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Dot maps
Dot maps
Definition
Dot maps are used to visualise distributions and densities of a big number of discrete distributed single objects whereas, in contrast to location maps, not every single object is depicted but one symbol represents a constant number of objects. For this visualisation simple or pictorial point symbols can be used. Widely used are points which leads to the name of this map type. Population density maps are often dot maps.
What is a dot map?
A dot map is a map used to illustrate geographic densities and distributions of a phenomena, where one dot has a value of a certain number e.g. 1 dot = 5,000 people.
The dot value of a map is the number of things each dot represents, so the map to the left has a dot value of 250,000.
The two types of dot map:
One-to-one (Dot value of 1) The dot is represented in its correct spatial location
One-to-many (Dot value of >1) The dots represent aggregate data and are often arbitrarily placed on a map
Geographical examples:
It can be used to represent anything, for example the distribution of farms across the Caribbean or the density of Jaguars in the Amazon rainforest.
Example datasets ideal for one-to-many dot maps include the population of the USA and the number of dairy cows in Wisconsin.
Example datasets ideal for one-to-one dot maps include the major cities in Europe or locations of recent earthquakes along the Pacific Rim.
To the right is a one-to-one dot map illustrating the population across Atlanta, USA using the 2010 census along racial and ethnic lines.
The areas with more dots close together have a higher population density. You can see where the more densely populated areas are because they are darker in colour.
Parameters for making a dot map
When constructing a dot map, two factors must be considered-
The graphical size of each dot
The dot value of the map
Make sure that the dots are equal in size and are not so close as to form solid areas, or so far apart as to make the variations in density difficult to see. Dot maps can be produced using mapping software such as Indie mapper where you import your data and alter the size of dots and dot value.
Quantitative symbols of fixed size
The most simple dot map uses a point symbol for a defined number of identical objects like in the previously shown image. The difficulty is to find an appropriate shape and size for the symbol as well as the value of it. The following interaction demonstrates how different a map can look like with different parameter settings.
Quantitative symbols of variable size
Quantitative symbols of variable size can be used if the map depicts an area where the object density is heterogeneous and where it is difficult to find and appropriate symbol of fixed size. Therefore different sizes of the symbol can be assigned to different values.
Maps with several thematic layers
Until now the shown examples of dot maps where all monothematic. But it could be different. It is also possible to combine different thematic layers which are related in some aspects. The differentiation of the objects is done by different shapes, sizes or colours of the symbols. It is important that the different layers do not compete with each other and the readability is still guaranteed.
Advantages and disadvantages of dot maps
Advantages
- Dot maps are easy readable, also for laymen
- Are perfectly suitable to show density distributions at a glance
- By counting the symbols it is possible to determine the original data
- You can map raw data
- They are easily understood
- They work well with all kinds of data
Disadvantages
- The data have to be georeferenced with coordinates
- The map design is time-consuming and expensive
- They can give the impression that some areas have nothing
- It can be hard to interpret when many dots are close together.
- Dot placement can be random.
- It is difficult to count large numbers of dots