Properties of line and vector related diagram maps

Specific aspects of line and vector related diagram maps are explained below.

Quality

In line and vector related diagram maps movements are distinguished by clear symbols. The symbols can vary in colour, line width, brightness and texture.

Quantity

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.

Direction, velocity and duration

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.

The duration is indicated by arrows (Spiess 2004)

Legend

The explanation of the lines and vectors in the legend is inevitable. The unit of value, line widths and the qualitative divisions should be illustrated. The line width can visualise both, a fixed value or an interval. The unit of values is related to the displayed dataset and the message which should be communicated via the map. To save space and to make the map more readable, progressive scale progressive scale are useful. Within a progressive scale the value interval increases progressively, but the change of the line width remains constant. The danger is that the user may incorrectly interpret the map. To read the diagrams correctly, the legend must be consulted.

Legend of a flow map (Lehmann 1972)

Design of line and vector related diagram maps

This section includes important aspects which should be respected when designing a line or vector related diagram map.

Precise representation in plan of the outline of a feature

Line and vector related diagram maps, unlike topographic line representations like railways or roads, do not claim to be precise in the representation in plan of the outline of a feature. Starting and ending points of movements are often the main content of line related diagram maps, but the precise course of the line is mostly of secondary importance. If this is the case, a straight or slightly bended line between the two points is sufficient. If the lines are directly related to the transport route (roads, sea route, power line, etc.) they are displayed heavily generalised in the place of the topographic symbols. Depending on the line width additional shifts in positions are necessary to provide a readable map.

Vector related diagram maps emphasise on the direction of the arrows which represents only a direction but not a real linear arrangement. If there is a relation to a linear object in the real world, but which is not important, only the approximate direction of a movement is depicted. The precise course can be neglected. This is the case in the following example. The commuter flows are not shown along a road but show only the rough directions.

Visualisations of trade and traffic statistics

Within line related diagram maps Imhof (1972) distinguishes between the visualisation of trade and traffic statistics. For visualisations of trade statistics only the start and ending points of a transport are important. The exact route between these points can be neglected. Therefore the start and ending point of movements will be connected when visualising trade statistics. The diagrams show the value of the moved goods. The exact route can not be understood. In contrast the exact routes play an essential role when visualising traffic statistics. Therefore not only a simple starting point to endpoint connection is visualised but also the route in-between. The transported volume is depicted for each section. This type of visualisation is often used to show traffic volumes. The following examples will show the difference between visualisations of trade and traffic statistics. Hover the mouse over the graphics to see the corresponding examples.

Nodes

If several lines of a line related diagram map connect to each other in a node (e.g. traffic streams into a city) it often results in graphical conflicts which impair the readability of the map. The closer you get to a city, the higher is the traffic volume which results in a thicker line width what makes it difficult to differentiate the particular lines. The principle to choose the line width cautious and restrained helps to avoid this problem. Another difficulty are junctions of point symbols and lines. A good quality of visualisation can be achieved by using simple point symbols like circles or squares. Furthermore a good readability can be achieved through an appropriate generalisation.

Map examples

Below four examples of different types of line and vector related diagram maps are shown.