Choropleth Mapping Joseph LukGoogle Maps, Google Earth, Google Maps APIh1. Choropleth Mapping with Color-It *Overview* Choropleth mapping is a v

Choropleth Mapping with Color-It

Overview

Choropleth mapping is a visual representation of data over a geographic area. A good example would be the 2004 Presidential Election Map Here, the data, blue for democratic or red for republican is overlaid on a map of the United States.

Typically, choropleth maps are created using mapping software like Microsoft MapPoint or ArcGIS. However, these programs have licensing costs and a significant learning curve. For simpler choropleth mapping tasks, particularly for students, a solution based on the Google Maps API is a good alternative. There were several options considered, one was to create a SQL database of shapefiles and overlay them on to Google Maps, a good example of this is Mapeteria. There are some drawbacks to this approach. The first was the size of the overall application: converted shape files are quite large, and the database size will grow depending on detail. Another option was to keep a database of latitude and longitude specifying polygons. While not as accurate as shapefiles, the size of the database is more manageable. One implementation in particular is Color-It, which was created by Zonums Solutions. Color-It allows users to overlay region shapefile polygons over Google Maps.

There are a few limitations to point out. Color-It uses a database to retrieve region divisions based on country. While the list is exhaustive, it is far from complete, so you may not find the country of interest. In addition, only one country can be mapped at a time, so you will not be able to compare one country to another.

To begin, you must select what type of coloring you want to perform. Color according to value will automatically select gradients between two colors and color regions based on the data values relative to the maximum and minimum values. Manual color selection will create a map that colors regions based on user input. Then select a country of interest from the list of countries sorted by continent.

Color According to Value

You will be presented with a table with all the colorable regions of a country and a corresponding data value. At this time, the data still needs to be entered manually, but offers the flexibility to easily manipulate data and change the map. If regions are missing data, you may also deselect regions by unchecking them in the table, Color-It will omit these regions. Once data is entered, you can select the colors for maximum and minimum values, the left color being the low value and the right being the high. You can also adjust the % opacity of the polygons as well as the width of the outline. Click on draw map to produce the choropleth map. If changes are required, you may adjust settings and click draw map again.

Manual Color Selection

This time, the data table will be replaced with an ellipsis button. Clicking on this button will bring up a color palette, where colors can be manually selected. The rest of the procedure is the same as Color According to Value.

Generating KML

For more detailed maps, Color-It also has the ability to convert your input into a Google Earth Compatible KML file. To perform the conversion, you can click on the generate KML file after clicking draw map. The file will be in KMZ (KML then ZIP) which you can extract into a KML file and open in Google Earth.