Hack 35. Load Driving Directions into Your GPS

From Google straight to your Global Positioning System receiver, thanks to GPSBabel.

GPSBabel (http://www.gpsbabel.org) makes it possible to convert driving directions from Google Maps into various formats usable by other software and devices. This is possible because Google encodes the coordinates of the entire route in the JavaScript that is sent to the Google Maps client and used to generate the route that is put on the map.

The first step is, of course, to get Google Maps to make some driving directions. We'll use one of the examples given on the Google Maps home page. Enter "JFK to 350 5th Ave, New York" and hit the Search button. After a short calculation, a map like that in Figure 4-18 appears, with the route highlighted.

Figure 4-18. Directions from JFK to Fifth Avenue

Now, we need to get the output into a predictable format. To do that, we first need a URL that contains everything we need to recreate our route. Fortunately, Google provides a tool to do just that! Click "Link to this page." The displayed map won't change, but the URL in the address bar will contain your query and the parameters needed to return to this map at this zoom level.

The last step is to modify that URL to output the JavaScript that the Google Maps client uses internally. To do that, just add &output=js to the end of the URL in your address bar and hit Enter. Your URL will look like this (but all on one line):

http://maps.google.com/maps?q=jfk+to+350+5th+ave,+new+york&spn =0.103861,0.199934&hl=en&output=js

The observant reader will note that the instructions "JFK to 350 5th Ave, New York" are included in that URL, with spaces having been replaced with plus signs. With this information you can skip the steps of looking up an address, clicking on "Link to this page," and changing the URL to include &output=js, and instead just enter the URL of the JavaScript page straight into your browser. Here is a link to the page containing the points for directions from JFK to San Francisco International Airport the hard way:

http://maps.google.com/maps?q=jfk+to+sfo&spn =0.103861,0.199934&hl=en&output=js

The resulting page will appear mostly blank, but the HTML source contains all the data we need to extract the coordinates of the route. Save that page to your local hard disk with the File Save As menu option. If you are using Internet Explorer, make sure to select Web Page, HTML Only from the "Save as type drop-down. Saving as Web Page, Complete will cause the file to be rewritten as non-compliant XHTML, which GPSBabel will not be able to read. Give the file a namewe'll call ours nyc.htmand hit the Save button.

Now we have a file to feed to GPSBabel. What can we do with it? One thing we can do is to load this route to a handheld GPS unit. These have limits on the number of points, so we can use GPSBabel to reduce the route Google Maps has computed for us to a smaller number of points. GPSBabel comes with a GUI frontend, but the real power lies in the command-line interface. For our example, we must use the command-line interface because the GUI does not support filters. Start a command prompt; make sure that GPSBabel is in your path, and switch to the directory where you saved your map file as nyc.htm. Connect a supported Magellan or Garmin GPS receiver to com1: or a serial or USB port. If you are running Linux, this is likely to be /dev/ttyS0 or /dev/ttyS1. If you are using Mac OSX with a USB adapter, it may be /dev/cu.usbserial0.

Assuming you used com1:, type the following all on one line and hit Enter. Otherwise, replace com1: with the name of your serial or USB port. For a Magellan GPS receiver:

c:> gpsbabel i google f nyc.htm x simplify,count=30 o magellan F com1:

For a Garmin GPS receiver:

c:> gpsbabel i google f nyc.htm x simplify,count=30 o garmin F com1:

That's it! GPSBabel will read the driving directions produced by Google Maps, pick the 30 most salient points along the route, and upload the new route to your GPS receiver. Now you can follow along on this route as you walk, bicycle, or drive!

One other thing you might want to do is to save those points to a GPX file so that you can include them on your own Google Map, or upload them to a site like http://gpsvisualizer.net/. The Topografix site at http://www.topografix.com/gpx.asp describes the GPX format:

GPX (the GPS Exchange format) is a light-weight XML data format for the interchange of GPS data (waypoints, routes, and tracks) between applications and Web services on the Internet.

This command will convert all of the points, without simplifying them, into a GPX file:

c:> gpsbabel i google f nyc.htm o gpx F nyc.gpx

You can also reduce the number of points from this GPX file and load them into your GPX. The following command will load the points from nyc.gpx, select the 30 best fit points and upload them to a Garmin GPS on com1:.

c:> gpsbabel i gpx f nyc.gpx x simplify,count=30 o garmin F com1:

Naturally, this is analogous to the earlier command we used to go directly from Google Maps output to the GPS receiver, except that this time the input file is in GPX format.

GPSBabel is such a great tool that any Google Maps hackers should consider becoming Babelheads!

4.8.1. See Also

Ron Parker

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