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MapInfo Monday: Normalizing Thematic Maps

  • 1.  MapInfo Monday: Normalizing Thematic Maps

    Employee
    Posted 24 days ago

    This is a follow-up to the article "MapInfo Monday: Analyzing Density Hotspot Coverage," which I wrote in June last year.

    I got some good feedback from a former police officer and colleague @Tony Maber. 

    Some of these comments concerned normalizing the data. This is a typical process for comparing polygons of different sizes. You can create a normal thematic map showing the population for your areas.

    To normalize such a map, you can divide the population by the size of the administrative areas and get a population per area unit.
    This makes it much easier to compare administrative boundaries of varying sizes.
    You typically measure crime by comparing the number of crimes to the population. Let's see how that can be done.
    Thanks for the tip, Tony

    Normalizing Crimes Against Population

    I have my map of boundaries with a population count and I have my crimes mapped as points.
    On the Theme tab, I select Join... in the Field dropdown. This will allow me to add data from another table to my population dataset. In our case, the count of crime points inside each polygon.
    In the Update Column for Thematic dialog, I can control from what table I want to bring over data. I select the Crimes table from the Get Value From Table list.
    As a next step, I click the Join... button to check the join condition.
    The Specify Join dialog allows the user to configure the join of the two tables. I set the points from the Crimes table to be within the polygons from the Population table.
    Back in the Update Column for Thematic dialog, I select Count from the Calculate list to count the number of Crime incidents in each population polygon.
    The resulting map shows the number of crimes in the population boundaries.
    The map above hasn't been normalized. It may be somewhat misleading as large polygons typically would hold more crimes.
    We will normalize this thematic map using the population. In this example, I will calculate the number of crimes per 10 inhabitants. Whether you use 10, 100, or 1000 in habitants to calculate the per capita value may depend on the number of crimes.
    From the Field dropdown on the Theme tab, I select Expression.
    In the Expression dialog, I can now modify the thematic expression from a single number to an expression normalizing the crime count by the number of inhabitants.
    The resulting map will look like this. Compare this map to the map above and you will see that some of the larger polygons have been moved further down in the ranges.
    Sometimes also showing a label with the actual value can be helpful.
    I hope you found this valuable.


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    Peter Horsbøll Møller
    Principal Presales Consultant | Distinguished Engineer
    Precisely | Trust in Data
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