![]() | ![]() | ![]() | Tracks |
Each track point has the following information: time stamp, position, altitude and depth in metres. In the computation results there are six fields for each point: distance to next point, cumulative distance to next point, altitude in metres (a negative number means depth instead), time to next point, speed in the line to the next point and bearing to the next point.
Track can be subdivided into segments
by some receivers at points in which the GPS fix was lost. GPSMan
keeps this information internally and in files; when displaying a
track with segments on the map, the segments will not be
connected.
The edit window for tracks allows some operations on tracks that may be useful to clean uninteresting start or end parts of a track, or to compose a single track from several others. They are:
An elevation graph for a track
can be plotted, as a side-view or a perspective, from the track
computation window if there are at least
3 track points with a valid altitude field. They are similar
to the elevation graphs for routes.
Creating a waypoint from a track point can be done by double clicking with the mouse left-button on a track point listed in a track window. This will open, for edition, a new waypoint having the same coordinates unless there is already a waypoint being edited. If the track is currently on the map the number of each track point together with the track name will appear in the help balloon when the cursor is over the point.
A waypoint with average coordinates can be created from a track window. Its latitude, longitude and altitude will be computed as the averages of the latitudes, longitudes and altitudes of the track points. This will be useful for obtaining more precise coordinates for a waypoint by recording a track with the receiver standing still.
A track can be converted into a route by a simplifying algorithm that keeps a certain number of track points as waypoints of the new route.
The algorithm that was developed for this may be seen as a variant of the Douglas-Peucker algorithm for finding critical points in polylines (see, e.g., [Heckbert and Garland, 1997] or [Li, 1995]). It starts from a straight line between the first and the last track points; if the number of points to keep is greater than 2, any point that stands furthest from the line will be retained, and the line is replaced by two new lines, those from the first to the new point and from it to the last one. This procedure is repeated always replacing one of the lines for which the distance to an intermediate point is maximum. The review of [Heckbert and Garland, 1997] describes an algorithm by Ballard and Brown (published in 1982) that seems to be very close to this one.
The sequence available here shows examples of routes with 3, 6, 11, 15 and 31 waypoints obtained from a track having 739 track points.
Although GPSMan lets the user fix the number of points to keep between 2 and the number of track points, there is a maximum number of points per route depending on the GPS receiver. It should also be noted that the time needed to find the route will increase significantly with the number of points (although keeping all the track points will take only the time to create the new waypoints).
So that a choice may be made between different numbers of points, GPSMan
may be asked to display the route and also the track on the map on
the fly. When the user clicks the Ok
button, the map will be
restored, new waypoints will be created (with names of the form
ZT
n, with n a 4-digit integer), and a window will be opened
for editing the route with these waypoints.
This operation will fail if there is already a route being edited; however the new waypoints will not be deleted. GPSMan will create a new group with all the new waypoints for easier access.
A simplifed
track
can be obtained by the same simplifying algorithm that is used for
converting a route into a track. This may be
particularly useful if the receiver has limited storage for
tracks.
An animation
of the movement corresponding to a track can be viewed in the map
window (Animation
button in the track edit/show window). A
control window will appear that allows for (re-)starting, pausing, or
aborting the animation, for skipping to the next track point, for
setting the speed (the scale changes are exponential), and for
choosing whether the last point shown will be centred on the map
window. The default speed is that in the track: the delay between the
presentation of two consecutive points is the difference between their
time stamps. If a time stamp is not defined the default delay is 30
seconds. The state of the animation and the total (real) time since
the beginning (if defined) are displayed.
A track can be created from a route from the route window.
mig_at_ncc.up.pt
, DCC-FC & LIACC,
Universidade do Porto, 2003![]() | ![]() | ![]() | Tracks |