Choropleth Mapping Using Quantile Method in CartoDB

Question

My understanding of choropleth maps that use the quantile method is that they will assign an equal number of items to each bucket (see http://www.ncgia.ucsb.edu/cctp/units/unit47/html/comp_class.html).

A recent map I made with CartoDB generated a map that did not do this. Instead, most items were assigned to one bucket. Choropleth Map Example

Is CartoDB using a different type of quantile method than what I described?

andrewxhill · Accepted Answer · 2014-01-27 21:43:47Z

4

Choropleth is using equal interval I believe. You can calculate the numbers for a couple different algorithms on your own by using some SQL helper functions we made (https://github.com/CartoDB/cartodb/wiki/CartoDB-PLpgSQL#wiki-statistical-functions). Then just use those results in your CartoCSS defined bins.

Doing it on the fly might take a bit of work in cartodb.js, but that should get you started.

answered Jan 27, 2014 at 21:43

andrewxhill

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We tried API query described in GitHub. It returned an error when we tried it with decimal values, but worked with integer values. Is this a possible explanation for why quantile would generate strange looking maps? Also - the default in CartoDB for choropleth is equal interval, but there is an option for quantile. When we choose quantile CartoDB generates a map like the one image, which could be due to the possible decimal issue?
– Nathan Storey
Commented Jan 27, 2014 at 22:13
But ... the choropleth wizard has a "quantification" pulldown that lets you select from Jenks, Quantile, Equal Interval and Heads/Tails. So one ought to be able to adjust the quantification and see that reflected on the map.
– Amanda
Commented Jan 28, 2014 at 18:59

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