Line and vector related diagram maps allow to visualised how
much of something can be transported in which time or speed on what way
and in which direction by means of lines and vectors. Most of the time
not all of these variables are visualised together but rather only
relevant information are represented. The difference of line and vector
related diagram maps lies, among others, mostly in the emphasis of which
information is depicted.
For line related diagram maps the emphasis is the quantitative or qualitative information about the transported or moved object. Furthermore starting and ending points of the movements as well as the course are important components. Line related diagram maps are often used in the field of traffic and trade relations.
Vector related diagram maps emphasise on the directions of movements and forces by using arrows to visualised them. For example they are used to show wind directions or historical processes. The arrows can also show quantitative information like number of persons or streams of commuters.
The differentiation of line and vector related diagram maps is not always easy. Often combined variants are seen. This is the reason why these two map types are discussed together in one chapter.
Specific aspects of line and vector related diagram maps are explained below.
In line and vector related diagram maps movements are distinguished by clear symbols. The symbols can vary in colour, line width, brightness and texture.
The amount of the moved objects is one of the main points of a line related
diagram. It can be visualised by varying the line width or the number of parallel
unit lines (Imhof 1972, p. 96). The transition of different
amount values can be done in two different ways: the line width changes continuously
or in steps. Stepwise changes comply more with real transport situations and are also
prefered regarding the graphical visualisation.
Continuous changes are used to
generalise high numbers of successive change in the line width. To show the transport
volume even more clear, symbols on the lines can be used in addition.
In vector related diagram maps the quantitative value of the movement is often
secondary in contrast to the direction and the quality of the vectors, like for wind
maps. But if quantitative values are relevant, they can be visualised with different
line widths.
The direction of a movement is the main point of a vector related diagram map. Usually it is visualised with arrows. Different speeds can be displayed by a number of arrows in a row. If several parallel arrows are displayed, a decreased distance between them relates to a higher speed. If the duration is relevant, it can be noted as text along the vector or with timestamps on the arrows. In line related diagram maps the direction is mostly secondary, but could be also indicated with arrows.
A description of the band diagrams or vectors within in the legend is essential. The legend describes the meaning of value scales, stroke widths and accumulations and qualitative aspects of the band diagrams or vectors in the map. A stroke width can stand for a unique value or for a defined interval. The representation scale depends on the dataset and the map's statement. To create a clearly arranged map reduced dimensions of vectors and band diagrams would be reasonable. Therefore, a progressive scale progressive scale is appropriate. Thereby, the interval values increase while the modification of the band widths remain constant. Progressive scales hold the risk of a wrong interpretation of the map. Therefore, it is always important, to consult the legend while reading a map.
This paragraph deals with some important aspects which should be followed during the construction of flow maps.
In comparison to topographical linear embodiments, as for example
railroad lines or highways, vectors and band diagrams in flow maps don't demand
for spatial accuracy.
Usually, start and end point of a
movement plays a decisive role within band diagrams, the accurate line
management is often of secondary importance. In that case, a connection of the
two points by a straight or slightly bent line is enough to represent the
movement. In case the band diagrams refer directly to the transport routes (for
example roads, sea routes or power lines), the bands are drawn, highly
generalized, in place of the topographic symbols of the transport routes.
Depending on the bands width, additional displacement can be necessary to keep
the map readable.
Vectors usually indicate directions of
movements or transports, like wind or water movements, and do not refer to a
real world construction. In case, there is a relation between a real world
construction and the vector, it depends on its relevance, whether it is paid
attention to or not. If it is not relevant, the vector only shows the direction
of the movement. The following example shows that case: The commuter streams
don't follow the roads but show the direction of the movement.
Imhof (1972) differs between trade and transport
statistic data representation. A trade statistic representation provides
information about start and end point of a transport. These points are connected
by a straight line. There is no information about interstations or about the
covered route. Transport statistic representations refer strongly to the covered
route. Therefore, the bands or vectors follow the routes covered by the
transport or movement. This kind of data representation provides information
about quantities per route. It is often used for the mapping of traffic volumes.
The following examples show the difference between
trade and traffic statistic data representation methods. Move the mouse over the
graphics to get corresponding map examples.
Especially, if you use band diagrams it often happens, that several bands coincide in one node (e.g. traffic streams in a city center). The overlapping of the bands close to the nodes can cause display and readability problems, because it gets hard to distinguish the bands form each other. To minimize the problem, it is advisable to use preferably small band widths (Imhof 1972, p. 195). But still keep in mind, that the differences between the bands should remain recognizable.
An other graphical problem is the visualization of conjunctions
of band diagrams and local positioning symbols, like cities or
municipalities. You can reach appropriate displays by using simple
symbols like circles or squares. Furthermore, generalisation and
displacement can help to improve the readability.
Below, four different types of flow maps are introduced.