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Interesting, Don.  For what it's worth, Bar H Boogie's first year  
mileage came from dirt bike odometer except for the park.  The second  
year's was verified with my GPS which also happens to be a Garmin Map  
76.  Of course on the two day ride, the second day was a reverse of  
the first, hence the matching mileage.  Aside from the bits of oak  
and orange groves, the route was in the open.

Lynne


On Mar 13, 2006, at 11:17 PM, Don Huston wrote:

> Hello Kathy,
> I use a Garmin GPSmap76S. I live in San Diego. I am a surveyor by  
> trade and use this unit at work and also measure endurance rides  
> with it. I always have it set on "distance 0.01mi" which means it  
> takes a reading every 53 feet +/-. I have 3 different types of  
> software that I use to store the data and also make maps. This unit  
> has extra good sensors and almost never loses signal. I plot the  
> data on Nat'l Geographic "State Series" California Topo Software,  
> also on Garmin "MapSource" software and sometimes on "OziExplorer"  
> software which uses very recent USGS Aerial Topo that must be  
> purchased specifically for each area you want to map but it has a  
> lot of detail. When the unit does lose signal it shows up on the  
> plot but the software topo is very detailed and often shows the  
> trail I have been riding so I can fill in the missing sections to  
> get the mileage as correct as possible. Every time I check the GPS  
> mileage against the mileage from following the trail on the  
> software topo using the mouse they agree within 2-4% (1-2 miles in  
> 50). I have GPS'd over 10 rides and several of those on 2-3  
> different years. Where the trails are the same I get the same  
> mileage so the unit is consistent. There has only been 1 ride "Bar- 
> H-Boogie" that measured 50 miles and I got 50 on both days. All the  
> rest have been 43-46 miles with the shortest being "Hog Wild" the  
> year it rained so hard they had to eliminate a loop and it measured  
> 38 miles both days. On 2 desert rides I got 45-46 and an Etrex unit  
> got 43-44. I have checked my unit with hwy mile markers, with my  
> truck speedo (calibrated at a speedometer shop), with 2 software  
> mapping programs and with various survey control points so I know  
> it is accurate.
>
> Suggestion: I think there could be a line on the vet card for the  
> type of GPS unit and a line under each vet check for mileage. Then  
> RM's could get a good average mileage from a lot of different units  
> for every loop on their ride and adjust things if they want.
>
> Nice topic, are you going to post some kind of results or ...?
>
> Don Huston

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