[OSM-talk] GPS scribble in JOSM

[Ed Davies]

In response to the reappearance of my message about GPS tracklogs appearing
as "scribble" in JOSM Andy Robinson wrote:
> You have "Draw lines between raw gps points" turned on in preferences and
> this feature does not work with downloaded gps points from the server via
> the API. 

Yes.

> This is because all points from the API are delivered in bulk and
> while there is in theory some logical order there is no way for JOSM to
> understand where one track starts and another finishes nor to guarantee that
> the order of delivered points is logical. 

Yes, I agree that it's not JOSM's fault.  It does seem to be a pity
though that the server knows which tracklog each point came from and
uses a download format which can distinguish those tracklogs (GPX)
but it doesn't happen that way.

E.g., I just downloaded (using wget) a tiny area which I mapped on the
Reading weekend.  I went through the area more than once, turning the
logger off in between (for lunch).  The resulting GPX file is:

<gpx
     creator='OpenStreetMap.org'
     version='1.0'
     xmlns='http://www.topografix.com/GPX/1/0/'>
   <trk>
     <trkseg>
        <trkpt lon='-1.000222648' lat='51.456581354'/>
        <trkpt lon='-1.00084492' lat='51.456404328'/>
        <trkpt lon='-1.001118505' lat='51.456039548'/>
        <trkpt lon='-1.00153693' lat='51.45601809'/>
        <trkpt lon='-1.001311624' lat='51.455959082'/>
        <trkpt lon='-1.001070226' lat='51.45588398'/>
        <trkpt lon='-1.000903929' lat='51.456028819'/>
        <trkpt lon='-1.000737632' lat='51.456415057'/>
        <trkpt lon='-1.000598157' lat='51.456629634'/>
        <trkpt lon='-1.001263345' lat='51.45675838'/>
        <trkpt lon='-1.001708591' lat='51.456828117'/>
        <trkpt lon='-1.00015291' lat='51.456897855'/>
        <trkpt lon='-1.000308478' lat='51.456699371'/>
        <trkpt lon='-1.000297749' lat='51.456753016'/>
        <trkpt lon='-1.000265563' lat='51.456543803'/>
        <trkpt lon='-0.999820316' lat='51.456495523'/>
        <trkpt lon='-0.99987396' lat='51.456495523'/>
        <trkpt lon='-0.999836409' lat='51.456490159'/>
      </trkseg>
   </trk>
</gpx>

(That's with line-breaks and indentation added).  I assume that if the
server split this into multiple <trkseg> or <trk) elements and put the
points in the right order then JOSM would join them up properly.

In a response to the original instantiation of my message, Richard
Fairhurst replied:

# They're supposed to be ordered by timestamp, as Ed theorised (line 663
# of dao.rb). I've committed a change to svn that will order them by GPX
# id _then_ timestamp, which should hopefully remove the problem.
#
# Steve/Nick - could you deploy?

So, at least in theory, the problem is solvable but it doesn't seem that
this fix has been deployed yet.  It's not quite clear from Richard's
message if the result of his fix will split the segments up but just
getting all the data in the correct tracklog sequences would be a big
improvement.  The odd extra lines from the end of one sequence to the
start of the next wouldn't be too much of a problem.

> The "Draw lines between raw gps
> points" is only really beneficial for individual tracklogs (your own or
> public ones downloaded from the "GPS traces" page). Its most beneficial use
> is for those individual tracklogs which have sparse coverage in a small
> geographical area. Make it much easier to see the route the log takes.
> Personally though since I log at 1 sec intervals I don’t find I need to use
> this feature very often.

I've been using a Garmin 12XL which seems to log at irregular intervals
as it sees fit to get the best trace for the least memory use.  With this
the lines between points are very useful.

Ed.