Using a Treemap to show gauged areas

According to Wikipedia a Treemap is a method for displaying hierarchical data using nested rectangles.  Its easier to explain with an example which also shows how Treemaps are useful in hydrology.

In this post we’ll use a Treemap to provide information on the catchment of Western Port, a bay to the east of Melbourne; in particular, the proportion of the catchment that is gauged.  By the way, according to the Vicnames database, the official name is Western Port, not Western Port Bay.

There are 4 main gauged tributaries draining to Western Port:

  • Cardina Creek
  • Bunyip River
  • Lang Lang River
  • Bass River

Figure 1 shows the gauged portions of these catchments overlaid on a shaded relief base map.


Figure 1: Gauged catchments draining to Western Port

We can also plot the the gauged and ungaged portions of these catchments on a treemap along with the remainder of the Western Port Catchment (Figure 2).  Or just the gauged portions compared to the rest (Figure 3).


Figure 2: Gauged and ungauged portions of the 4 catchments


Figure 3: Gauged portions of the four catchments compared with the rest of the Western Port Catchments

If we are going to estimate flow or pollution loads to Western Port than we need to factor up the results from the gauges.  A treemap shows how much extrapolation is necessary.  Code to produce these treemaps is available as a gist.


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