkpt lat="51.604711" lon="0.021709">

    again implying that elevation is "accurate" to 2 decimal places. However, I doubt that very much. In the best aviation barometric altimeters, plus or minus 50' is added by pilots to allow for error, so I would expect the reproducible error from the Garmin to be no better than a handful of feet at the most optimistic. I just tried it walking down some stairs and back up, it certainly came back to its original reading, but as baro pressure changes quite fast, especially when a low pressure system is approaching, very frequent re-calibration is required. Pilots have the current pressure passed in most RT calls (QNH), even if just set, because it can change quickly.

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