The accuracy of GPS location depends upon the operator being able to read the instrument to the desired level of precision. I don't own a GPS.
If I want to know where I am, what will I read on my GPS? Degrees/minutes/seconds/fractional seconds? Fractional degrees? How many decimal places?
Considering that one degree of latitude at the equator is a bit over 100 kilometers, it will take a pretty precise readout to get a location within a few centimeters.
Can anyone provide information on this?
It is pretty precise. You can select the data format you prefer. I use minutes with a resolution of 5 decmil places for both lat and long. I also like the UTM coordinates which has a resolution of 1 metre in both directions. Given that the GPS accuracy with a good view of the sky is about 5m a 1m resolution is quite adequate.
AIMS/IAAF "A" Measurer
My unit reads decimal degrees to five digits to the right of the decimal point, or degrees, minutes, and seconds to the 0.01 of a second.
This may be slightly off topic but it is related. As you may know trail races are difficult, if not near impossible to measure using conventional road measurement techniques. However some of the rocky Tucson Trail Race series races have been measured using GPS.
This has prompted me, with help from Ken Young, to draft the article below.
The degree of accuracy obtained from GPS need not be to a centimetre, or even a metre. Even a ball park figure within 50 metres would be a major improvement on what is available at present.
Thoughts and comments welcome. The article is in draft form.
Measurement of Trail Races
Until recently the difficulty of measuring a trail course to any degree of
accuracy has proved a major problem. The advent of Global Positioning System (GPS) has enabled some trail runners in the Tucson
Trail Race Series to obtain the distance of specific trail races, even those which are on very rocky and mountainous trails.
This essentially uses a runner instead of a bicycle as the measuring vehicle. A serious runner will tend to run the shortest available route.
According to different sources the maximum range of error due to different factors would be as follows:. Ionospheric effects +/- 5 metres, broadcasting satellite orbital position errors, +/- 1-5 metres, satellite clock drift
+/- 2 metres and measurement noise +/- 0-10 metres, multipath distortion +/- 1 metre, Tropospheric effects +/- 0.5 metres, Numerical errors +/-1 metre Allowing for my very limited understanding of this technology, and
the possibily that by using different sources I have double counted [different sources using different terminology], an accuracy of +/- 25
metres would appear to be possible. According to another source, the accuracy of horizontal coordinates will be within 100 metres of truth 95 percent of the time. [i.e +/- 50 metres] "This specification will hold true
regardless of the manufacturer or model of receiver."
If the short course prevention factor currently used for road courses were doubled for trail races, [from one metre per kilometre of distance of the race course, to two metres per kilometre, over the marathon distance from 42 m to 84 m,] that should be sufficient to ensure the course to be at least the
If several runners in a particular race used GPS technology that would give a very good indication of the actual distance of the course. Such ongoing re-evaluation would take care of any year-to-year variations in
The GPS gives total distance, splits, times, elevations, total climb and descent, amongst other kinds of data. From this a detailed profile of the course can be produced, and much of the uncertainty of trail running
can be resolved. Whether great performances have been set, or if the course was just short.
Based on my tests
adding 2m/km probably wouldn't be enough to ensure the course is not short if you are planning to use a single GPS measurement. If you are planning to take several measurements and average them, you are probably safe with 2m/km.
But I believe the accuracy of the distance measurements do depend on what receiver you are using. Current models appear to be much more accurate than older ones.
Actually, I have more data than is shown in that chart. I'll post an updated chart on that thread in the next couple days.
This would likely work reasonably well in Tucson, but in an area where there is a significant tree cover I doubt that the accuracy would be sufficient. My experience using a GPS on the trails is that the signal and the accuracy degrades significantly in the trees.
AIMS/IAAF "A" Measurer
I'm trying to reconcile the thought that 2 meters per km would be a sufficient SCPF using GPS, with the fact that runners run marathons and routinely complain that they've run something like 27 miles. OK, I don't know if it's "routinely" but it happens.
In another stream of conversation on this forum Pete said that the very accurate surveyor-grade instruments use a stationary ground base in addition to satellites. So I'm wondering if there's any way to mimic that system using affordable units-- or is it not possible without access to secret military systems/signals/decoders?
Oops, I meant to say that runners "measure" marathons with their GPS devices (and then report our mismeasurements to race directors!).
I'm wondering if, in places where one can walk or run but not ride a bicycle, if a CALIBRATED measuring wheel or a calibrated walked bicycle might be just as accurate as a consumer grade GPS unit.
I have had the same thought as Jay about using a calibrated measuring wheel on rough trails. And this past weekend my daughter and I assembled one from old mountain bike parts, plastic tubing and a counter. The photos show the results. We haven’t tried it on a measurement yet, but it seemed to work well in the back yard. Hopefully we will have a measurement test before long and I’ll let you know well how it works.
Measure Wheel 2
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Wayne Nicoll was the first I knew of to convert a bike front wheel to a measuring wheel. It worked quite well. I made one too, just to try it out, but never used it much.
Tom McBrayer wrote a short article which appeared in Measurement News #56, November 1992. See below:
I've still got the "wheel" and it's still used from time to time..In fact I made a 2nd one and used both as demos at the Measurement Class held at the RRCA Convention in Houston in 2006. It was a very short class and bikes were out of the question. This was just to show everyone what a Jones Counter looked like mounted on the bike and how it functioned.
The two main problems I see with measuring trail courses with a calibrated bicycle are wheel slip and calibration. It's easy to imagine the former is an issue on loose, rocky trails, especially on turns. The latter issue is basically, what surface do you calibrate on? A handheld measuring wheel might eliminate some of the first issue, but the calibration issue would still be there.
I have had trouble receiving satellites with my Garmin 60csx inside buildings, inside cars, inside planes, and outside around tall buildings. Tree cover has never been an issue for it. When I compared GPS measurements against Jones measurements with heavy tree cover I got similar error as my experiments with a clear view of the sky. This was also true with the one experiment I did with a Forerunner with heavy tree cover.
There is a Bluetooth module available but has been out of stock. My understanding is the part number is R859N. The service manual pictures aren't as clear as they might be and it does look like a SIM card. However, it is like most internal Bluetooth modules with connectors and a short cable.
The hi-def screen is a different screen, not just different drivers or controller (which is embedded anyway).
I use GPS via USB GPS dongle and Google Earth or Earth Bridge.
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