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Oldboy Pocket GPS Moderator
Joined: Dec 08, 2004 Posts: 10643 Location: Suffolk, UK
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Posted: Tue Jan 01, 2008 2:36 pm Post subject: |
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Andy_P2002 wrote: | I think I added a nought that I shouldn't have... | I won't sleep tonight now ...... _________________ Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1135 |
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Snudge Lifetime Member
Joined: Aug 22, 2007 Posts: 211 Location: Peterborough
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Posted: Tue Jan 01, 2008 3:09 pm Post subject: |
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Andy_P2002 wrote: |
I think I added a nought that I shouldn't have...
I did it for a 10x1 triangle which gives 10.04987562112089027021926491276 and scaled up (badly!)
How about 100.4987562112089027021926491276? |
That's more like it but you hadn't added a nought - you had just got the decimal point in the wrong place. Easy mistake to make when using a slide-rule but with your number of decimal places I'm guessing you used one of those modern calculators?
Back to school for you I'm afraid Andy.
The true figure of 100.4988 of course is significant. |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 01, 2008 3:26 pm Post subject: |
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not very....
Still less than 1/2% unless I'm even more hung-over than I thought! |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Tue Jan 01, 2008 4:04 pm Post subject: |
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Jeez, I'm loosing sleep over this as well. What I want to know is how much slow my GPS will be if I am going up a 1 in 3 hill at a lattitude of 51.0000N 0.0000 W travelling on a heading of 257.5 degrees with a 4.478 degree drift (It's icy) Please use the WGS84 spheroid.
PS the hill is 213.5m long. (the road measurement hypotenuse) |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 01, 2008 4:49 pm Post subject: |
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I'm off to the pub.... |
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M8TJT The Other Tired Old Man
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
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Posted: Tue Jan 01, 2008 6:25 pm Post subject: |
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Yo, Dude. Good idea. I wish that I had thought of that before posting my last. But as I didn't, I have done the calculation and find that the difference is so small that I don't really give a d***.
Happy new year to everyone who visits this thread. . In fact, not only them, but everyone |
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teamgers Occasional Visitor
Joined: Dec 20, 2007 Posts: 10
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Posted: Tue Jan 01, 2008 8:46 pm Post subject: |
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Phew, see what i've started................ |
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DennisN Tired Old Man
Joined: Feb 27, 2006 Posts: 14901 Location: Keynsham
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Posted: Tue Jan 01, 2008 8:54 pm Post subject: |
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A far more accurate method was used when I were a lad. You counted the number of clicks as you went over the joins in the rails, then divided them by something and it told you how fast you were going. (You sort of said "Diddly dee, diddly dah, diddly dee, diddly dah and your pal counted them). Now that I've got demented, I can't remember what it was you divided them by, nor how long you had to count for. Anita could tell you. _________________ Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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DennisN Tired Old Man
Joined: Feb 27, 2006 Posts: 14901 Location: Keynsham
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Posted: Tue Jan 01, 2008 9:05 pm Post subject: |
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Andy_P2002 wrote: | I remember at school a long time ago, having to work out the curvature required on a snooker table to stop all the balls running into the middle! |
When I were a lad, snooker tables hadn't been invented - I suppose some crafty would-be snooker table manufacturer had all you boys working at this because it saved him paying somebody to work it out. _________________ Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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Anita Pocket GPS Moderator
Joined: Mar 15, 2006 Posts: 3219 Location: Windlesham, Surrey
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Posted: Tue Jan 01, 2008 9:38 pm Post subject: |
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DennisN wrote: | A far more accurate method was used when I were a lad. You counted the number of clicks as you went over the joins in the rails, then divided them by something and it told you how fast you were going. (You sort of said "Diddly dee, diddly dah, diddly dee, diddly dah and your pal counted them). Now that I've got demented, I can't remember what it was you divided them by, nor how long you had to count for. Anita could tell you. |
'Fraid I've got no idea, Dennis. When I were a lass we couldn't afford to travel by train. If it was too far to walk we went by bus or coach. _________________ Anita
TomTom VIA 135 - App 12.075
UK map 1130.12368
Samsung Galaxy S21 |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Tue Jan 01, 2008 9:38 pm Post subject: |
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DennisN wrote: | When I were a lad, snooker tables hadn't been invented - |
SURELY not?
