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!

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. Spatial Information Technology Center

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 Spatial Information Technology Center

A Classification of Dot Maps http://lazarus.elte.hu/hun/buszke/visegrad/particip.htm 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 Spatial Information Technology Center

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 Spatial Information Technology Center

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 Spatial Information Technology Center

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.” Spatial Information Technology Center

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. Spatial Information Technology Center

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. Spatial Information Technology Center

Dot Value and Size Dot Value – The numerical value represented by each dot in dot mapping; sometimes referred to as the unit value. Spatial Information Technology Center

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. Spatial Information Technology Center

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. Spatial Information Technology Center

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. Spatial Information Technology Center

Dot Placement General rule locate the dots as close to the real distribution as possible, using the center of gravity principle. Spatial Information Technology Center

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 Spatial Information Technology Center

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 Spatial Information Technology Center

Visual Impressions of Dot Maps Reader perception of dot densities is not a one-to-one or linear relationship. Spatial Information Technology Center

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 Spatial Information Technology Center

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. Spatial Information Technology Center

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 Spatial Information Technology Center

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. Spatial Information Technology Center

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 Spatial Information Technology Center