GPS drift

ben@31

I cycled a loop earlier with my Garmin 800. When I looking at the data, the route overlay was spot on for most of the way around the loop, with the overlay staying on the roads marked on the map.

With the exception of one single road contouring down the side of a hill. Here the GPS overlay has me swerving off the road, descending scrub land on side of a hill going off-piste, through thick overgrown forests and across two rivers on my fragile road bike. Then rejoining at the end of the road and carrying on my merry way.

It baffles me how it can be accurate on every other road this except one. Are the small hill and some pine fir blocking NASA's finest GPS signal with it's 20ish satellites?

I busted my balls cycling uphill to the start of that road :-(

sloppyschleckonds

ben@31 wrote:

I busted my balls cycling uphill to the start of that road :-(

Herein lies the problem

paul_onabike

GPS is a very weak signal and can be affected by hilly terrain and trees. You have to remember that the satellites are very far away and your garmin is computing your position from the signal from several satellites at the same time. If just one signal is lost temporarily this can cause problems in the accuracy of the tracking.

fudgey

Go and do the ride again now to see if it works properly the second time.

ElectronShepherd

Remember you don't have 20 satellites - some are always blocked due to there being an Earth-sized planet in the way.

My Garmin (not an 800) has a basemap installed and a "lock to roads" feature, which keeps the position indicator on a road, even if the actual signal gives a slight variation. I see the effect on the MTB; cycle down a road and turn off onto a bridleway and the position marker carries on moving down the road. Eventually the discrepancy gets too much, and it "jumps" to the bridleway route. If you do the ride again, perhaps try disabling the lock to roads option, and see what effect that has? It sounds to me as if the lock to road was in place for most of the ride, but due to weather conditions and/or tree cover, going down the hill the errors in position were too great, and it "unlocked" from the roads, and "relocked" once it started receiving a better signal again.

redvee

My 800 showed me crossing a river on a non-existant bridge on the ride home last week, I had it on the map screen and the trace was 1/4 mile off to my right as it picked up the road on the other side of the river.

kajjal

On the 500 you have to set it to update position every second to make it much more accurate. Maybe this is your problem as well.

kettrinboy

Had my 800 for over 4 yrs and its lost its bearings for a mile on maybe 4 or 5 rides out of ~1100 rides , just something that happens from time to time , anyway if it happens its easily corrected on in my case Sportracks and other ride logging sites.