[OSM-newbies] new to OSM editing - advice on coping with gpx/POTLATCH offset

[Andy Robinson (blackadder-lists)]

It's totally normal. Each time you go out the satellites are in a different
orientation and you may be seeing an entirely different set from the
previous time you went on the route. Thus taking a trip exactly in the same
position you did previously will not result in the same track log as before.

If you get lots of tracks for the save road or feature then you will soon
find you can very accurately place it in the landscape. Both Potlatch and
JOSM allow you to see all the previous tracks that have been uploaded, which
is why we encourage everyone to upload their traces.

Newer GPS devices with high sensitivity receivers are also better than those
which are not or do not have SBAS (WAAS/EGNOS).

So you are right about the 5 to 10m accuracy, but then that's more than good
enough for OSM's use.

Another point to note is when walking. If you hold the GPS in your hand you
may find your trace is quite ragged. This is because without thinking about
it as you walk you actually turn sideways a lot. This changes the satellites
that are in view and degrades the quality (relative position) of the track.
On a bike or in a car it's much less noticeable because the rotation is
rarer. To get around the worst of it when walking try attaching the GPS to
the top of a rucksack or the arm strap of the same as this seems to reduce
the problem.

Cheers

Andy 

>-----Original Message-----
>From: newbies-bounces at openstreetmap.org [mailto:newbies-
>bounces at openstreetmap.org] On Behalf Of Mike Harris
>Sent: 17 January 2009 10:28 AM
>To: newbies at openstreetmap.org
>Subject: Re: [OSM-newbies] new to OSM editing - advice on coping with
>gpx/POTLATCH offset
>
>Hi!
>
>Personally, I wouldn't trust the gps signal (even with WAAS corrections) to
>better than 5-10 metres. I have tried walking the same route several times
>and this confirms the level of consistency. As a walker-mapper I often find
>it necessary to do a little interpretation. I tend to adjust an OSM feature
>where it seems sensible to do so. Many existing OSM traces have been done
>by
>cyclists (or even from a car on major highways) so a walker's record is
>likely to be more accurate. Nevertheless I never adjust by more than a few
>metres and at the most to half way between the original OSM position and
>the
>gps trace. If I walk the route again later I apply the same sort of rule -
>so there is a process of successive approximation! Of course, if I know
>from
>surveying that the existing OSM trace is seriously in error I would then
>make a correction and, if appropriate, added a note= to say what I have
>done
>and why.
>
>Hope this is a sensible approach!
>
>
>Mike Harris
>
>-----Original Message-----
>From: JW [mailto:jameswillmer at googlemail.com]
>Sent: 15 January 2009 00:48
>To: newbies at openstreetmap.org
>Subject: [OSM-newbies] new to OSM editing - advice on coping with gpx
>/POTLATCH offset
>
>Hi all
>
>I started small by adding some .gpx data (Blackberry curve / bbtracker.org
>app)
>to local maps in areas which i know well.
>This area has been mapped
>previously by someone using potlatch and the yahoo available satellite
>photos (UK)
>
>It turns out that there seems to be an
>offset between potlatch  locations and
>.gpx data i record - maybe 5 to 10m or so.
>
>
>There are a few courses of action
>
>1) trust my phone gps .gpx data is correct
>and change all potlatch locations as
>i can..... this is not practical for
>a small edit added to a large mapped area
>
>2) don't worry about it this is
>within normal gps errors ..... just "interpret"
>where the footpath is and add it beside the potlatch road.
>
>3) something else?
>
>What do others do?
>
>Ciao
>JW
>
>
>
>
>
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