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Wednesday, March 09, 2005

Getting Longitudes and Latitudes with Google Maps: Why?

A friend asked (post a comment next time!) what the point of the Getting Longitudes and Latitudes with Google Maps was, and without any context, I can see how this might be confusing. My bad. Here's why I think it's a handy trick.

In order to create a GeoPDF with the MAP2PDF for Acrobat plug-in, you have to know the locations and coordinate values of two points the map you're georegistering (known also as georeferencing (Why we call it georegister is a discourse in pedantics better left to another time... (Yes, I hack lisp.))). The GeoPDF creation process in brief:

  1. Open map in Acrobat

  2. Click on the GeoRegister button

  3. Click on the Map Coordinates button

  4. Pick your projection from the list (more on this later) and OK

  5. Click Registration Points

  6. Click on two points on map and enter corresponding longitudes and latitudes, or map coordinate values and OK

  7. Click OK

That's it. Experience has shown that one of the tricky bits can be getting the longitudes and latitudes of known points -- an that's where Google maps comes in. But first, let me address the "more on this later" of picking a projection. For many maps of practical interest, the differences between how different projections represent the same area may be tolerably small. When playing around with different maps of dubious pedigree, we've used Lambert Conformal Conic or Transverse Mercator, and have found that just using the default values often does a good job. What's a good job? Patrick rounded up an Atlanta road map from who knows where that contained no projection information at all, and georegistered it using the process described here. He's got the Acrobat and the plug-in on a laptop with GPS, and one of his favorite tricks is to turn on GPS tracking while driving -- showing his location on the laptop as he goes. When you stop for a red light, the icon is at right intersection and on the road. You're traveling South on I75, the icon is on the Southbound side, etc. The point is not that this is possible -- plenty of navigation systems out there, -- but that he was able to whip one up on his own quickly, easily, and without knowing the first thing about the origin of the map.

While many maps provide a graticule (lines of longitude and latitude drawn on the map) or a grid of map coordinates from which it is easy to extract real world coordinates for the point you're looking at. But street maps often do not present such data. The nice thing about the Google hack is, you can just enter a street address or an intersection and voila! -- your coordinates have arrived.


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