An important aspect of any set of Mapping Data Polygons is the location of their centroids. The centroid is where the label for the geography gets placed, and also where each pie chart (or other "info graphic") gets placed.
Historically, the position for the centroid is half-way between the maximum/minimum latitude, and half-way between the maximum/minimum longitude. When I started in this business, that was the only specification for the centroid - this resulted the label for Florida to be out in the Gulf of Mexico! The only improvement since then has been the restriction that the centroid be within the physical boundary of the geographic entity. There is still no algorithm accounting for the shape of the polygon. This results in the situation illustrated below:
(clicking on a map will take you to a larger version)
Although the upper map illustrates the confusion of mis-placed text in Census Tracts in Washington, DC, the lower map shows you specifically which Census-Tract-Centroids have been moved (the original Centroid locations are red, and the moved Centroids are yellow). The improvement in readability and data-comprehension will make your maps look more professional. Similar to Editing in a newspaper article, it is noticed most when the effort is absent - your maps will look unprofessional and unsophisticated. (When the label falls on a boundary line, the result will be confusion, and a lack of confidence in your product)
Unfortunately, this attention-to-detail does not come for free; it takes approximately 36 hours to adjust the centroids of the 66,450 Census Tracts in the United States.
Dennis C. Dixon
Geo-Information Specialist
Dixon Spatial Consulting
dennis.dixon@dixonspatialconsulting.com
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