Cheap GPS with powerbooks; What Works

Some time ago (just before Christmas, oddly enough) I was looking around to find a comparatively cheap way to investigate GPS systems. The main requirement was for a cheap but reliable GPS system that could transfer data to my powerbook in an open format, that could be used for photo-geographic correlation and shared with projects such as and Colour display and on-screen mapping weren't required.

It turned out that the low-end Garmin systems (eg the etrex range) almost all perform this task quite adequately, as they support RS232 (serial) interfaces with a range of protocols, including Garmin's own standard and the NMEA interface that many applications use. Powerbooks (and, as far as I know, all new Macs) don't actually come with a serial port but (and this seems to surprise many) you can easily add one via a USB connection. The hardware generally recommended is the Keyspan USA-19HS - and I can vouch that this works perfectly with the Garmin etrex (the yellow base model) on an Apple Powerbook 12" with OSX 10.4.3. Naturally, you'll also need the Garmin PC serial cable. (It's slighty ironic that the signal traverses the first 12000 miles through the ether with no problem, and then you need 2 cables and an adaptor to get the signal to cover the last 3 feet. But it works).

Once you've got the etrex wired up to the powerbook, the next trick is to get the data out of it into a usable file. There are numerous pieces of software for this, but it can be hard to tell which support this strange pseudo-serial interface. One that I've found to be very simple and effective is Link2GPS from HikeTech - a $12.95 piece of shareware that will download files from the etrex in the open GPX XML format. Once you've extracted your track (and you can just leave the etrex to record as you wander around), you can use it as input to any app you care to write.

It's worth noting that the etrex has a certain degree of trouble capturing locations in city centres ("concrete canyons") where you can only see a small sector of sky. In narrow streets with high buildings you simply won't get a location. But for suburbs, rooftops, and open farmland you can fairly easily acheive a 10-metre resolution or better, enough for most puposes. It also depends how you carry the device; the external pocket of a leather jacket gets zero signal, but the top pocket of a rucksack gets a good signal in most circumstances. You can also get a reasonable signal from within a car in open country, and it appears (though I've only partly tested it) that Route 66 can use the NMEA interface to track your location in this situation. And, when Google Earth gets a proper Mac release, it should be possible to have some fun with that.

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Posted by parsingphase, 2006-01-08 18:43

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