Inconsistent units on elevations from .gpx file

vieuxdelamontagnevieuxdelamontagne Global Mapper UserPosts: 66Trusted User
edited August 2014 in Technical Support
I have opened a .gpx file into a pre-existing GM workspace, and am finding that the elevations are being incorrectly interpreted as being expressed in medieval units (something GM calls "feet"). The definition of the .gpx schema states that "GPX uses the following conventions: all coordinates are relative to the WGS84 datum. All measurements are in metric units." Now, when I start an independent instantiation of GM and open the same file (with nothing else loaded), the elevations are correctly interpreted as being expressed in metric units (metres).

I think I know several ways that I could fix the problem, but I would rather understand what caused it, so that I can take preventive measures to assure myself that it won't happen again. I'll be glad to send the .gpx file on request, but it seems likely that by itself, that would not be very helpful. The pre-existing GM workspace in which the problem manifests itself is a mere 721 KB, but the files that it loads are numerous and large.

I am running GM 15.2 b052714, 64-bit.

Will be grateful, of course, for enlightenment.

--Vieuxdelamontagne

Comments

  • Thea_bmgThea_bmg Global Mapper User Posts: 38
    edited August 2014
    Hi Vieuxdelamontagne,

    By default, the coordinate system and units for a workspace are set to the coordinates of the first file loaded into it. The elevation units might have been previously set to feet in your old workspace, so it is displaying that elevation data as feet. Could you go to the Vertical Options tab of the configuration dialog box, and check if the Elevation Display/Export Units were set to feet? If so, try changing it to either metric or native overlay units. Metric would display all elevation data for points in your workspace as meters, and 'Native Overlay Units' would display all elevation data in the units native to that data layer. Does that correct the problem in your old workspace?
  • vieuxdelamontagnevieuxdelamontagne Global Mapper User Posts: 66Trusted User
    edited August 2014
    Thank you for this explanation, which I think is probably correct, although I don't know for sure--because all of the layers I have currently loaded use only metric units. However, I do have an elevation grid file--in metres--which was derived (using a separate instantiation of GM) from a lidar data file in which the units WERE medieval. So, although all of the layers and features currently included in the workspace are in metric units, I cannot be sure that GM--and the workspace--are not haunted by the ghost of a departed layer. This, obviously, may be an insidious problem, necessitating care on the part of the user.

    If I may be permitted a little whine, however, let me suggest that, whatever default GM may have established in its current workspace, one might expect that for a file in which units are specifically called out, GM would use that specification to override the default, for that file. While it is true that a .gpx file does not explicitly specify the elevation units, that is because the schema definition REQUIRES that elevations be in metres. In other words, if they ain't metric, it ain't a .gpx. Contrapositively, if it's a .gpx, the elevations are in metres.
  • MykleMykle Global Mapper User Posts: 446Trusted User
    edited August 2014
    Once a file of data is loaded, it is no longer bound by the import format. Otherwise, you should not be allowed to load a GPX file into a project that is defined in units other than GPX units (medieval or otherwise!).

    One of the primary benefits of Global Mapper is the ability to import data of any supported projection (and any others that you can properly define), and display all loaded layers in ONE projection (and units) of your choice.

    Yes, the first loaded layer will set the initial display projection. However, I would not like to see elevation units of a set of layers to use more than one unit. I expect to be able to set the display projection as needed, and to change the display as needed.

    Many export formats are rigidly defined, like GPX requiring WGS-84, Decimal degrees, elevations in metres. Other formats can be exported in the current display projection. I frequently use GPX and KMZ (rigid formats), and I frequently use formats where other units are required and appropriate (CSV and ASCII).

    Yes, I frequently check to ensure that my current display projection is set as specified for the current project. And, that varies between projects. Woe to me, and a lot of rework, if I don't check. Frequently.

    We have project managers who don't define the required projection at the start of a project. Guess who gets to re-process the data when you didn't guess correctly.

    Many parts of the world have an extensive library of legal documents that use measurements in older units of measure. Many of them continue to require new measurements in those units. The same is true of datums (data?). Your perspective (and mine) is not relevant.

    I suspect that many of us would have a lot less work if the world could agree on one datum and units of measure. Job security!

    (There is a major highway in my area that is the only one in the USA that is signed in kilometers. It is sounding like the politicians are going to get their way and force this roadway to comply with all of the others.)

    Your whine is noted and appreciated. However, ...

    Cheers,
    Mykle
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