Symbolization of Transport Features in a GIS

Author: Dr. Jean-Paul Rodrigue

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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:

• 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.

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.

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 highway system 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 improved.

2. Visual Resources

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:

• 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.

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 strategies:

• 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 or ownership.
• 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.