IEEE Robotics & Automation Magazine - March 2018 - 72

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covariance_psi_bpsi_b="0.1"/>

Code 3. (continued) The file representing the geometric map of Figure 3. After the initial information and the metadata, the
coordinate system of the map is specified, and the points and line segments composing the map are listed (see the text for further
details, and the complete file is available as supplementary material accompanying this article in IEEE Xplore).

Y
ψa

0
ψb

ρ
α
0

Figure 4. A line segment representation. In the XML data format,
t is rho in the standard XML format, a is alpha, } a is
psi_a, and } b is psi_b.

2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Figure 5. The example environment represented as a topological map
(nodes are the light blue squares and edges are the light blue line
segments). The local coordinate system is shown (the unit is meters).

of a line segment is expressed by listing the elements of the
covariance matrix relative to the four parameters.
Note that the geometric representation presented is just for
illustrative purposes and is not optimized. It is redundant
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IEEE ROBOTICS & AUTOMATION MAGAZINE

March 2018

because the points representing the corners of the walls and of
the obstacle could be derived as the points at which the line
segments touch each other (or, more realistically, at which the
distance between the line segments is less than a threshold). It
is possible to represent geometric maps composed only of
points or only of line segments.
We compared the size of the files containing scans acquired
by laser range scanners (which are point-only geometric
maps) represented either in the Player format [20] or in the
standard format. Results show that the size of the files represented with the standard is approximately three times larger
than that of the Player's files due to the overhead introduced by
the XML representation. Note that the current version of the
standard does not prescribe any way (similar to the blocks of
cells for grid maps) to limit the size of the geometric maps.
Representing Topological Maps
Consider a topological map of the same environment as in
the previous sections, as shown in Figure 5. The topological
map models the environment as a graph with some places
(which correspond to nodes) and their connections (which
correspond to edges). Code 4 shows the file representing
the topological map in the XML standard format. The
id is TopologicalMap, maptype 3 refers to topological
maps (line 7), and its coordinate system is aligned to an
arbitrarily placed global coordinate system (lines 11-14).
In the standard, each node is described by a symbolic id
and, optionally, by its location and its properties (lines 16-32).
In our example, we specify the locations of all nodes (as coordinates x and y in the coordinate system of TopologicalMap).
Custom properties of a node can be added by the user as a list.
Typical examples of properties are the sensor data collected at
the node location and the features extracted from them. The
algorithm shows an example of a property associated to node5
(lines 22-31), which represents the distance between the location of the node and the nearest obstacle in the environment
(lines 24-29). Note that the number of properties associated to
a node is represented explicitly by property_num.
Finally, (oriented) edges are specified by their ids, their tail
and head nodes (starting and ending nodes, respectively),
and, optionally, their properties (lines 34-56). Our example

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2018