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Beautiful Data by Toby Segaran, Jeff Hammerbacher

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Data Checking

It is generally worth spending a significant amount of time at every stage of an analysis to make sure that the data is accurate, and geocoding was no different. Errors in geocoding came from a number of sources: there are typographical errors in the addresses, new buildings are often not listed in public databases, and zip codes may be reassigned over time. We further suspect that the USC software included a bug during the period we used it, because large numbers of addresses were falsely assigned to the Los Angeles area and elsewhere around the state; we remapped these addresses using another free online service at http://gpsvisualizer.com. Our debugging process included using R to draw simple maps of latitude versus longitude for each county and most towns to identify the addresses that had been located far outside the Bay Area.

The addresses in San Jose posed an interesting geocoding challenge. Sales are listed for several "towns" that are not recognized by any mapping sites we could find, so we assume they are informal names for neighborhoods: North, South, East and West San Jose, Berryessa, Cambrian, and a few others.

Where possible we tried to correct any errors. When that was not possible, we used R's missing values to indicate that we do not know the exact latitude and longitude. This is a better approach than throwing out bad matches, because we need varying levels of accuracy for different purposes: when we map the data at the level of county or city, we can ...

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