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#21
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Wireless on recumbents (RANS Stratus)
"Cletus D. Lee" wrote in message T... In article k.net, says... "Cletus Lee" wrote in message Another choice, a Garmin Geko. uses GPS for speed and distance. No wheel magnet. It does under report speed by about 1.5% over a well calibrated wired computer. My Garmin Geko doesn't under report speed--it's right on. I am sorry, You are correct about the speed. Mine is accurate to the 0.10 mph. Except for the time it registered 888 mph in downtown Houston while I was stopped at a traffic light. It is distance that is under reported by ~1.5%. The reason the distance is off is because the Geko measures distance from point to point. A route with a lot of turns will be in error. Also I have found that false signals are causing route track errors in city canyons. -- Cletus D. Lee Bacchetta Giro Lightning Voyager http://www.clee.org - Bellaire, TX USA - Some of that tracking error may be avoidable, one problem is they are always off by a few feet anyway and if I select power saver mode on my emap it checks position every 2 or 3 seconds instead of 1, small difference but still a difference. Even then both gps and computer are pretty spot on in the end. Odd thing is mine always shows me at least 100 feet off in my home town, anywhere else is fine |
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#22
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Wireless on recumbents (RANS Stratus)
"brian hughes" wrote in message hlink.net... Well, I'm not so sure about the trip distance being under-reported. When I first bought it, just for an experimentation I brought it along and watched it update while my wife was driving down the freeway. Assuming the survey mile markers were correct, I didn't see any noticeable error at all. I also used it when I rode a century (actually a 108 mile ride), every rest stop was exact on my Garmin, as was the final distance when compared to the cheat-sheet. It was not 1.5 miles off, it wasn't even .15 miles off. But heck, I guess you could also argue that they probably used some kind of GPS to calculate and make the cheat sheet to start with. Maybe you know for sure, but I theorize the Garmin does not simply triangulate between points (thus cutting corners as you indicate). I believe the Garmin somehow integrates velocity, and thus doesn't necessarily cut off corners. The reason I believe this is I've seen the Garmin continue to update trip distance for a several seconds (at least) after the GPS signal is lost. Brian My only gripe about Garmin is they are wayyyyy too slow updating maps, I often ride on roads I know have been around for at least 10 years that still don't show up on the gps. |
#23
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Wireless on recumbents (RANS Stratus)
"brian hughes" wrote in message link.net... "bentcruiser" wrote in message om... Rvc wrote: I've just purchased a Stratus and wonder if any improvements have been made in the wireless computer world ? If you are talking distance trouble, try putting the computer on the derailleur tube. That's what I did on my RANS Tailwind, it works just fine on the derailler tube. Except last Tuesday when the weather was quite cold for these parts (about 15 deg F) on my morning commute. My wireless only reported about 1/2 the distance I really traveled--kept jumping between my actual speed and zero. The rest of the week the temp was in the mid 20's and it worked fine--I guess I could try a new battery but this one isn't that old. Seems like under 20 deg F it doesn't work dependably. I don't think poor cold weather performance is a wireless thing. On my V-Rex and my DFs I have wired computers and they don't work well either when it gets well below freezing. Anyone know of a computer that works well in the bitter cold? Some of that may be atmospheric conditions instead of the temperature |
#25
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Wireless on recumbents (RANS Stratus)
I agree with Cletus about chords being shorter than arcs. I would try his
