A GPS won't cut it for TSD rallies.  The position refresh rate is something like once per second and it considers that you've been driving in a straight line during that, so the more turns & hills you have (and the higher the speed), the more the calculations are screwed up.  Google TSD and gps, there's a good article about that.

We have a cheap Garmin Legend eTrex GPS that comes in really handy if we get lost.  Sometimes you don't have a clue where you actually are on the map, or even which direction you're pointed in.  So when they say just head to the nearest recovery point, it helps a lot if you can figure out where you're starting from.

It also sometimes helps, if for example you know you're approaching a Caution Acute Right, to see an overhead view of the intersection coming up on the GPS map display before you arrive there.  It can give you a bit of a heads up on what to expect.

The GPS is mostly handy only in that it keeps a log of where we've been that I can later download and study on a map on my computer.  So when they're all talking about Lavant Mill Rd later on, I can actually locate that on the map afterwards to see where it was.

Beyond that, I wouldn't trust a GPS for accurate distance measurements on anything other than a nice straight highway.

Here's a sample of our log plotted during the Lanark Highlands Rally following Lavant Mill Rd.  The grey line is where the mapping software suggests the road is located.  The dotted blue line was our run heading north, and the yellow dotted line was coming back south.



Note how far our actual track diverges from where the mapping software thinks the road is located.

Lesson #1: NEVER trust mapping software created by a company in the USA to accurately depict the path of any remote forest roads located in Canada.

I have the GPS configured to 'snap' our location to the nearest displayed road.  Note how, at the top of the picture, the GPS plots our track along the lake shore, then suddenly snaps over to the mapped road, then gives up again and accurately plots our location next to the lake.  I think this is why the North-bound track doesn't match the South-bound track.  I'll turn off the 'snap-to' feature next time.

In dense forest where the trees cover the road, the GPS will frequently lose satellite reception.  When that happens the log will sometimes show a mysterious spike in our track, suddenly branching several hundred meters off into a field before snapping back to the road.

Note that I mark each Checkpoint as a waypoint on the GPS.  Evil crafty organisers like Craig Hamm will deliberately move checkpoints so that people like us, who tag the CPs on our GPS, will be surprised on our return trip to find that the CP is no longer in exactly the same spot.

Like Ferdinand I use my GPS for providing a constant heads-up tulip diagram showing what's coming up. It will also give you your average speed if you remember to reset it at every CAS, so you can get a general idea of whether you need to go faster or slower between known points.

If you turn off the snap-to-nearest-road function you will usually get a more accurate picture of your route. This is useful on your second pass, although two passes of the same road can produce different tracks, particularly if there is a lot of tree cover.

A GPS's cannot be adjusted to correct to the rally distance so you will have to do odo corrections anyway, and because it's possible for the GPS to loose reception occasionally it's probably better to use your cars odo.

Naturally, Steve is correct, I don't know how they zero stuff down there to the 0.01 (no grace, either).  I like the GPS to tell me where I am, and what way I'm pointed, that has really helped me when you go off course.

Some people hook up bicycle odos for cheap rally odos.  You can buy rally odos cheap (they're just odo, though, no CAS, no timiing, but you can calibrate them, reverse them etc. pretty handy, MUCH better than the stock odo!).  Ask around, some guys might have moved up to an Alpha from a TerrorTrip or Brantz, maybe they'd sell it to you for cheapish.

Based on comments after last year's Lanark, I have to be much more crafty with CP placement this year.

Craig