<div dir="ltr">Thanks for explaining why my android phone says I am at +38m (+/- 3) in my backyard when in fact it is at Dutch sea level -4.4m.<br clear="all"><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br></div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">Best, Peter Elderson</div></div><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">Op ma 4 mei 2020 om 02:39 schreef Greg Troxel <<a href="mailto:gdt@lexort.com">gdt@lexort.com</a>>:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Martin Koppenhoefer <<a href="mailto:dieterdreist@gmail.com" target="_blank">dieterdreist@gmail.com</a>> writes:<br>
<br>
> I’m asking for comments on <a href="https://wiki.openstreetmap.org/wiki/Proposed_features/ele:regional" rel="noreferrer" target="_blank">https://wiki.openstreetmap.org/wiki/Proposed_features/ele:regional</a><br>
<br>
Two big comments:<br>
<br>
First, the current wiki documentation about ele and Altitude should be<br>
really straigthened out, so that we have a basis for what we are<br>
comparing to.<br>
<br>
Second, the notion of a single regional vertical datum doesn't really<br>
work. In the US, that could be the old NGVD29, or the current NAVD88.<br>
Plus, we are about to get NATRF2022. However, all of these are within<br>
a meter or so, and in terms of vertical data in OSM, that's not really<br>
a big problem. So if there is going to be precision, then we should<br>
follow GIS and have an explicit datum. I would say an EPSG code is<br>
sensible -- see the proj package for canonical values.<br>
<br>
<br>
<br>
As for ele/Altitude, there is great confusion out there about "WGS84"<br>
and two separate concepts:<br>
<br>
height above the ellipsoid. Often written HAE. The ellipsoid is a<br>
mathematical surface that is NOT a surface of equal gravity. While<br>
geodesists and geodetic surveyors use it, and while it's part of the<br>
calculations within GPS, I am not aware of a single vertical datum in<br>
use in any country that is relative to the ellipisoid. Note that<br>
water does not flow "downhill" when "down" means to a lower value of<br>
HAE. Water is hugely important in elevation and mapping.<br>
<br>
height above geoid, or height above a reference equal-gravity surface,<br>
or height above sea level. (Yes, I realize that "sea level" is a huge<br>
can of worms.) This is more or less what every height system uses or<br>
intends to use.<br>
<br>
<br>
In WGS84, one gets from the base computation lat/lon and a height above<br>
the ellipsoid. This is purely a geometric answer and is totally<br>
disconnected from grravity. Then, GPS receivers use a gravity model to<br>
compute the offset from the ellipsoid and the reference gravity surface<br>
(which is more or less the "sea level surface"), and they them use that<br>
to get a "height above sea level". Receivers that display to humans<br>
display this sea level height. NMEA has that same sea level height.<br>
<br>
(Android stands alone in that the API returns height above ellipsoid.<br>
That's not wrong, but it is unusual. IMHO how Android defines an<br>
interface is irrelevant to OSM's definitions.)<br>
<br>
<br>
When people say "WGS84 altitude", they mean the height above the WGS84<br>
equal-gravity surface as computed from the ellipsoidal height and the<br>
gravity model. This is sort of 0m at sea level. Note that the<br>
ellipsoid can be 100m different from this equal-gravity surface, and is<br>
often significantly different. It's ~30m in Boston and I hear it's 50m<br>
in Switzerland. Nobody who says "WGS84 altitude" really means "WGS84<br>
ellipsoidal height". If they did, they would say "WGS84 ellipsoidal<br>
height".<br>
<br>
<br>
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</blockquote></div>