Fun with GPS #1

For the past week I’ve been playing with a Garmin eTrex Legend GPS unit that shekhark got me. It’s a lot of fun!

I had used a GPS unit before, in 1998, when working on the Bangalore Street Atlas Project (I built a CD-ROM viewer interface to what was otherwise a paper-only map). That thing weighed a few kilos and did nothing more than provide current coordinates over a 2400 bps serial line. Thanks to the signal scrambling policy in effect at that time, it was also wildly inaccurate. In a test with a borrowed laptop and the antenna on the roof of the car, it would jump several metres every few seconds, making it unusable for navigation.

Things have changed radically since. The eTrex is a handheld unit about the size of a mobile phone. It—like most consumer GPS devices these days—not only displays coordinates, but tracks and plots motion against a base map, allows waypoints to be recorded, can save a track and provide directions to navigate it again, and other features I haven’t explored yet.

Accuracy is still dodgy, due to various issues. GPS signals don’t travel through buildings and humans. You can’t put the device in a pocket and expect it to track where you go. It doesn’t work tucked away or indoors. It has to be out in the open, with a clear view of the sky. This makes it somewhat difficult to use when riding a bike, unless you have some kind of a mount (I don’t). Even when in the open, the best accuracy I’ve seen is seven metres. That means the actual location could be anywhere within seven metres of where it thinks it is. On a road hemmed in by buildings with no satellites currently overhead (they’re not geostationary), accuracy drops as low as eighty to a hundred metres.

To cut it short, here are my tracks from the past week, projected against a 30-metre resolution Landsat satellite image. For more fun, see the SVG version.



The area covered is Bannerghatta Road, Jayanagar, JP Nagar and bits of Koramangala. If you look carefully, you’ll see Lalbagh just above the CSCS label top-left, the Airport runway top-right, and Hosur road running from centre towards bottom-right. The tracks are colour-coded: darker shade indicates lower elevation. This map was generated using the free GPS Visualizer online service.

Waypoint names are limited to 10 characters, uppercase only. This is quite a bit considering the eTrex has no keypad, but forces you to come up with cryptic abbreviations. In my labels above, ‘BG’ is Bannerghatta Road, ‘HM’ is Hulimavu, where I live, ‘JSC’ is the Jayanagar IV block shopping complex, and a trailing ‘X’ or ‘JN’ represents a crossing. The GPX exchange format allows descriptions and notes to be added, but I haven’t gotten around to computer-side annotation yet.

There is no base map available for any part of India, and that is what makes this device interesting to me: I’m working on building a free, community edited map.

The first project is the Mumbai Free Map. Check it out. The folks at CRIT collected hand drawn maps from various public corporations in Bombay, put them together (nowhere as trivial as it sounds), vectorised it, and are now using GPS to stretch it into shape. The Web UI (built by Locative Tech) is very basic at the moment, but that will change soon. The background is a 15m Landsat image—the best that is available for free. Because of its limited resolution, the map gets very pixellated when you zoom in. We intend to replace it with a vectorised image representing land and water boundaries.

The Bangalore Free Map project is only an idea at the moment. We’re looking for supporters. If you think you can contribute (code, data or money), here’s the proposal we sent out to funding agencies for the Mumbai project.

BTW, if you have any interest in this sort of thing at all, you should do yourself the favour of checking out Schuyler, Rich and Jo’s awesome new book, Mapping Hacks, published by O’Reilly.