General Cycling Discussion - GPS - the ultimate yuppie toy?

Guys, I think I might have been through this a while back, but I got chatting to a chap I know about the problem I've had getting a reliable computer setup on my Gemini. I don't want to get into the use/don't use, need/don't need discussion of the pros/cons of bike cimputers but he mentioned an interesting angle about the use of GPS to do the same.

As he explained it to me. Bill Clinton scrapped Selective Availability when he left office. That eliminated the GPS signal scrambling that varied its accuracy around +/- 50m and most units are now better than 3m accuracy. The UK Ordnance Survey uses a 6 digit reference, like TQ401354 but the accuracy of the GPS units now give a 10 digit accuracy, like TQ4018935472. I digress.

There are GPS units that also display altitude. Take a map, draw a straight line, disregard contour intervals, and all you have is a straight line distance. A bike computer would do a reasonably good job of the true distance travelled. An altitude GPS unit should in theory be able to do the same. Ignoring cost, battery life, and other issues, has anyone actually used one in such a context?

I have a reason for asking. The OS maps, like all maps, are never as uptodate as you'd sometimes like them to be. The UK Forestry Commission is quite tolerant about people riding on their land, providing you don't stray into areas that are unsafe, such as where they are doing felling/logging operations. The forest tracks change. Some fall into 'disuse', and new ones open. Maps can't keep up with that. GPS though, if you use accurate coordinates, will let you add the course of tracks to the maps, and that might be useful for exchanging routes with your pals.

Any idea or comments about this?

Have you used GPS in such a case?

Or is still just a yuppie toy?

I have a GPS unit at work that provides horizontal accuracy to within about 3 or 4 feet (1-1.5metres). It cost about $10,000 CDN. It's vertical accuracy is around 30' or 10 metres.
Generally GPS really sucks for vertical information.

Generally GPS really sucks for vertical information

That's quite close to what I'd heard as well. I was not sure if it was just a single opinion, but you've confirmed that there are still some questions about it.

as far a altitude, GPS is not so great... b/c the satellites are pretty high up, 3 can be used to get a decent 2-d location and 4 a pretty precise measure. i think it's minimum 4 for altitude and then it's often bad b/c the sattilites may all be near each other and are all obviously above you, so the triangulation error is MUCH higher than for 2-d positioning. that said, IF you can locate enough satellites, in theory the Altitude should be good enough for what you are wanting - i think +/- 50m or so

as i understand, for really accurate altitude measurements a fixed ground reference point is created and then with this AND the satellites GPS can get a really good GPS measure --- i think this is what is used in site plotting and other precise-measurement things --- but not cheap and not fast


There are GPS units that also display altitude. Take a map, draw a straight line, disregard contour intervals, and all you have is a straight line distance. A bike computer would do a reasonably good job of the true distance travelled. An altitude GPS unit should in theory be able to do the same. Ignoring cost, battery life, and other issues, has anyone actually used one in such a context?

so i'm not sure that i understand you question: so you're asking if a GPS could be used to measure NOT the 2-D distance but the true 3-D "distance travelled"? first, unless you're in a really mountainous area i don't think the elevation difference from 2 points will matter much so just computing from start-end won't work. So what you're really asking is if the GPS can use real-time 3-D measurement to compute "true 3-D distance" travelled? if so, in theory it shouldn't be too hard to do, but i don't know of any GPS that does and of course you have the problem similar to the barometric Altimeter vertical counting devices that you have to define a metric and it makes a big difference whether 5m or 10cm "counts" as vertical change... (for instance, i have 3, a Sunto, a Ciclo and a Cateye and they all use different methods and the Cateye ends up counting about twice the actual vertical change while the Suunto seems to be a little high (like 2%) and the Ciclo a little low (like 3%))

so i'm not sure that i understand you question: so you're asking if a GPS could be used to measure NOT the 2-D distance but the true 3-D "distance travelled"? first, unless you're in a really mountainous area i don't think the elevation difference from 2 points will matter much so just computing from start-end won't work. So what you're really asking is if the GPS can use real-time 3-D measurement to compute "true 3-D distance" travelled? if so, in theory it shouldn't be too hard to do, but i don't know of any GPS that does and of course you have the problem similar to the barometric Altimeter vertical counting devices that you have to define a metric and it makes a big difference whether 5m or 10cm "counts" as vertical change

That's what I was getting at. Could a GPS unit replace an existing bike computer? I know some/all GPS units let you review the course (waypoints) travelled? But I'm not sure if that is an automatic recording, like saving current position every 30 seconds, for example, or if you have to record it manually by pressing a button to record the snapshot?

Bike computers are not as accurate as we'd like ot think they are. If you set the computer with the wheel circumference then that might change a bit when you are sitting are on the bike, and it might also change again depending on tire pressure, though I think the difference in a typical 26" wheel will not be too noticeable?

I see GPS eliminating the calibration/mechanical aspects of existing bike computer distance recording, but it now seems if you opt for GPS, then you introduce different problems into the calculations.

Another problem with GPS is it measures distances in straight lines only. You would have to make a waypoint at every curve and intersection along the route to get an accurate distance. A bike computer measures the actual distance travelled.

Another problem with GPS is it measures distances in straight lines only. You would have to make a waypoint at every curve and intersection along the route to get an accurate distance. A bike computer measures the actual distance travelled.

That was my concern. Do GPS units record the waypoint by themselves, or do I have to punch a button to record it? Different makes and models do different things. I notice that Garmin make a unit called eTrek Summit that does vertical calculations but to a climber, distance is not so relevant.

I'd like to see the straight line distance measurement represent the 3D view not just the 2D.

hmm... yeah i think theoretically it could work but i don't know of any current GPS device that calculates 3-D distance... there are also some with integrated barometric altimeters which i think qould be a better solution for this type of thing.

yes, i think there are GPSs that automatically tkae sample points (i think my eTrex does so every few seconds) and this is really only a memory/battery issue --- a GPS could sample a few times a second and store it all if you have the memory and battery.

so i think it would all work, it would just be a matter of someone developing and testing the unit... and i'm not sure if there's really much of a market for such a thing...

but yes, i think it is likely in the not-too-distant future that we will all have GPS units on our bikes or wrists...

i haven't used mine much on the bike, (Garmin eTrex), but the 2-D speed is pretty cool in either a car or on the bike... i've actually thought of calibrating my bike computer via the GPS but i was too lazy to really do it...

I saw a review recently of a watch (Timex I think) that used GPS signals to continuously calculate current speed, distance etc. The idea is that you can use it regardless of which bike you are riding or even if you are running. At $400 it looks like a very sophisticated solution to something that isnt a problem. It wont tell you your current position.

i think my eTrex does so every few seconds

Which one do you use? I see there are about four or five in the eTrek range.