GIS3015 - Module 10 - Dot Mapping

In this week's module we learned about Dot Mapping.  Dot Maps are used to show conceptual raw total data for enumeration units.   Dot size, value, and placement is important to show that the data occurrences are not uniform throughout the enumeration unit.  One dot is assigned a certain amount of occurrences of the data.  Dot maps differ from proportional symbol maps because they use multiple dot symbols to show variation in the spatial distribution of the data.  Ancillary information is used to create a detailed map of the raw phenomena.   Attributes that are either limiting or related can aid in the correct placement of dots.  Selecting the correct dot size and value can affect the perceived distribution of the phenomena.  If dots are too small, then the data appears to have a sparse distribution and if they are too large then the distribution appears too dense.  A ‘nomograph’ can be used to determine dot size and value.

This week's assignment tasked us to create a dot map displaying the population data of the 23 southern counties of Florida.  The main focus of this map was the dots.   The dot density menu under the Symbology tab was used to create the dots and determine their size, value, and placement.  Dot sizes between 1 and 5 are normally used and I selected a size of 3.5.   This dot size seemed to create dots that appeared to begin to coalesce without overlapping.  The dot value I selected was 1 dot represented 20,000 people.   This value in combination with the dot size appeared to best represent the data.   Using the dot symbology properties menu we fixed the placement of dots so they would remain static and also applied a mask.   Masking allowed us to exclude dots from being placed in the surface water layer.   There were two options but selecting to place the dots only in the Urban Land layer provided the correct dot placement.  I ran into some trouble with the masking option as it seemed change after I had rearranged the ordering of layers in my map later on.  

For this map I chose to not include outlines or boundaries on the map.   I felt  the map's purpose was to show the spatial distribution of the dots and the boundaries would interrupt the visual flow of the map.   My first South Florida layer containing the dots had a hollow fill and was at the top of my Table of Contents (TOC).  I chose to show the Urban Land layer so the map viewer could see that people congregated in urban areas.  I added a second South Florida layer to show the areas of South Florida that were not surface water or urban land.   Per the lab requirements I categorized the surface water layer to show rivers, lakes, and wetlands.  I also provided an inset map of Florida for reference.

I chose to create a legend detailing the dot value.  In order to create this additional legend I used the Draw Tool in ArcMap to first create three rectangles.   I then placed an appropriate number of points in each box to represent different population values.  I created text with the same tool to label each square and then grouped the individual items as one rectangular unit.  I chose to use the same urban land color as the background for my dot value legend.   I had a few trial/error moments using the Draw Tool but overall it was relatively easy to use.   I also used this tool to type my major city names and leader lines.   Originally, I labelled the cities using the label tab for the Major Cities layer but found that the labels were covering dots on the map.   I chose a hollow circular symbol for the cities so the dots would not be hidden.

Overall, I enjoyed making this map and reading/viewing other dot maps when learning about this method, especially maps such as those created by the Cooper Center (http://demographics.coopercenter.org/DotMap/index.html).  Visually, dot maps can be a great way to communicate with the viewer.  Pattern variations of a phenomena over a range of time is easily seen with dot maps (http://www.nationalgeographic.com/clean-water-access-around-the-world/#select/TOT/total).