Jean-Paul Rodrigue (2017), New York:
Routledge, 440 pages.
Symbolization of Transport Features in a GIS
Author: Dr. Jean-Paul Rodrigue
1. Cartography and Symbolization
Cartography is a communication tool that conveys a message
to a public through a medium; the map.
Cartography is the art and science of expressing graphically
the physical, economic and social features of the earth.
The better the cartography, the more likely that this message will
be conveyed effectively. Some forms of communication are better than
others, so all maps are not equal, even if they could be representing
the same features. Since many transport projects have a high visibility
and significant capital costs, it is surprising that the usage of visual
resources, particularly of cartography, is often neglected or not used
properly. The cartographic quality of many transport analyses is commonly
poor. This stems from the fact that many transport practitioners are
engineers or economists by training, disciplines in which cartographic
expression is not emphasized or even considered. Among transport geographers
using GIS-T (Geographic
Information Systems for Transportation), the cartographic output is
also commonly neglected, again an outcome of the priority placed on
analytical methods. Even if cartography does not appear to be a feature
which is analytically strong (in contradiction to the GIS packages that
produce them), proper cartographic expression has become of crucial
element of transportation research, particularly because of the following:
Maps are using visual communication tools, thus implying that cartography
is at the same time an art and a technique. It is an art since
it is a visual expression; every map is to some extent a form of art
that seeks to esthetically please its audience. Considering maps as
an artistic expression is often seen with a level of suspicion among
practitioners. It is often perceived that the quality of the container
is inversely proportional to the quality of the content. Cartography
is also a technique since it abides to a set of rules and methods pertaining
to the visual symbols it uses; their placement, the choice of colors
and their size for instance. Cartography is a process of abstraction,
also referred as symbolization, which uses a set of defined graphical
elements to communicate a message.
- Transportation systems, notably networks, are complex entities
and the map offers a powerful medium to visualize them. Thus,
cartography can be seen as a synthetic tool.
- Transportation is a field of application which is often planning
driven. As such, many projects require the approval of various private
(funding) and public (regulation) entities, and sometimes with the
general public involved. Maps are thus a medium that can be used
to explain the nature of a project and help persuade an audience.
Symbolization is the set of graphic methods used to convert
cartographic information into a visual representation.
Symbolization implies that the features on a map to be generalized
and simplified since not all possible elements are relevant to the
message a map conveys. It thus helps the message to be easier to understand.
For instance, a map depicting an
often ignores all the roads of lesser importance, thus underlining the
feature it seeks to emphasize.
With the maturation of GIS in recent years, the generation of maps
has become a simpler and straightforward process. Graphic design capabilities,
which were found lacking in earlier packages, are more extensive. GIS
enable to produce maps at a very low cost and in large quantities. In
addition, more information is available from a variety sources, particularly
in numerical format. Several databases and basemaps are made available
at virtually no cost. The Internet has become a massive distribution
medium of graphical images such as maps and enables access to a wide
array of publicly available databases from international, national and
local institutions. Many public or private agencies, from newspapers
(e.g. the New York Times) to government offices employ professional
cartographers and the quality of the cartographic output has considerably
GIS automate several aspects of the cartographic process and assist
cartographers for tasks that previously took a lot of training, time
and manual expertise. The creation and revision process of maps is improved
since previously created maps can be stored, retrieved and modified
to suit new purposes. The layout, the composition and the symbolization
can be modified at will. It is important to stress that GIS do not per
se make good or bad maps, cartographers do. Consequently, the appropriate
usage of visual resources is the
first step in the efficient cartography of the transport phenomena.
The rapid diffusion of GIS and the improvement in computerized visualization
techniques offers transport practitioners many opportunities to improve
the visual quality of their work. This begins with the usage of visual
resources, mainly two basic ones:
Raster information, since it is grid-based, can only modified through
its color hue and intensity. For
cartographic purposes, visual
resources can be used to represent location, direction, distance, movement,
function, process, and correlation. On most maps, including those related
to transportation, several elements, such
as title, scale and legend are almost always present. How all these
elements are positioned on a map, also known as map composition, depend
on the nature of the message as well as the potential audience. Each
cartographer has his/her own visual style.
3. Symbolization Strategies for Transport Attributes
Transportation deals with a set of issues that rely on a specific
range of symbols. Most of the symbolization deals with networks, which
are features that are commonly represented with lines and points (see
graph theory). Other
symbolization strategies, such as choropleth maps, are common with standard
cartographic methods. The following are the most common symbolization
- Color resources. Considers the hue, texture and intensity
of colors. A hue refers to the gradation of color within the optical
spectrum (visible spectrum) of light. The texture is the variety
of patterns that can be used to fill a shape, such as hatches, cross-hatches
or dot density. The intensity is the relative saturation of a color,
on a scale from bright to dull. Color resources are particularly
useful for category ranges.
- Shape resources. Considers the wide variety of geometric
figures available. In a vector-based GIS, shapes are mainly represented
as points, lines and polygons. These shapes can be modified in terms
of their nature, size and orientation.
- Nominal. It includes only names, which are the result
of a classification. These names are not ordered in a specific way;
rather they describe different categories of the same rank. So,
the only conclusion to be made is the inequality of each class.
Transportation infrastructures are particularly suitable for nominal
representations. Networks and terminals can be classified by function
- Ordinal. Result of placing descriptive categories into
a formal order which enables a comparison of rank without providing
any information about the extent of the difference. There is thus
an implicit qualitative order between classes. Networks and terminals
can be classified by size, level of importance or congestion.
- Interval. An interval scale is the result of arranging
values on a scale with point of reference and a unit of measure.
These scales are quantitative, which means some computations are
allowed, namely how the ranges between classes are set. The level
of traffic on networks and terminals can be categorized.
- Proportional symbols. The size of a symbol is a function
of a quantitative variable. Thus, the radius of a circle or the
thickness of a line can be set according to a variable. For transportation
systems, proportional symbols are particularly important to express
flows in a network or at terminals.
- Labeling. Involves the positioning of descriptive text
over specific geographical features. The labeling process which
is particular to transportation mainly concerns assigning identification
symbols (dominantly numbers) to road segments.