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Re: [RC] Newer GPSs and accuracy - Don Huston

Hello Nancy,

I use the GPSMAP76S. It is the first of the newer models and I find it to be very accurate. I use it in my surveying business to find existing property corners and it is consistently within 1 meter when used on open ground (no heavy tree cover).

IMHO accuracy depends first on signal strength (antenna), second on maintaining constant reception (no loss of signal because of heavy trees, deep canyons, rider body interference, etc), third on the method used to record your track and fourth you must have WAAS "Enabled" on your unit ( manual pg 55, System Setup).

The GPSMAP units have the best antennas and can pull in signal much better than any of the other units so yours is good on that point.

Signal loss shows up when to analyze the track on the Garmin MapSourse software.
Where you place the unit on your body has a huge effect on maintaining signal (preventing signal loss) in tough areas like trees and deep canyons. I found that putting my unit on the top of and outside of my small CamelBak worked the best. I found a large cellphone case that would hold the GPS unit and attached it permanently to the CamelBak. The other place that might be even better would be on the top of a helmet.


Page 44 of your manual shows how to set up the tracks. I always keep the track log "on". I never use the save tracks function because my gps does not save all the individual speed data which I like to have. I just download any track I want to keep and then clear the unit so it's ready for next time. Under "Setup" and then "Record Method" I use "Distance" and then "Interval" I use "Most Often". On my unit "Most Often" is every 0.01miles or 50-60feet so a 50 mile track will use about 5000 points.

I like the "Distance" method because it makes a very complete and IMHO the most accurate record of the trail with points every 50-60 feet no matter what. Also, when I stop, like for water or a vet check, the unit does not waste any memory recording stuff unless I move more than 50-60 feet.

Like you, I have checked my unit with my truck odometer (which has been calibrated by a speedo shop because I had a problem and is now very accurate) and with several mileage check signs along freeways and it has always been right on.

On real crooked sections of trail there will be some error because the distances will go straight between points and not follow the curve and of course there is another slight error because the distances go level instead of following the up or down slope. IMHO a very generous correction for those errors would be to add 2% to whatever the gps says, to get true trail miles. That means if a GPSMAP type gps, that did not lose signal, recorded 49.0 miles of trail, plus 2% = 1.0mi, that would be a true 50 mile trail.

Someone correct me here if I'm wrong but I think AERC rules allow a 50 mile ride to have a 5 mile error. So, assuming my 2% curve and slope adjustment are okay, then a ride that gps'ed at 44.1 miles could still be considered a 50 miler (and the riders will be much happier).

Don Huston




At 06:12 AM 7/24/2008 Thursday, Sluys Guys wrote:
Hi folks, I would like some opinions from experienced GPS users on the accuracy of the new GPSs. I have a Garmin GPSmap 60cs that I have been using to mileage a trail for a new ride. In the past my old GPSs have had a pretty good margin of error due to elevation, tree cover, mountains etc. and I felt uncomfortable with using the mileage without some adjustments but I am finding this new model to be quite accurate. I have checked the mileage on some steep sections of road against my odometer and it came out exactly the same in cloudy and sunny conditions.. In some steep sections of the trail I have rounded up to the next 10th just to make sure that I wasn't measuring long. I have also measured by tracing my track on the GPS map to make sure it wasn't cutting corners and I feel pretty comfortable with the final mileage as I rode the whole thing last weekend. For those of you who using the new GPSs what is your experience with the accuracy??.

Thanks, Nancy Sluys

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Don Huston
donhuston @ cox .net
SanDiego, Calif




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[RC] Newer GPSs and accuracy, Sluys Guys