Choropleth Google Maps

Abstract

Introduced in 2005, Google Maps offers 18 maps of the world at different scales, varying from approximately 1:85 million to 1:4,800 at the equator at a screen resolution of 100 dpi. Each map has been tiled into individual raster squares that are downloaded separately, often from different servers. A typical Google Map might download map tiles from seven or eight different IP addresses, each associated with a different server that could be located in different Google data centers. Subdividing the map into tiles improves the perceived map download time and allows the map to be easily panned. Google Maps also makes use of the Asynchronous JavaScript and XML (AJAX) server/client technology that maintains a constant connection to the map server, a major improvement in server/client performance. Maps and imagery in Google Maps have been projected with the Mercator projection. The limitations of this projection have been well-documented, and its distorted depiction of the world has been a major cause for concern. For example, Greenland is represented as being larger than Africa when, in fact, Africa is 14 times larger than Greenland. Scale varies continuously from the equator to the polar areas. Changes in scale in the Google Maps display can be observed by examining the scale bar when moving north or south from the equator. The change in map scale is particularly noticeable at the extreme latitudes. The distortion caused by the Mercator projection is not noticeable with larger scale maps. In 2006, Google introduced an Application Programming Interface (API) that includes a series of functions that may be invoked by the user. These functions control the appearance of the map, including the scale, position, and any added information in the form of points, lines, or areas. The API makes it possible to incorporate Google Maps on Web sites, and to overlay information from other sources – a process referred to as a “map mashup.” One application of the Google Maps API is the construction of choropleth maps by super-imposing shadings. Current examples include maps of London by the UCL Centre for Advanced Spatial Analysis (CASA) and election result maps by county or state (see Web Resources). The UCL CASA provides Google Map Creator, a freeware application for thematic mapping with Google Maps (see Web Resources). One advantage of choropleth mapping with Google is that the underlying map can remain visible, providing some geographic context to the representation of the data. Normally, thematic maps lack the necessary background map to properly interpret the locational component. While it can be argued that stripping background information may result in the better formation of spatial patterns by the map user, providing more locational information may be viewed as a necessary component for all thematic maps. The purpose here is to demonstrate how choropleth maps can be made with Google Maps.

Cite this work

No public copy is stored for this record, so only the metadata is cited.

Stored only when both papers are in the corpus — 41 citations exist upstream.