method of testing the distance of a GPS unit at a High School track. My Garmin GPS III+ takes readings when I change speed or direction. The following excerpt from one of my journeys (this was in an uphill portion of the route) shows how frequently it will mark a point on a track. When I change direction or speed it stores a "Trackpoint", provides the latitude and longitude of the point, shows the time it created the point, then shows the distance, elapsed time, speed, and heading to the next "Trackpoint" Trackpoint N47 48 39.4 W122 00 44.8 3/15/03 9:10:33 AM 36 ft 00:00:02 12.2 mph 278° true Trackpoint N47 48 39.4 W122 00 45.1 3/15/03 9:10:34 AM 17 ft 00:00:01 11.7 mph 270° true Trackpoint N47 48 39.0 W122 00 46.2 3/15/03 9:10:39 AM 88 ft 00:00:05 12.0 mph 242° true Trackpoint N47 48 38.8 W122 00 46.6 3/15/03 9:10:41 AM 39 ft 00:00:02 13.2 mph 227° true Trackpoint N47 48 38.6 W122 00 46.8 3/15/03 9:10:42 AM 20 ft 00:00:01 14.0 mph 220° true The following excerpt shows the frequency when I am in flat and straight sections of a ride Trackpoint N47 50 18.4 W122 04 25.5 3/15/03 9:24:04 AM 570 ft 00:00:18 21.6 mph 326° true Trackpoint N47 50 21.2 W122 04 30.4 3/15/03 9:24:17 AM 438 ft 00:00:13 23.0 mph 310° true Trackpoint N47 50 23.9 W122 04 36.1 3/15/03 9:24:32 AM 476 ft 00:00:15 21.7 mph 306° true Trackpoint N47 50 26.2 W122 04 39.6 3/15/03 9:24:41 AM 332 ft 00:00:09 25.1 mph 313° true Trackpoint N47 50 26.7 W122 04 41.0 3/15/03 9:24:44 AM 113 ft 00:00:03 25.6 mph 300° true Trackpoint N47 50 29.8 W122 04 47.1 3/15/03 9:24:58 AM 520 ft 00:00:14 25.3 mph 307° true Trackpoint N47 50 30.8 W122 04 49.4 3/15/03 9:25:04 AM 188 ft 00:00:06 21.3 mph 302° true As you can see it does a fairly good job of reporting the changes in direction and speed. If my direction and speed remain constant, the frequency of reporting get stretched out. Most roads have a fairly large radius curves. If you have an 11.5 degree arc with a 500 foot radius the difference between the arc length and the chord length is about 0.16 feet. This is in a nominal 100 section of road. Most roads that I work with and ride on have a radius that is usually measured in the thousands of feet. In cases like that the difference becomes even less. Having said something about the theoretical side I would advise a person to use the approach suggested by Cletus. That should give you a good reference for the accuracy of the unit you use. William Higley, Sr. Vision R-50 RANS Rocket "Cletus D. Lee" wrote in message T... In article k.net, says... "Cletus D. Lee" wrote in message T... In article k.net, says... "Cletus Lee" wrote in message Another choice, a Garmin Geko. uses GPS for speed and distance. No wheel magnet. It does under report speed by about 1.5% over a well calibrated wired computer. My Garmin Geko doesn't under report speed--it's right on. I am sorry, You are correct about the speed. Mine is accurate to the 0.10 mph. Except for the time it registered 888 mph in downtown Houston while I was stopped at a traffic light. It is distance that is under reported by ~1.5%. The reason the distance is off is because the Geko measures distance from point to point. A route with a lot of turns will be in error. Also I have found that false signals are causing route track errors in city canyons. -- Cletus D. Lee Bacchetta Giro Lightning Voyager http://www.clee.org - Bellaire, TX USA - Well, I'm not so sure about the trip distance being under-reported. When I first bought it, just for an experimentation I brought it along and watched it update while my wife was driving down the freeway. Assuming the survey mile markers were correct, I didn't see any noticeable error at all. I also used it when I rode a century (actually a 108 mile ride), every rest stop was exact on my Garmin, as was the final distance when compared to the cheat-sheet. It was not 1.5 miles off, it wasn't even .15 miles off. But heck, I guess you could also argue that they probably used some kind of GPS to calculate and make the cheat sheet to start with. Maybe you know for sure, but I theorize the Garmin does not simply triangulate between points (thus cutting corners as you indicate). I believe the Garmin somehow integrates velocity, and thus doesn't necessarily cut off corners. The reason I believe this is I've seen the Garmin continue to update trip distance for a several seconds (at least) after the GPS signal is lost. What I know is from observation. Take a look at the track produced by your Garmin. Compare on a map against the roads actually taken. I think you will find the route is a series of chords. The distance along those chords is the distance measured and reported by the GPS. The Chord is always (by definition) shorted than the curve that it spans. This is the error that I refer to. It is not much but it accumulates. More accurate on straight roads, less so on routes with lots of turns. As for your 108 mile Century, if the sponsors did anything other than actually chain off the actual distance, then it is an approximation. If you want to really see how accurate your Garmin really is at distance, take it to a High School track. At cycle speeds, do several miles of a known distance (caution the track may be metric). See how that compares to the known distance. I did this with my Magellan once and was lucky to get three points per lap. I would have sampled more often if I had been walking. -- Cletus D. Lee Bacchetta Giro Lightning Voyager http://www.clee.org - Bellaire, TX USA - |
