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Truman, wow, thanks, that was a lot of information. I have been 
measuring this trail since winter and have gone over it at least 3-4 
times, some sections more. I have also compared some sections to 
surveyors wheel and have driven any sections that I can. Most of my 
measurements with GPS were done with no leaves on the trees, although my 
recent data with leaves is very close to the winter measurements. I also 
wear an external antenna on my helmet to get the best reception 
possible.  As I also suspect some inaccuracy in hillier areas I have 
been very careful with this. Your information is very useful to me and I 
will be checking the mileage further with a wheel just to be confident. 
I do feel like I'm getting it as accurate as possible with my multiple 
methods of measuring. I surely don't want it to be out of whack since I 
also hope to be able to ride a day of the ride myself :-). Do you think 
I'm covering all my bases?

Thanks, Nancy


Truman Prevatt wrote:
> The issue with elevation is not elevation it is coverage. A GPS unit 
> will solve for altitude if it is locked to a sufficient number of 
> birds. Where you have a lot of elevation changes you normally have 
> blockage to some available birds. The 3 dimensional fix is quite good 
> since it was designed to provide three dimensional fixes for military 
> aircraft. If not it uses the WGS84 datum to calculate location. This 
> datum is a model for the earth that defines a mathematical surface 
> (ellipsoid) that is the general shape of the earth. Hence if only 3 
> birds are available it has to force elevation (which is height above 
> the WGS84 ellipsoid) to be used as zero. It has to do this go get any 
> fix at all with 3 birds. Because of this if only three birds are 
> available, the location will be off - hence, any use of that location 
> in a distance will be off. The same is true independent of the reason 
> you don't have sufficient birds - canyons, tree cover, mountains.
>
> The newer GPS units have more sensitive receivers and a better 
> antenna. Some are equipped with the ability to use some form of 
> differential GPS correction, e.g. WAAS, which will help with the 
> accuracy in some places. The receiver sensitivity will go a long way 
> helping provide better accuracy. There are still things one needs to 
> be aware of. First is situations where the receiver can lock onto a 
> reflection of the satellite signal rather than the direct path because 
> you could not see the direct path but could see the reflected path. 
> Radio signals bound off of things just like billiard balls bouncing 
> off the bumper. This could happen in a canyon, mountain valley, etc. 
> This wasn't much a problem for the low end commercial units until the 
> more sensitive receivers came out. This has always been an issue, 
> however, an issue of a active research because of its impact on 
> military use with a lot of papers in the technical literature looking 
> at how to detect it and mitigate it.
>
> For example in the "urban canyons" as well as natural canyons this can 
> present a significant problem to law enforcement using GPS or to our 
> military in the case we have to send in troops into an urban area. By 
> the GPS locking to a reflected path rather than a direct path (which 
> it can't see) the pseudorange estimate used in the location algorithm 
> can be significantly off (light travels at 1 foot per nanosec 
> (billionth of a second) and the difference in the path length of the 
> direct and reflected path can be hundreds to thousands of nanoseconds. 
> That will cause an error in the fix.
>
> Clouds are not a major issue but tree cover of deciduous trees with 
> foliage will always be an issue since the GPS frequency is above the 
> cut off frequency for foliage penetration and leaves on the order of 
> two inches up make dandy reflectors for the signal. Conifer forest 
> probably won't be an issue with the new receivers.
>
> So I would say that if you measured the trail in two directions and 
> compared you would be fine on relatively flat open terrain. In heavy 
> deciduous foliage I'd still measure by hand. I would check canyons and 
> narrow valleys several times at different times a day to see if the 
> results were consistent. If any were more were more than a 1 to 2% 
> difference, I'd measure by hand to be safe. It is probably best to 
> maintain the fix points in memory and download them to a tracking 
> program. I don't know about the lower end commercial units, but some 
> of the better units have the capability to store every NMEA sentence 
> (1 per second) produced by the GPS receiver. That includes the error 
> ellipse (a measure of how good the fix was) and GPS DOP (Dilation of 
> Precision which is a measure of best quality that particular satellite 
> configuration can give you). If the error ellipse (or DOP) suddenly 
> goes from small, e.g., 10 meters to large 200 meters, then that's a 
> good sign that there is a problem in that area.
>
> The thing to always remember about the GPS units is the GPS 
> constellation nor signal structure was never designed for measuring a 
> continuous distance on the earth. It was designed for global all 
> weather navigation and time reference transfer for a global reference 
> time accurate to several nanoseconds for the military. It generates 
> point estimates. Some will be bad - depending on a lot of things. For 
> it's intended use that's fine since for navigation, GPS simply 
> enhances the vehicle INS by removing drift and for time transfer, it 
> enhances the atomic clocks in military systems again to remove the 
> drift to keep all the atomic clocks synchronized with the global time 
> standard. Every GPS point does not have to be accurate to accomplish 
> that - just statistically "on the average" they need to be accurate.
> .
> GPS was never designed to support any specification of accuracy for 
> distance estimation on a random route. Can it be used for that - yes 
> with some care in understanding were the issues are and how to address 
> them. I've been playing around with location technology for close to 
> 40 years now have worked on at one time or another most space based 
> locations systems from TRANSIT (a.k.a. NAVSAT 
> http://en.wikipedia.org/wiki/Transit_(satellite)) to GPS and 
> personally I've very skeptical of any trail entirely measured with a 
> low end commercial GPS unit. Not saying it can't be done, just saying 
> it needs to be done with great care. The only exception might be on a 
> flat open desert - but that would be a pretty boring trail.
>
> Truman
>
> 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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