Although they use slate beds for the tables now, I guess they used sandstone in your day...
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Hello1024 Occasional Visitor
Joined: Jan 01, 2008 Posts: 3
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Posted: Wed Jan 02, 2008 12:26 am Post subject: back on topic... |
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By my understanding, a GPS has theoretically 100% accuracy if you average it over enough distance in a straight line, because it measures time and distance from start and end points to calculate speed. Over enough distance the accuracy of the start and end points becomes irrelivant, and with atomic clocks the time is accurate to many more significant figures than is relevant.
The downsides are that there may well be rounding internally in the GPS, rounding for displaying the figure, and most units only calculate speed on a sliding 3 sec (or so) moving average, which is nowhere near as accurate as a minute or so in a straight line.
I believe GPS units will always under-estimate speed when going up and down hills or round bends, because they calculate the distance as the distance between points where they got a fix from the GPS satellites. When you draw straight lines between a finite number of points on a curve, the sum of the lengths of the lines is always less then the length of the curve. |
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Andy_P Pocket GPS Moderator
Joined: Jun 04, 2005 Posts: 19991 Location: West and Southwest London
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Posted: Wed Jan 02, 2008 12:56 am Post subject: Re: back on topic... |
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Hello1024 wrote: | By my understanding, a GPS has theoretically 100% accuracy if you average it over enough distance in a straight line, because it measures time and distance from start and end points to calculate speed. |
Not according to this:
Quote: |
Vehicle Speed
GPS velocity is calculated based on the frequency shift (or Doppler shift) in the carrier band. Accuracy of the velocity depends on the effect of multi-path, city sky-scrappers, DOP etc. The comparison of Speed data from GPS and Speedometer or Tachometer has been studied detailed by many researchers. The GPS speed is more accurate than speed from speedometer/tachometer. Witte et al, has studied that the speed determined by the GPS receiver was within 0.2 metre/sec of the true speed measured for 45% of the values with a further 19% lying within 0.4 metre/sec.
(Intelligent In-Car Navigation System
Narayanan Ramanathan
Project Manager
GIS &GPS CoE )
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and:
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5. Conclusion
The GPS is accurate for the determination of speed
over ground (about 10 times more accurate than a car
odometer) when moving at relatively constant speed in
straight lines and is competent at determining speed on
curved paths, although some overshoot does occur
during transitions. Absolute error increases slightly at
higher speeds but in percentage terms is less. In addition,
when the system is tested under conditions of sudden
changes in speed some inadequacies become evident.
The system smooths the peaks and troughs of rapid
accelerations and decelerations, which is attributed to
inherent smoothing within the mathematical algorithm
and the one sample per second output of the system.
GPS is therefore ideal for the determination of speed
under conditions of constant speed and steady acceleration
over several seconds, however, is unable to resolve
rapid changes in speed.
(Witte TH, Wilson AM. “Accuracy of non-differential GPS for the determination of speed over ground”, 2004)
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Hello1024 Occasional Visitor
Joined: Jan 01, 2008 Posts: 3
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Posted: Wed Jan 02, 2008 1:20 am Post subject: |
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Using doppler shift from multiple sources is great for an instant measure of velocity, but if there is no need for instant results (ie. you can average it over time) doing the old way of measuring time and distance has to be more accurate.
The report that doppler shift is used surprises me, because if that is the case, there is no significant reason to delay or average the displayed velocity, since at over a GHz it would be theoretically possible to measure the shift (by lots of possible methods) in at least a matter of miliseconds. |
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DennisN Tired Old Man
Joined: Feb 27, 2006 Posts: 14901 Location: Keynsham
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Posted: Wed Jan 02, 2008 1:30 am Post subject: |
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Hello1024 wrote: | Using doppler shift from multiple sources is great for an instant measure of velocity, but if there is no need for instant results (ie. you can average it over time) doing the old way of measuring time and distance has to be more accurate. |
Like I said, count the Diddly Dee, Diddly Dahs for a few minutes. _________________ Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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