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Sounds to me like you have it under control.

Truman

Sluys Guys wrote:
> 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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