Endurance.Net Ridecamp Archives

Susan,
I don't know why you are sorry but I did notice that you failed to tell
how you knew that the GPS mileage was wrong. Just because it felt longer
or someone noticed that it was longer (whatever that means) or discussed
it at the convention does not prove that the GPS mileage was wrong. If
you or anyone else that has a GPS track for a ride that they would like
to have checked for accuracy just email it to me and I will run it thru
my software and post the results. 

Personally I don't care what the mileage is for a particular ride, I ride
for fun, it's my hobby. I try to make it very clear that I'm not
complaining, just giving information and  tell only the ride manager
what my GPS mileage was and offer to make them a map showing all the
water stops and vet checks with distances so they can decide what if
anything they want to change for next year. Some rides are harder than
others because of steeper terrain, difficult footing, severe weather,
etc. but just because a particular ride takes longer and tires you more
does not mean that the mileage is wrong. 

GPS is just another tool and should be checked by the user to see if
their particular tool is correct (within limits). Most newer car
odometers are very accurate. Take your GPS for a 30-40 mile drive on some
windy back road and check it out. If you assume your GPS is short 10-20%
because someone says so but it is really correct, then all your estimates
on fitness and the ability of your horse to complete a 50 are going to be
short by 5-10 miles....not a good idea. I train for an accurate mileage
then when the ride is over and my GPS says 42mi I am happy to get credit
for 50mi but I also know that if the ride had been a true 50 we would
still have finished in fine condition with no complaints because after
all this is endurance and according to some studies the endurance part
for horses doesn't even start until the 35 mile mark.

Check your tools. You wouldn't use a heart monitor that was off 10-20%
would you?

Don Huston


At 09:27 PM 3/21/2007 Wednesday, you wrote:

Sorry...I've been to too many rides that were measured with a GPS unit and the mileage was wrong.  I'm not the only one that has noticed this and there was even a man speaking on this topic at the AERC convention.  The documentation is correct.  I've never said ALL GPS are wrong.  Obviously, the satellite reception was broken.  Bottom line: the measurement was wrong.

Don Huston <donhuston@xxxxxxx> wrote:

THE DOCUMENTATION IS WRONG. Some GPS units including the Garmin GPSMap 76 series and 60 series are within 2-5% when satellite reception is not broken up. I am a surveyor. I use a handheld Garmin GPSMap76S to find lost survey markers. Here are some tests of accuracy that I have done. First: the "location" test and I have done this many times before. Monday I calculated the GPS coordinates for a survey done on some property 60 miles from my office. At the jobsite the property owner pointed out a corner he had found and the coords on my GPS unit were only 1 meter (3 ft) off from the calculated true position (had maximum satellite reception). Therefore this GPSMap76S can in fact "locate" where you are very accurately.  Second: the "distance" test. I have software (nRoute by Garmin) that will follow roads and highways from one address to another and give me the mileage. The software said 486 miles from my boarding stable to the basecamp at Hurricane. I drove the exact route, the GPS never lost signal and got 490 miles. The extra was probably from stopping for gas etc. I have GPS'd Sunland 50, Fireworks 50, Warner Springs 50, Eastern Mojave 50, Bar-H-Boogie 50, Git-R-Done 100 just to name a few that used trails that show up on my Topo! software and OzieExplorer software using GIS digital photos. On those rides my GPS got the same mileage (within 2-5%) of the mileage generated by following the visible trails on the software with the mouse. Therefore this GPSMap76S does in fact measure the "distance" along a highway or a trail very accurately. So...if the locations are accurate and the distance between locations is accurate (2-5%) then the overall accuracy is 2-5% for this GPSMap76S. The key to accuracy of course is reception and how the GPS is set to record the trackpoints. I use a CamelPak that has a pocket top and center to hold my GPS. That way nothing blocks the reception except heavy trees or deep canyons and when that happens I can use my software (if the trails are visible) to fill in the blanks to get a very accurate (2-5% short) trail mileage. I set my GPS to record trackpoints based on distance (0.01mi = 53ft). The great thing about a distance setting is whenever I stop for water or a vet check it shows up on the software as a large block of time with no distance and no wasted trackpoints. So I never have to touch the GPS during a race and I still get a record of time and location for every stop.

Don Huston at cox dot net
SanDiego, Calif