What ellipsoid or geoid for GPX file elevations-and how do you know?

I have been poking around and have been unab开发者_如何学编程le to find a standard for what ellipsoid or geoid corresponds to "0" elevation in GPX files. Is there a standard? Do I just assume the WGS84 ellipsoid? Or if different standards are used, is there a way of figuring out from the file which one was used? Or making an educated guess?

And what is the source for your answer? I would love to have an authoritative reference.

I clipped the following from the beginning of the the official GPX 1.1 Schema

<xsd:schema targetNamespace="http://www.topografix.com/GPX/1/1" elementFormDefault="qualified">
    <xsd:annotation>
        <xsd:documentation>
          GPX schema version 1.1 - For more information on GPX and this schema, visit http://www.topografix.com/gpx.asp  GPX uses the following conventions: all coordinates are relative to the WGS84 datum.  All measurements are in metric units.
        </xsd:documentation>
   </xsd:annotation>

which tends to support your assumption. Later in the schema explicit reference is made to the vertical datum to be used.