[Tagging] Micro Mapping, was Race track

John Smith deltafoxtrot256 at gmail.com
Wed Feb 3 00:39:06 GMT 2010


On 3 February 2010 10:26, Anthony <osm at inbox.org> wrote:
> I'm not sure what GPS software you're talking about.  For recording of
> altitudes, I'm not sure GPS is not accurate enough to be very useful.  Also,

I already made that point.

> there's the issue of what vertical datum is being used.  For most GPSes it
> is easy to set the datum for the lat/lon to WGS84, but the vertical datums
> vary quite a bit, they're often hard to determine, and in many cases they
> can't be changed.

Can the elevation be set on any GPS units?

I assumed anything without an altimiter would simply use GPS derived
information.

> If we had measured the roads separately and given them absolute elevations,
> we might be off by 4 meters on overpass A, and 4 meters in the opposite
> direction on road B, and not even know which one is above the other.  And
> then when we drive on the road our GPS readings might be off by another 4
> meters.

Errors are irrelevent, as long as point A is > than point B you can
guess which one is higher than the other.

The point is giving GPS devices help, when said GPS devices are
capable of knowing where they are elevation wise, to know which layer
they are on, relative information won't help unless they already know
which way they are located on.

Think of the Japanese situation where there is multiple decks, how
does the GPS "know" which one the user is travelling on to tell them
the correct speed limit for example.




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