Dot Mapping Lecture Transcript
3. Welcome to the dot mapping module, your next thematic mapping method to conquer. Dot maps
indicate location of one or more occurrences of a geographic phenomenon. This is typically done by
using a dot symbol where one dot equals a certain amount of the phenomenon and then placed in the
relative geographic location where it is most likely to occur. These maps can also be called dot density
or areal frequency maps. The map we are looking at depicts the population of France in 1830. It is also
the first dot map to ever have been created, owing tribute to the French Cartographer Montizon. In
this map 1 dot equals 10K people. You should zoom in on the image to get a better look. You can see
this mapping method allows one to easily get an idea of the amount of people living in France as well
as the relative geographic distribution and density of the population.
4. What makes a dot map? It uses small dots as symbols to show overall spatial distribution and the
number of dots changes from place to place in proportion to the number of objects being mapped.
5. You must use conceptual data (or raw-count data) in order to utilize a dot map. As you can imagine
creating a dot map by hand is very time consuming and is now done by computer software. The use of
this type of thematic mapping has been declining. It’s most common usage is for agricultural maps.
6. When is using the dot mapping method applicable? Some criteria:
a. when you have discrete data occurring at points;
b. when you want to compare distributions of related phenomena,
c. and/or want to portray variations or patterns in density.
You should not use a dot map when you are making large scale maps or when your enumeration unit is
large, or if your data is continuously distributed.
7. There are some advantages to this type of map--It is a very easy concept to understand; it is effective
for showing variations of phenomena that exist in large quantities, ideally you could recover the
original data from the map by counting the dots, dot placement takes into account the distribution of
other phenomena (such as land use), and it can be used in combination with other map types.
8. Some disadvantages to using this type of map would be that it is hard to estimate density, the map
reader could interpret the dot to represent a single occurrence, if your dots are dense it is impossible
to recover your original data, and if you are not careful computer placement of dots is random.
9. Dot maps can be represented as one to one or one to many. Here we are looking at a famous map
produced by Dr. John Snow in 1854. This is one of the first geographical analyses of disease data. Dr.
Snow mapped the incidents of cholera outbreaks in relation to well sites. By doing so it was apparent
the cholera outbreaks were closest to the broad street pump, so they decommissioned that pump,
which helped to end the epidemic. This map is an example of a one to one map, as each dot
represents one cholera related death. Using the one to one method works here, as the map
represents a very small geographic area. An example of a one to many dot map would be the French
Population dot map shown to you on the first slide. You recall, each dot represented 10K people. If
you tried to create that map using the one to one method it would be uninterpretable.
Map Considerations
11. When constructing a dot map you should consider the scale of your enumeration unit, the dot value
and the dot size.
12. The smaller your enumeration unit, the more accurate your dot map placement must be. It is best to
map your data using the smallest enumeration unit that is available. So for example it is best to use
county level data over state level data.
13. The dot value is the numerical value represented by each dot. The dot value can be established by trial
and error or by using a nomograph. A nomograph shows the relationship between dot size and
density. We will be using Arc to determine this for us instead of using a nomograph.
14. Keep in mind if your dot value is too small you will have a large number of dots which could give the
impression of high accuracy. If your dot value is too large you will have a small number of dots and any
patterns in the data may not be visible.
15. Choosing your dot size has some similar issues. If your dot is too small your distribution will be
insignificant. If your dot is too larger they could coalesce and give an impression of excessive density.
Both of the dot value and dot size issues are why we use trial and error to determine these values.
16. In order to choose a correct balance between the two, remember: density increases when dot value
decreases and dot size increases. Yet, perceived accuracy increases when dot value and dot size
decrease.
17. That leaves the question of how many dots should be on your map? Once you have determined your
dot value, the number of dots in any enumeration unit is calculated by = dividing the total occurrence
by the dot value. We do not normally map any remainders. You can sum the remainders and
distribute them between the enumeration units.
Dot Placement
19. Let’s move on to placing the dots on the map. There are many methods for this the first being a
uniform placement. This is where dots are placed uniformly throughout the enumeration unit. It’s not
a first choice as geographic phenomena are not uniform.
20. A second method for dot placement is geographically weighted placement. In this method dots are
shifted toward neighboring enumeration units. This is based on spatial autocorrelation, for example
that high values tend to be located near other high values.
21. The last method is the geographically based method. This is when dots are placed based on
geographic knowledge of the area. This method produces the most detailed dot map and is what we
will be doing in the class.
22. In order to place dots using the geographically based method we must use information about the area.
We do this by applying “filters” or layers of information about the area to the map to guide in dot
placement.
23. If you are placing dots by hand you want to avoid placing dots in geometric arrangements, creating
false voids or clusters, or lining up dots on boundaries or even avoiding boundaries. Again we will be
using software in order to place our dots but these are things to keep in mind if you are adjusting your
dot placement.
Design Issues
25. The main design issues for these maps are mostly related to the legend itself. Most dot maps usually
only include a statement indicating a dot value. 1 dot = 10 people. If you choose you can include visual
anchors. These are small representations of three different dot densities on the map (such as a low,
medium. And high values).
26. So as you can see in this example we have a properly designed legend. You see the statement 1 dot =
250 cows. You then see three dot densities showing you a small, medium, and high density unit. The
size of the square for the dot densities follows the map scale and density unit – in this case 1 square
mile on the map.
27. The other main design issue for this type of map would be the projection that you use. It is best to use
an equivalent projection so that you do not distort the area for your density measure. This is more
important when you are mapping small scale.
This concludes the dot mapping lecture. Be sure to read the associated chapter in your textbook to reenforce these concepts prior to taking the module quiz.