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[RC] Accuracy of GPS - k s swigartApril said: Yeah, I'm saying that if properly set up, the GPS will measure the distance traveled into the ravine and out of it. If the GPS is taking a waypoint measurement every 5 minutes instead of every 5 seconds, you're going to get very different mileage. And the 5 second one is more accurate because it will follow you down into the ravine. No, it won't. The GPS distance traveled measurement does not take altitute into account in its calculations. It assumes a straight line between the two points on a flat plane that it measures. So it won't "follow you down into the ravine" it will assume that each point it took (even if it takes you longer than 5 seconds to get across) that you were not going down and back up again, but rather that you went "as the crow flies" between each of its measurements. And it is a good thing that it does this, because it would be significantly less accurate if it tried to take change in altitude into consideration. The altitude function of a GPS unit is grossly undependable. As an example, I was sitting on the beach on harbor island in San Diego with my feet dangling in the water at medium tide and while I was SITTING there GOING NOWHERE, over a span of about five minutes it told me that my altitude was anywhere between 50' and 90' above sea level. Obviously, with my feet IN the ocean I was AT sea level (it can't be explained by the tides, tides in that area do not vary that much, high tide is about 8' higher than low tide and even so, it wasn't high tide). Additionally, despite the fact that the GPS (as evidenced by its constant changing of its reading of my altitude) did not tell me that I was moving. If it did take change in elevation into account, it would have told me that I was moving an average of 20'/second. (do the math, that works out to just under a quarter of a mile per hour). Which is different from what happened to me at the Fullerton Train station (elevation about 300') on the same day. Turned it on and while I was waiting for the train to take me to San Diego to go sailing, and while I was standing there it told me that my elevation was 5,680'---NOT EVEN CLOSE. And while I was standing there, not moving at all, it told me that my speed was .6 MPH. It wasn't the altitude reading that was changing while I was standing there not moving, it was the lat./long. measurements of my position that were changing despite the fact that I wasn't moving at all. Since (between my trip to the docks I stopped at West Marine who happened to have a rep from Garmin there talking about how to use a GPS as a navigational aid...sailers are big into navigation) the admitted accuracy of a hand held GPS is "within about 15' on ocean, but within about 45' in hilly terrain," in the area where I ride, a difference of 45' feet (as the crow flies) would put you either at the top of a ride or the bottom of a 500' ravine. Further, you have to guard against lost satellites. The antenna must be as high as possible and have the largest view of the sky possible. The more satellites the GPS can see, the more accurate the readings. And therein lies the problem with the accuracy of much of the use of them. Every step that you move changes the ability of the unit to "aquire" satellites. And if I am moving along riding a horse, the last thing in the world that I'M going to be doing is making sure that my GPS unit is still seeing enough satellites to be able to continuously accurately gauge exactly where I am. So I won't know if the GPS has been "seeing" satellites all along, or if it has been making assumptions (which is what it will do if it can't see satellites). These assumptions may or may not be accurate assumptions. As for Pigeon Mtn folks measuring the trail distance and putting up markers on the GPS say-so...well, I don't have my doubts so much on the accuracy of a properly used GPS, rather I have doubts on the knowledge and ability of the user of the GPS. Just handing somebody a GPS and saying go mark the trail without any education on how to properly do that will result in inaccurate readings as well. And therein lies the other problem with them. It is, indeed, true, that a tool is only as good as the idiot that uses it. Considering that most people are idiots :), if a tool takes a rocket scientist to use it properly, then it isn't a very good tool...especially if its proponents give them to idiots to use. I do think that used properly a GPS is a very good tool for training. And it's not necessary equipment either. But I like it. :) And therein lies its advantage. If it gives you slightly inaccurate information along your training ride, it doesn't matter. Because the information itself is, as you admit, "not necessary." For me, in endurance riding, it comes under the heading of "fun toy" (kind of like my heart rate monitor), as in, if the fun toy gives me information that tells me something that is way off from what I already know without having it, I assume that the data coming from the device is wrong. :) All this doesn't mean that the GPS isn't a REALLY handy tool for navigating on a sail boat, especially since "within 15 feet" out on the ocean doesn't make a hill of beans of difference. It is also a better fix than anything else you are going to get, which could also be said out in the hills. A GPS isn't a useless piece of equipment, it just has its limitations. One being that it is of limited accuracy as it moves into and out of terrain where it can "see" satellites and one is that it requires a certain amount of expertise to operate it properly (part of that expertise is understanding its limitations and not using it in situations where it matters if it is going to be inaccurate). Ironically enough, the fact that a GPS is piss poor at accurately determining altitude is not one of its major limitations, since few, if any (there may be some, but I have never seen one), of them use this altitude information for anything other than display. And, it doesn't really make all that much difference in the distance travelled calculation to assume that the distance travelled was flat, even if it wasn't. Few people ever negotiate grades that are more than about 10% and certainly not for extended periods of time. The average slope for most people riding around these parts (which is pretty hilly) would probably be between 2% and 4%, so that's how much (assuming all the other calculations were accurate) the distance travelled measurement would be "off" by. Tevis, the grandmother of all "hilly courses" (with a total of ~40,000 of elevation change...22,000 down and 18,000 up in 100 miles) has an average grade of about 7%...so, if you GPSed Tevis, and the GPS had perfect acquisition at every turn in the trail (unlikely) and the Tevis trail actually is exactly 100 miles (also unlikely), then the GPS would tell you that you had gone about 93 miles because it wouldn't take into account the average slope of ~7%. However, few trails are a hilly as Tevis and even hilly trails usually have long sections that are flat, which brings the average slope down quickly, so rarely does change in elevation make a big difference, despite the fact that the GPS doesn't take that into consideration. So while it is an obvious limitation, April is right, it isn't a particularly significant one. kat Orange County, Calif. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Ridecamp is a service of Endurance Net, http://www.endurance.net. Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp Subscribe/Unsubscribe http://www.endurance.net/ridecamp/logon.asp Ride Long and Ride Safe!! =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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