# GPS "trip meter"

I just got back from kayaking. For those who know the area, I put in at the boat ramps in Marina del Rey, paddled around to Mother’s Beach, then went back the other way to the end of the jetty. From there I paddled up to the Venice Beach Pier, and a little beyond. I turned back there and went back to the boat ramps.

I’ve used my GPS to find out how fast my motorcycle is going, since the speedometer reads fast. It showed the commute to work to be about 43 miles (I don’t remember exactly), which is about what it is.

I took the GPS on the kayak today, and the trip meter said I’d traveled 8.1 miles. I don’t think the route I posted in the first paragraph is that far. True, I was out for three hours; but there must have been a good 20 minutes just drifting along with the breeze (very relaxing).

Is the GPS measuring vertical movement as distance traveled? Out in the ocean the swells were running about 3-4 feet and the ocean leg counted for at least half of the trip. Did those swells add “distance” to my trip? Or did I actually paddle eight miles?

Oh – I’ve previously estimated the trip to be around 5 five miles. Maybe six. I’ve heard that it’s a mile from the end of the breakwatter to the pier.

Probably not. Any GPS I am familiar with uses purely the horizontal component of your travel to compute distance. It samples every second or so, and notes the distance traveled from the previous second. It is often inaccurate at slow speeds, and some GPS units won’t even count the distance traveled when below a certain speed threshold.

If you look at the error the system gives, you need to make sure your sample rate is slow enough that the distance travelled will be large compared to the error.

If your error is +/- 3 ft, for example, and you are travelling at just a few feet per second, the you better not sample too fast or your readings will be pretty meaningless.

I’ll try reducing the sample rate.

I don’t know much about GPS devices, but I bought one designed for runners. It fits on an armband and transmits to a wristwatch. It only displays time and distance (you can’t use it to navigate). I was skeptical about its accuracy, but I thought I’d give it a try. I have been astounded by how accurate it seems to be. When I run over marked routes, or use it to measure “out and back” it seems to come within spitting distance of where I think it should be. Even when walking (say, 16 minute miles) it keeps dead on.

You say you were out 3 hours and are suprised you traveled only 8 miles? What were you paddling with, a teaspoon?

I drifted around Mother’s Beach to see if I could see the sharks and stingrays. I probably spent a good ten minutes there, but there was a thin overcast that prevented me from seeing very far into the water. On the way out I was fighting the headwind. Out in the ocean the swells reduced my speed a bit. Out past the pier, I chatted with another paddler for a couple of minutes. On the way back in, I put my feet up and drifted the lenght of the final lane to the ramps since the wind was directly at my back and I was in no hurry.

Get a good map and plot your course and check distance that way. I have a GPS in the car and have found it to be extremely accurate, using highway mile markers as a benchmark.

I took it in a commercial airliner once and got a reading of 457 MPH at one point (it doesn’t transmit so I figured this was OK, if the guy next to me was using a laptop). No way to tell how accurate that was, though.

There’s always the mundane possibility that you forgot to reset the trip meter before starting, or that multiple tracks got concatenated together. You can download the tracks to your computer and see if they make sense. My GPS seems pretty accurate as far as kayaking goes. Kayaks are surprisingly fast, so 8 miles in three hours is not unreasonable, even with some dawdling. What did your GPS say for speed?

Actually, being a radio receiver, there is a small possibility it might cause an error on the aircraft’s navigation radios. Cabin personnel will often specifically state that you cannot turn on anything that receives a radio signal including cell phones (FCC regulation, not FAA), radios, and GPS receivers.

I wouldn’t say I paddle fast at all. And this time I was really dawdling because I can usually do the route in an hour and a half. I checked the trip meter to make sure it was 0.0 before I took off. I didn’t check the speed.

FWIW, I put the GPS in a Pelican case and put it in the seat pocket.