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Old March 20th 17, 01:48 AM posted to rec.bicycles.tech
John B.[_3_]
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On Sun, 19 Mar 2017 10:29:45 -0700, Jeff Liebermann
wrote:

On Sun, 19 Mar 2017 08:34:51 -0700, Joerg
wrote:
Miles is bad. That means a position indication is useless for any
serious trail riding.


Not exactly. It depends on what you're doing. If you try to plot
your ride, you might get some screwed up data points mixed in with the
data. For example, my last hike into the bottom of a local canyon
yielded a maximum altitude of 3,000 feet higher than the ground. There
was only one or two bad data points, but it was enough to screw up all
the statistics. Same with maximum speeds traveled, where the distance
covered between a real position indication and a bogus point or two is
high enough for me to claim breaking the sound barrier.

Mapping software authors know about all this and do their best to
compensate. The most common and best method is to do a sanity check
on all positions. If the GPS suddenly claims you've instantly moved
many miles, that point gets dropped. You probably won't see garbage
data on your smartphone or mapping GPS because of this feature. You
will see garbage if you use raw NMEA-183 data in some application. If
you want to see if you have a potential problem, just connect a data
logger to the GPS and collect some $GPGLL sentences. Write a program
that looks for large changes in adjacent sentences. The glitches, if
present, should be drastic and obvious.


I know nothing of U.S. mapping but I can assure you that using marine
charts there is a definite problem, in some cases, in comparing GPS
positions with existing charts. One of my friends favorite anchorages,
in the S. Philippines, is on dry land according to the current marine
chart of the area :-)

To the extent that some, perhaps many, charts include offset
information for use with GPS.
--
Cheers,

John B.

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