
658 IEEE TRANSACTIONS ON INTELLIGENT TRANSPORTATION SYSTEMS, VOL. 13, NO. 2, JUNE 2012
As has been discussed in a previous section, objects might
be partially obscured due to occlusions or range failures. For
example, when a car is only partially observed on one side, if
we do not assume any strong model on the car or we do not
even know that the object is a car, an estimation of its size and
center point could be quite unreliable. Partial observation could
greatly affect the reliability of feature parameter estimation and
could subsequently reduce accuracy in tracking moving objects.
Developing an object model to support robust feature parameter
extraction on partial observation data is key to this procedure,
as well as for building a tracking module.
In addition, a desirable detection result is that all moving
objects be successfully detected with a minimal number of false
alarms. When a moving object enters the area of laser coverage,
software can easily find the object as long as it gives a reflection
to the laser beam. However, because an object might be simul-
taneously measured by different laser scanners and because the
contour points of an object might be spatially disconnected due
to occlusions or range failures, multiple alarms could arise from
a single moving object. To reduce multiple alarms and have
more complete knowledge for feature parameter estimation,
developing an algorithm of grouping the measurements from
different laser scanners into the same object is another key to
this procedure.
In the succeeding sections, we define an object model, we
address a method of grouping the measurements from different
laser scanners to detect moving objects in the environment
and extract their feature parameters, and we conclude with an
experimental result that verifies the algorithm.
A. Object Model
Fig. 5 describes a special characteristic of laser measure-
ments. Suppose that a laser scanner performs counterclockwise
scanning, and the horizontal contour of a car is measured by a
sequence of laser points from s to e [see Fig. 5(a)]. Simplifying
the shape of a car using a rectangle, edges that represent two
vertical sides of the car can be detected through a corner
detector and a line fitting on the laser points. A directional
vector u
i
that is associated with each edge is defined according
to the scanning order of laser points, e.g., from a point measured
later to one measured earlier.
Let u denote a directional vector that is extracted from the
data of a single laser scanner after its alignment to a reference
frame. We found that no matter where a laser scanner is placed,
directional vectors u are equal if they are observations on the
same side of the object [see Fig. 5(b)]. Suppose that v
i
s, i =
1,...,4arethedirectional vectors defined on each side of a
car, and they compose a counterclockwise loop. By matching u
with v
i
s, we can find which side of the object is measured so
that the laser points that correspond to u are used to update the
estimates of that specific side. In this work, we call u a support
vector of the side v
i
.
Based on t he aforementioned considerations, an object model
is defined in this work with the feature parameters shown in
Fig. 5(c), where the shape of an object is simplified using a
rectangular model. Developing a more accurate model for each
kind of object will be addressed in future work. In addition,
Fig. 5. Definition to an object model. (a) Measurement to a car from a single
laser scanner. (b) Measurements to a car from a network of laser scanners.
(c) Feature parameters of the object model.
TABLE I
P
ARAMETERS IN AN OBJECT MODEL
a reliability item is defined for each feature parameter for the
sake of partial observations (see Table I). Currently, reliabilities
are estimated with binary values, i.e., true =1or false =0.In
the case of a directional vector, the reliability denotes whether
the side has a support vector (reliable)ornot(unreliable).
In the case of a corner point, if both neighboring sides are
supported, the corner point is a (reliable) one; otherwise, it is
a guess through other feature parameters on the object model
(unreliable). In the case of dimensional size, reliability tells
whether the corresponding feature parameter represents a full
dimensional size (reliable) or perhaps a partial one (unreliable).
In the case of a center point, which cannot be directly observed,
the reliability denotes whether the coordinates are estimated
from other reliable feature parameters. A more detailed de-
scription of each parameter in an object model can be found
in Appendix A.