Nicolas Cadieux <njacadieux.git...@gmail.com> writes: >> For elevation, I read the spec as saying that the datum is "WGS84 >> orthometric height", meaning that one takes WGS84 ellipsoidal height and >> uses EGM2008 to get a height that is sort of "above sea level". The >> notion that the height is ellipsoidal height is to me unreasonable. > > If the standard says orthometric height, it means that it takes the > ellipsoïdal height and then applies the geiod model (in this case > EGM2008 or Earth gravitational model 2008). This is the height where > the average sea level would be given the local gravity on land. > Orthometric height is the geiod height or the height above the average > sea level.
Agreed but what it says is: <xsd:element name="ele" type="xsd:decimal" minOccurs="0"> <xsd:annotation> <xsd:documentation> Elevation (in meters) of the point. </xsd:documentation> </xsd:annotation> </xsd:element> To me, "elevation" always means some kind of orthometric height. I have never heard anyone call an ellipsoidal height elevation. Given the notion of WGS84 in GPX, and that WGS84 defines orthometric height, I find this unambiguous -- but not comfortably so. >> I would suggest to the Jeremy to understand the delta from "WGS84" to >> GDA94. I'm not a geodesy.expert.au, but my impresssion is that it's >> only a few meters and that it is therefore unlikely that points from a >> Garmin unit have errors that are small enough to notice that. I have >> not been able to notice the NAD83(2011)/WGS84(G1762) shift (about a >> meter) with L1-only navigation solution GPS. I can resolve it very >> clearly with dual-frequency multi-constellation RTK. > > In North American, most devices do not make a difference between Nad83 > (revised models) and WGS84 (revised models). I imagine this is > probably the case with GDA94, specially if GDA94 was identical to > WGS84 original in the beginning (i’am not sure this is the case, I > really don’t know here). Agreed. I think what you are saying is that when one asks a device to datum transform from WGS84 to NAD83 it will use null transform. >> Despite "GPX is WGS84", if the GPS receiver was receiving differential >> corrections, either locally or via SBAS such as WAAS, then the output >> coordinates are no longer in WGS84 and are instead in the differential >> system's frame. WAAS is I believe in something like ITRF2005, but it's >> very hard to figure that out precisely. (My understanding is that at >> least most of Australia currently has no available SBAS, but almost all >> measurements made in the US with navigation-grade equipment are with >> WAAS.) > > Weird... I would expect the coordinates to be a simple corrections of > whatever version of WGS84 is currently in use... I expected that too. It seems not to be though. The reference stations that generate the coordinates don't have a way to get precise WGS84(G1762) coordinates. And, GLONASS/Galileo/BeiDou don't use WGS84. It all amounts to a bunch of frames which are for practical purposes equivalent (ITRF2008 is a good overall description today, I think, but that and ITRF2014 are really close). My theory is that until you get to RTK, you just aren't going to get sub-meter. So worrying about which modern (>= 2005) flavor of WGSF84/ITRF/IGS is academic. If you can find a clear statement of what frame any SBAS uses, I'd love to see a URL/pointer.
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