Monday, April 2, 2007

Hawaii Population Data & New Features

As promised, my break gave me a new burst of energy. I spent my week off in Hawaii and figured it would be a good region to experiment with a few new map features. I tackled two items that were on my to-do list: labels & logarithmic color scales. I have generated a new map of population by County Subdivsion for Hawaii to demonstrate these.

First, on labels. I've found that Google Earth is quite powerful except when it comes to labeling polygons. You can provide "names" for polygons, but these only appear in the "Places" panel on the left of the map view. I suppose this may have to do with the difficulty in figuring out where to put these names on the map display (given how oddly shaped a polygon can be), but I would have thought there was some good default behavior for this (if I am missing a feature of KML, please let me know!). To place a label on the map display, you have to create a "Point" placemark. Out of a desire to keep moving, I pushed forward without labels. The Census Bureau's shape files do actually include a center point for each polygon, so I have now gone back and updated my code to generate "Point" placemarks for each of these center points. With these points, I can now have labels appear on the map. This is very helpful, particularly when dealing with geographies below County (County Subdivision, Block Group, etc.).

Second, on logarithmic color scales. One of the challenges in mapping any sort of data that has a very wide range and is not very evenly distributed across the range is that in can be hard to find a color scheme that provides clarity at either extreme. I have talked about this in a couple of previous posts, but finally gotten around to implementing a logarithmic scheme that more evenly distributes the data across the range. I'm not entirely happy with what I've implemented, so I plan to work on it further.

On to some screenshots. Below you'll find 3 perspectives of population, by County Subdivision, from the 2000 Census for Hawaii. Every meter in height represents 5 people; Greener represents lower population, Red higher population. The labels come in handy because I'm not that familiar with the islands. The new logarithmic color scheme comes in handy because Honolulu has a much higher population that all of the other County Subdivisions. I've used the same Green to Red color scheme I've used before, but with the logarithmic scaling it now does a much better job of helping one to distinguish between the Subdivisions on the lower end of the population range. Without this new scheme, Honolulu would be red and everything else green.

From directly overhead:


From an angle, looking North:


From an angle, looking South (so you can see the northern side of Oahu):


One other note of interest: I was perplexed for a few minutes because the Midway Islands and the other islands west of Kauai all showed up with tall, red polygons - meaning they have a high population (I excluded them from the screenshots for this reason). It turns out that the Honolulu County Subdivision includes all of these islands, hence they get the data for the entire Subdivision. I suppose this demonstrates one of the perils of geographic aggregation when working with an island chain.

1 comment:

Alden said...

Aidan - great work and the log scale colors make a lot more sense. One small technical note on Hawaii - it is the islands west of Niihau that are part of Honolulu County. Niihau is just west of Kauai and is part of Kauai county. Niihau is privately owned and very sparsely populated so likely is overlooked by the census data.