![]() | ![]() | ![]() | Tracks |
Tracks have a datum for all its points, a
colour
and a width (in pixels) used in displaying it in the map.
The colour will be sent to the GPS receivers supporting it, in which
process a colour matching algorithm will be applied if the original
colour is not in the set of colours accepted by the receiver. Any
colour matching algorithm may give unexpected results and the one used
in GPSMan is no exception.
Each track point
has the following information: time
stamp,
position
(always shown in the DMS format),
altitude and
depth
in either metres or feet, depending on the existing option for
this
. In the
computation results
there are six fields for each point: distance to next point,
cumulative distance to next point, altitude in
the unit
selected by the user
(a negative
number means depth instead), time to next point, speed in the
line to the next point and bearing to the next point.
A track can be subdivided into segments
by some receivers at points in which the GPS fix was
lost. The first track point always starts a segment, and any
other track point can be marked as a segment starter from the track
edit window by using the mouse right-button on the last column of the
points list.
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,
although the side-view graph can be plotted against time
instead of total distance if there is valid time
information.
A speed graph for a track
can be plotted from the track computation window if there are at least
3 track points with valid time information. It is similar to the
side-view elevation graph and can also be plotted against time instead
of total distance.
A climb rate graph for a track
can be plotted from the track computation window if there are at least
3 track points with valid time information. The graph is plotted
against time, the vertical units being (user selected) altitude unit
per second. If there is enough points a noise-reducing filter, kindly
provided by Paul Scorer, is applied to the data.
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 other line items
by a simplifying algorithm that keeps a certain number of the track
points as points of the new line, which can be a route, another track,
or a polyline. When converting to a polyline information on
segments is only used if all the points are kept.
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 simplified line will increase significantly with the number of points (although keeping all the track points will take only the time to create the new item).
So that a choice may be made between different numbers of points, GPSMan
may be asked to display the simplified line and also the original
track on the map on the fly. When the user clicks the Ok
button, the map will be
restored, the simplified line is used in forming a new item (in the
case of a route, with new waypoints having names of the form
ZT
n, with n a 4-digit integer), and an edit window will be
opened for editing the new item. If the edit window for the item type
was already in use, then the item is created under an
automatically generated name. For a route GPSMan will create a
new group with all
the new waypoints for easier access.
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 or a polyline
from the route window or the polyline window.
mig_at_ncc.up.pt
, DCC-FC & LIACC,
Universidade do Porto, 2004![]() | ![]() | ![]() | Tracks |