[josm-dev] Display issues with downloaded raw GPS data

[Frank Cremer]

Hi,

I seem to have a problem with the downloaded raw GPS data. I am relative 
new to OSM and JOSM (only a few days), but have quite a bit of 
experience with mapping/surveying/GIS. So, it may be that I am doing 
something wrong, or missed the obvious...

When I download raw GPS data within JOSM (directly from the server), it 
displays the tracks, but also seem to be connecting track points that 
should not have been connected. This is very confusing as there may be 
many long lines crossing most of the area. I know that it is possible to 
switch of the drawing of all lines, but for mapping it helps to know 
which points are connected within a track.

By saving these downloaded tracks, I have identified the problem.  The  
GPX file contains only one track (<trk>), whereas the underlying data 
appears to be constructed from several tracks. So what happens is that 
the last point of one track is connected to the first point of the next 
track. This connection is drawn by JOSM as a line, which actually should 
not have been there.

I am not sure about the cause of this problem. It may be that the server 
is providing the GPX data correctly in independent tracks and JOSM does 
not handle this properly. Or it may be that the problem is within the 
server.

As a work around I have created a little Octave (a open source Matlab 
clone) script that load the GPX data and if there is a jump of more than 
50 m between track points it creates a new track. The script is provided 
below.

If need be, I can help to implement a solution (either as plug in or 
change of code), but before that it's better to know what is the actual 
root of the problem. So if anyone has an idea (or can give tips on how I 
should use JOSM to avoid the problem) that would be great. Thank you.

Best regards,


Frank


------------------------------------
# fix_gpx.m
#
# This Octave script loads an existing gpx file and creates an output file
# that has new tracks if the distance between track points is larger 
than 50 m.
# It can be used to clean up the downloaded raw GPS data in JOSM.
#
# To run it, use e.g.:
#    octave --silent fix_gpx.m Track.gpx
# It will create a file named "Track_fixed.gpx" as output.
#
# Please note that:
#   1. It requires Linux/Unix (for loading/processing GPX files with 
grep/cut).
#   2. It uses a brainless method for reading GPX files that assumes 
that the GPX
#      file contains lines with 'lat="<data>" lon="<data>"'.
#   3. The "headers" are copied, but all other data is ignored (like the 
time).

# Create the output file
fname=argv{1};
i=findstr(fname,'.gpx');
fno=sprintf('%s_fixed.gpx',fname(1:i-1));

# Copy the header of the input file to the output file
fi=fopen(fname,'rb');
fo=fopen(fno,'wb');
start_track=0;
while !start_track
    line=fgetl(fi);
    if isempty(findstr(line,'<trk>'))
        fprintf(fo,line);
    else
        start_track=1;
    end
end
fclose(fi);

# Read the file using grep/cut (much faster than processed line by line)
cmd=sprintf('grep lat= %s | cut -d\'"\' -f 2,4  --output-delimiter=\' 
\'',fname);
fi=popen(cmd,'r');
p=fscanf(fi,'%f');
p=reshape(p,[2 length(p)/2])';
fclose(fi);

# Approximate conversion factor from degrees to meters
pm=mean(p);
a=[1 cos(pm(1)*pi/180)]*40e6/360;

# Estimate the distances between subsequent track points
dp=p(2:end,:)-p(1:end-1,:);
dp=[a(1)*dp(:,1) a(2)*dp(:,2)];
d=sqrt(sum(dp'.^2)');

# Get a list of all distances that are larger than 50 m
f=find(d>50);
f=[f;size(p,1)];

# Output new tracks if the distance is larger than this 50 m
s=1;
for k=1:size(f)
    r=s:f(k);
    fprintf(fo,'<trk>\n<trkseg>\n');
    fprintf(fo,'<trkpt lat="%.6f" lon="%.6f"></trkpt>\n',p(r,:)')
    fprintf(fo,'</trkseg>\n</trk>\n');
    s=f(k)+1;
end

# Properly close the file
fprintf(fo,'</gpx>\n');
fclose(fo);