#26
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Wireless on recumbents (RANS Stratus)
"William Higley, Sr." wrote in
: I agree with Cletus about chords being shorter than arcs. I would try his method of testing the distance of a GPS unit at a High School track. My Garmin GPS III+ takes readings when I change speed or direction. The following excerpt from one of my journeys (this was in an uphill portion of the route) shows how frequently it will mark a point on a track. When I change direction or speed it stores a "Trackpoint", provides the latitude and longitude of the point, shows the time it created the point, then shows the distance, elapsed time, speed, and heading to the next "Trackpoint" Trackpoint N47 48 39.4 W122 00 44.8 3/15/03 9:10:33 AM 36 ft 00:00:02 12.2 mph 278° true Trackpoint N47 48 39.4 W122 00 45.1 3/15/03 9:10:34 AM 17 ft 00:00:01 11.7 mph 270° true edit for brevity 00:00:06 21.3 mph 302° true As you can see it does a fairly good job of reporting the changes in direction and speed. If my direction and speed remain constant, the frequency of reporting get stretched out. Most roads have a fairly large radius curves. If you have an 11.5 degree arc with a 500 foot radius the difference between the arc length and the chord length is about 0.16 feet. This is in a nominal 100 section of road. Most roads that I work with and ride on have a radius that is usually measured in the thousands of feet. In cases like that the difference becomes even less. A couple of observations if I may. Altitude is bizarre in GPS speak. For giggles google geoid. Beyond that, the earth model used by a particular GPS can have an impact on accuracy, particularly given your surface route is in 3 dimensions. Further be aware of the distinction between "taking a reading" and "storing a datum." While there is an entire newsgroup dedicated to GPS, here are some basics. Garmin supports two families of near real time reporting protocols, one proprietary and one per the not-as-tight-as-it-could-be NMEA standard. Per NMEA, it will report each "sentence" every 2 seconds. I don't remember the frequency of the proprietary messages, but it's something similar (not a useful improvement for most applications. Its chief advantage seems to be in a more tightly written spec). Given the RAM inside the device is finite, if you were saving vectors it would stand to reason to skip any successive vectors where the data (excluding timestamp) were the same, and let the plotting software figure it out. Think of it as a finely detailed cue sheet - the changes you don't make aren't recorded. In any event, you'll have no saved updates closer together than some arbitrary minimum time which is quite likely some multiple of the actual calculation frame. Anything less than 500ms would surprise me on a low-cost unit, with 1 sec being a more expected delta time. My hunch is that they use the same "engine" on most of their GPS receivers, but dumb 'em down for non time critical applications (read non aviation, non-NASA, and non-military). I'd suspect the device is running through the position calculations every 20-1000ms; that's just a hunch though. "guess where you are now!" Howard bitshift blah blah. |
#27
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Wireless on recumbents (RANS Stratus)
In my property description, there is a description of a rail road right
of way. It starts 1/2 miles south of my house, 1/4 mile west, going east, and 2 degree turn to the north until some station number, and somehow comes across my property. The only way that makes send is if it is about 1/4 radius turn. "William Higley, Sr." wrote: snip Most roads have a fairly large radius curves. If you have an 11.5 degree arc with a 500 foot radius the difference between the arc length and the chord length is about 0.16 feet. This is in a nominal 100 section of road. Most roads that I work with and ride on have a radius that is usually measured in the thousands of feet. In cases like that the difference becomes even less. snip William Higley, Sr. Vision R-50 RANS Rocket |
