1. Mapping Point Phenomena: The Common Dot Map
SP 240 Cartography
Alex Chaucer
Notes: This presentation works good supplemented by images from the Dent Textbook in this chapter.
Towards the end, have students try to estimate the amount of dots in given maps as an example of underestimating dots. You might even want to have some slides of dot density with the values canceled. It’s a fun exercise to do…we really do underestimate!

2. The Common Dot Map Dot Mapping
A method of producing a map whose purpose is to communicate the spatial variability of density of discrete geographic data; also called areal frequency mapping.
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3. Mapping Technique Dot distribution maps have been around since 1863.
Dot mapping technique involves
Choose an point symbol
Number of points increase in proportion to objects represented
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4. A Classification of Dot Maps
Dahlberg’s Classification
Dot equals one
Ie. One dot = one earthquake epicenter
Dot equals more than one
Ie. One dot = 25 acres of harvested crop
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5. Advantages of Dot Mapping
The rationale of mapping is easily understood by the map reader
It is an effective way of illustrating spatial density, especially suitable for showing geographic phenomena that is discontinuous (consisting of discrete elements)
Original data may be recovered from the map if the map has been designed for that purpose
More than one data set may be represented on the same map. This is recommended only if there is a distributional or functional relationship between the sets.
Final execution by computer means it is quick and rather straightforward and presents few production and reproduction problems
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6. Disadvantages of Dot Mapping
Map interpretation is not one-to-one; perception of relative density is not linear
The method is time-consuming if done by hand and therefore costly. Considerable research into factors controlling dot distribution is necessary, and provision must be made for the acquisition of ancillary resource materials
When the map has been designed for maximum portrayal of relative spatial density, it is practically impossible for the reader to recover original values
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7. Michael R. C. Coulson quote
“It is not the dot size, or value, or even placement, that give the real power to the dot map-assuming some reasonable decisions have been made. Rather, the power of the dot map is in the overall pattern of the distribution that is revealed.”
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8. The Mapping Activity
Spatial Proxy – each dot represents at a point some quantity that actually exists
Dot Polygons – the geographical territory that each dot “covers,” which makes up the dots enumeration district
The map designer must make decisions on scale, dot value, and dot size.
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9. Size of Enumeration Unit
The smaller the statistical unit in relation to the overall size of the map, the greater will be the accuracy of the final dot distribution.
Strive for the smallest enumeration districts possible.
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10. Dot Value and Size
Dot Value – The numerical value represented by each dot in dot mapping; sometimes referred to as the unit value.
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11. General Guidelines: Dot Value and Size
Choose a dot value that results in two or three dots being placed in the statistical area that has the least mapped quantity
Choose a dot value and size such that the dots just begin to coalesce in the statistical area that has the highest density of mapped values
It is preferable to select a dot value that is easily understood. For example, 5, 500, and 100 are better than 8, 49, and 941.
Select dot value and size to harmonize with the map scale so that the total impression of the map is neither too accurate nor too general. This will require experimentation.
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12. General Steps Adopt a unit value for each dot
Divide this value into the data total in each enumeration area to obtain the number of dots to be placed there
Select a dot size
Proceed to place the dots
Dots should “coalesce” in the densest part of the map.
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13. Counting Up Method
Counting Down – If your dots are for 4000 units, and you have 4500, then you might place just one dot, causing map error by not including the other 500.
Counting Up – Use a smaller enumeration units, and the population of each is added until the dot value is reached. The “remainder” is then added to other adjacent units until another dot value is reached.
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14. Dot Placement General rule
locate the dots as close to the real distribution as possible, using the center of gravity principle.
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15. Legend Design Dot-distribution map legends should have:
A statement indicating the unit value of the dot
A set of at least three squares that illustrate three different densities from the map
Finally, the legend should be CLEAR
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16. Dot Map Production Most dot maps of the past were created by hand.
When creating dot maps by hand, it is useful to use suitable art materials to ease in the map production process.
Today most are created by computer, so this is less of a concern
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17. Visual Impressions of Dot Maps
Reader perception of dot densities is not a one-to-one or linear relationship.
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18. Questions of Numerousness and Density
Apparent density
the subjective reaction of the map reader to the physical stimulus of the actual density of dots per area
Numerousness
the subjective reaction to the physical number of objects in the visual field, without actually counting the objects during perception
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19. Underestimating Dots
Cartographic research has demonstrated that map readers tend to underestimate the number of dots on a dot map.
Ie. Perception of an area 10 dots to area of 1 dot, it is likely to be more like 15 to 1 in reality.
Design Ideas
Include a legend with low, mid, and high densities
When recovery of original data is important, don’t use the coalescing dots.
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20. Rescaling Dot Maps Rescaling
Method used to enhance dot maps by assigning new dot values to compensate for the reader’s underestimation of density
Method is time-consuming and may be more applicable to computer-generated maps
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21. Computer-Generated Dot Maps
There are programs specifically created for dot mapping, and they have been used to map out the the Census of Agriculture in the past.
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22. Microcomputer Dot Mapping
Programs can do dot mapping these days (Maptiude, Arcview, Map Viewer)
Most programs allow the user to specify dot size and value
Random distribution of dots by computers may not give desired results
Look for data with the smallest enumeration district
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