Data visualization is part of my everyday work-flow. More often than not, I’m playing around with my data in a GIS to tease out interesting or informative spatial patterns, or to ensure that I’m getting the results that I’m expecting. As a result, I am constantly trying out different classification schemes to help me generalize spatial patterns, highlight outliers and/or patterns, or just plain mess around with my data.
Unfortunately, QGIS (which has been my primary GIS for several years
now) only has had ‘Equal Interval’ and ‘Quantiles’ classification algorithms
implemented. While these classification schemes are no doubt useful and
revealing when used in the right context, I often need something that
better represents the ‘actual’ distribution of values in my data. For
this, I usually turn to the Jenks Optimisation (or Natural Breaks)
classification. Essentially, this classification algorithm generates
class intervals that minimize within group variance, and maximize
between group variance. In this way, given a certain number (k) of
classes, we arrive at an ‘optimal’ classification of our data into k
classes. In the past, I would import my data into R, and calculate
class intervals using the very handy classInt package. However,
moving data between QGIS and R, while slightly easier using my manageR
tool (shameless plug!), is not optimal when all I really want to do
is fiddle around with different classification schemes. So I decided to
reimplement the Jenks algorithm in Python so that I could do things
directly from the Python console in QGIS.
Obviously I didn’t really want to implement this algorithm from scratch,
so I had a look at the R code from the classInterval function in the
classInt package (ah open source!), as well as the handy Python script
from here. Once I had the code in hand, it didn’t take long to have
a nice Python script ready to be run on my data directly from within
QGIS. While I was at it, I also implemented a few other classification
algorithms to play around with, including ‘Equal Interval’, ‘Quantiles’,
‘Standard Deviation’, and R’s ‘Pretty’ algorithm. For those of you who
don’t know, R’s pretty algorithm basically computes a sequence of about
‘n+1’ equally spaced ‘round’ values which cover the range of our input
data, such that the class breaks are 1, 2 or 5 times a power of 10. The
Python script is available here, and has a version of the pretty
algorithm based on code from the labeling package.