#28
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Wireless on recumbents (RANS Stratus)
Railroad curves are treated a bit differently than road curves. A 2 degree
railroad curve will have a radius of approximately 2865'. The 2 degrees refers to the interior angle that well be subtended by a 100' chord. Most modern highway curves use a 100' arc length definition. The older highways used the railroad definition because at the time, most Engineers with route layout experience came from the railroads. I may have caused confusion in my original statement when I spoke of an 11.5 degree arc. More properly stated it should have read "a curve with a 500' radius and a central angle of 11.5 degrees." This would provide an arc length of approximately 100'. William Higley, Sr., P.L.S. Vision R-50 RANS Rocket "Mike Schwab" wrote in message ... In my property description, there is a description of a rail road right of way. It starts 1/2 miles south of my house, 1/4 mile west, going east, and 2 degree turn to the north until some station number, and somehow comes across my property. The only way that makes send is if it is about 1/4 radius turn. "William Higley, Sr." wrote: snip Most roads have a fairly large radius curves. If you have an 11.5 degree arc with a 500 foot radius the difference between the arc length and the chord length is about 0.16 feet. This is in a nominal 100 section of road. Most roads that I work with and ride on have a radius that is usually measured in the thousands of feet. In cases like that the difference becomes even less. snip William Higley, Sr. Vision R-50 RANS Rocket |
#29
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Wireless on recumbents (RANS Stratus)
"William Higley, Sr." wrote:
Railroad curves are treated a bit differently than road curves. A 2 degree railroad curve will have a radius of approximately 2865'. The 2 degrees refers to the interior angle that well be subtended by a 100' chord. Most modern highway curves use a 100' arc length definition. The older highways used the railroad definition because at the time, most Engineers with route layout experience came from the railroads. I may have caused confusion in my original statement when I spoke of an 11.5 degree arc. More properly stated it should have read "a curve with a 500' radius and a central angle of 11.5 degrees." This would provide an arc length of approximately 100'. Hmm, two radii of 2865' with outer ends 100' apart = two degrees of subtended arc, right? Now, two radii of 500 foot length and included angle of 11.5 degrees has 100' arc length? Scribble, scribble, erase, erase, scribble... Tune in next year for response, if I can find my way back! |
#30
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Wireless on recumbents (RANS Stratus)
Dean Arthur wrote: "William Higley, Sr." wrote: Railroad curves are treated a bit differently than road curves. A 2 degree railroad curve will have a radius of approximately 2865'. The 2 degrees refers to the interior angle that well be subtended by a 100' chord. Most modern highway curves use a 100' arc length definition. The older highways used the railroad definition because at the time, most Engineers with route layout experience came from the railroads. I may have caused confusion in my original statement when I spoke of an 11.5 degree arc. More properly stated it should have read "a curve with a 500' radius and a central angle of 11.5 degrees." This would provide an arc length of approximately 100'. Hmm, two radii of 2865' with outer ends 100' apart = two degrees of subtended arc, right? No. At 100 feet distance from starting point, the curve is 2 degrees to one side of the straight ahead line. Repeart 45 times for 4500 feet along the circumference, you will have turned 90 degrees. 90 times for 9000 feet 180 degrees, 180 times 18000 feet 360 degrees. C = 2 PI r 18000 /2 = 9000, 9000 /3.1415926 = 2864.789 feet radius. That distance makes sense for this railroad to touch my property. Now, two radii of 500 foot length and included angle of 11.5 degrees has 100' arc length? At 100 feet, the curve is 11.5 degrees to one side of the straight ahead line. Reapeat for 15.65 times for 1565 feet for 180 degrees divide by PI 3.1415926 = 498.15 feet. |
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