[Tagging] Micro Mapping, was Race track

Anthony osm at inbox.org
Mon Feb 1 03:31:20 GMT 2010

I've heard that before about GPS equipment, but I'm not convinced it a) is
true; and b) isn't easy to workaround, even if true.  The raw data received
by a GPS is timing data.  How can they mess up the altitude without messing
up the lat and lon?  And even if they can (presumably by lying about the
altitude of the satellites?), can't some sort of DGPS, using known altitudes
of fixed locations, be used to counteract the deliberate errors?

Granted, I don't think GPS is ever going to be a good way to get precise
altitude information.  As I understand it this is a matter of geometry and
that altitude error will always be worse than lat/lon error.  But I think
that's a good argument for not recording absolute elevation but rather
recording some sort of relative elevation.
http://www.na-motorsports.com/Tracks/NY/images/glen/elev.gif is a good
example of relative elevation.  Even if we just used a relation among the
nodes making up the track that would be quite useful.  We could go back
later and find out a precise elevation for one point on the track to convert
that to absolute elevation, but in the mean time relative elevation would be
quite useful.

On Sun, Jan 31, 2010 at 9:42 PM, John F. Eldredge <john at jfeldredge.com>wrote:

> It may be difficult to obtain GPS equipment that has accurate altitude
> data.  The GPS satellite system is maintained by the US military, and I have
> read that the altitude information available to civilian equipment has
> deliberate errors, in order to make it harder for terrorists, or non-US
> militaries, to use that equipment to plan out artillery attacks in advance.
>  The equipment used by the US military makes use of a more accurate, but
> encrypted, altitude signal.  In the US, at least, such equipment is
> classified and not legally available to civilians.
> --
> John F. Eldredge -- john at jfeldredge.com
> "Reserve your right to think, for even to think wrongly is better than not
> to think at all." -- Hypatia of Alexandria
> -----Original Message-----
> From: John Smith <deltafoxtrot256 at gmail.com>
> Date: Mon, 1 Feb 2010 12:10:59
> To: Tag discussion, strategy and related tools<tagging at openstreetmap.org>
> Subject: Re: [Tagging] Micro Mapping, was Race track
> The only other problem left to solve is the 3D bit, elevation could be
> added to nodes as well, but all we need then is GPS equipment that has
> more accurate elevation.
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