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xda
Lifetime Member
Joined: Mar 11, 2004
Posts: 1199
Location: Park Gate
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 Posted: Wed Jan 02, 2008 7:47 am Post subject: |
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Doesn't work any more Dennis, they have done away with the old standard length of rail joined by fish plates, which gave you the expansion gap every 100 ft, which gave you the Diddly Dee, Diddly Dahs. You have to find other games to keep the kids busy now.
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Graham.
TT Go720, App:9.510(1234792.1) OS:842337
GPS: V1.20, Boot: 5.5279, Home: V2.9.5.3093
Map: Europe V910.4892
Map: Europe_Truck V870.3421, Kingston 8GB SD
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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: Wed Jan 02, 2008 10:08 am Post subject: |
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I think that counting Denis' 'Didly Des' over time is rather similar to measuring the delay of a pseudo random code to determine distance to satelite. The only problem with the GPS scenario is that you need at least three 'didly de didly dah' combinations from different sources to get a 2D fix and 4 to get your clock right to withn a few nano seconds.
To assist using the didly de method, as far as I can remember the rails are 66 feet long. Unfortunately, the modern practice of welding rails has scuppered this method, so GPS it has to be then. |
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DennisN
Tired Old Man
Joined: Feb 27, 2006
Posts: 14892
Location: Keynsham
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| Posted: Wed Jan 02, 2008 10:15 am Post subject: |
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| M8TJT wrote: | | Unfortunately, the modern practice of welding rails has scuppered this method, so GPS it has to be then. |
There are still two alternatives ..
1. Count telegraph poles.
2. Count sheep.
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Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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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: Wed Jan 02, 2008 10:18 am Post subject: |
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| How far apart are the sheep. I need to know to use this principle. |
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DennisN
Tired Old Man
Joined: Feb 27, 2006
Posts: 14892
Location: Keynsham
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| Posted: Wed Jan 02, 2008 10:35 am Post subject: |
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Ah, now you only count the ones which are
This .......................................................................................... far.
Do NOT count those which are closer together, or further apart, as that would insert a pseudo random code to determine distance to calculate.
You need to count fairly quickly, if I remember rightly.
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Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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Oldboy
Pocket GPS Moderator
Joined: Dec 08, 2004
Posts: 10642
Location: Suffolk, UK
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| Posted: Wed Jan 02, 2008 11:05 am Post subject: |
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| DennisN wrote: | | 2. Count sheep. |
Used to be cows when going past Milton Keynes. 
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Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1120 |
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Oldboy
Pocket GPS Moderator
Joined: Dec 08, 2004
Posts: 10642
Location: Suffolk, UK
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| Posted: Wed Jan 02, 2008 11:13 am Post subject: |
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I think THIS makes good reading.
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Richard
TT 910 V7.903: Europe Map v1045
TT Via 135 App 12.075: Europe Map v1120 |
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philpugh
Lifetime Member
Joined: Dec 28, 2005
Posts: 2003
Location: Antrobus, Cheshire
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| Posted: Wed Jan 02, 2008 11:40 am Post subject: Re: back on topic... |
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| Hello1024 wrote: |
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.
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I don't know about the sliding average. Certainly my GARMIN GPS60 doesn't do this. On a walk this hols in the Cheshire Peak District half way round we called into the Cat and Fiddle Pub for a pint and to get out of the wind and rain. I left the GPS60 and logger on in the rucksac. On return to home the GPS60 had logged a single erroneous point some distance away from the pub. The GPS60 indicated the highest speed achieved on the walk to be 824mph - which is explained by the erroneous point some 1/2Km away from the pub. Also my GPS Logger recorded a multipoint (taking 20 secs) error over a very similar distance and direction. I suspect that it was caused by electronic interference of some sort - perhaps having the two units adjacent for that period.
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mikealder
Pocket GPS Moderator
Joined: Jan 14, 2005
Posts: 19638
Location: Blackpool , Lancs
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| Posted: Wed Jan 02, 2008 11:55 am Post subject: |
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Lets take a quick look at the previous three pages, this thread has kept me amused over the last few days, it starts with a simple and often asked question:
| teamgers wrote: | | Hi, can anyone tell me which is the more accurate, the speed indicated on your sat nav? or, the speed indicated on your speedo in the car? |
Some contradictory answers before the question was answered:
| Anita wrote: | | PaulB2005 wrote: | | The Sat Nav will be more accurate as speedos tend to under read the speed. |
I think you'll find speedos tend to over read, i.e. the speed shown is faster than the actual speed, in my Ka by about 10%. |
Some mathematics (part one):
| Andy_P2002 wrote: | | Reap wrote: | | going up or down hill will make it read slower than it is. |
Quick bit of Pythagoras...
On a 1 in 10 hill, for a horizontal travel of 100yds that the GPS registers, you actually travel 100.04998750624609648232582877001 yds - meaning an error of -0.05% on the speed indicated. Hardly significant! |
Some redefinition of the shape of a triangle:
| philpugh wrote: | | The 'base' of your triangle needs to be curved to match the spheroid of the datum (eg WGS84). So the amount of under-reading will depend upon if you are going N-S or E-W (and directions in between) and where on the surface of the earth you are travelling. |
Some more Mathematics, and proof that slide rules can still beat the calculator:
| Andy_P2002 wrote: | | snudge wrote: | | I had a new slide-rule for Xmas and I can't get the figure that Andy does? |
Also very possible!
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? |
Insomnia starts to kick in:
| Oldboy wrote: | I won't sleep tonight now ......  |
| M8TJT wrote: | | Jeez, I'm loosing sleep over this as well. |
The evils of drink take over:
| Andy_P2002 wrote: | I'm off to the pub....  |
| M8TJT wrote: | | Yo, Dude. Good idea. I wish that I had thought of that before posting my last. |
How the heck does a Snooker Table fit in to this lot?
| DennisN wrote: | | 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. |
Discussions about railways:
| 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). |
Rail Length:
| xda wrote: | | Doesn't work any more Dennis, they have done away with the old standard length of rail joined by fish plates, which gave you the expansion gap every 100 ft, which gave you the Diddly Dee, Diddly Dahs. You have to find other games to keep the kids busy now. |
Counting objects (we are back to combating insomnia)
| DennisN wrote: | There are still two alternatives ..
1. Count telegraph poles.
2. Count sheep. |
| Oldboy wrote: | | DennisN wrote: | | 2. Count sheep. |
Used to be cows when going past Milton Keynes. |
Now I was going to ask if we can keep this “On Topic” or should it be moved to the “Playground” – But I see Phil has pulled it back on track for now - Mike |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Wed Jan 02, 2008 4:22 pm Post subject: |
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A great summing up by mikealder!
I think the prize for the best answer goes to robin2 who wrote
| robin2 wrote: | | Satnav speed is very accurate if, and only if, you are travelling at a steady speed, on the level and in a straight line. Under those circumstances you can rely upon it to much better than 1 mph to calibrate your speedo. Cars sold in Europe must have speedos that are zero to 10% fast. To allow for changes due to tyre wear, under inflation etc most manufacturers aim for a few percent fast, to ensure that if using the speedo to keep to the limit there is no possibility of unknowingly exceeding it. |
The only point missing from robin2's first sentence is, as philpugh observes, that you must also have a good satellite reception or you may get some wild results.
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Gone fishing! |
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DennisN
Tired Old Man
Joined: Feb 27, 2006
Posts: 14892
Location: Keynsham
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| Posted: Wed Jan 02, 2008 4:36 pm Post subject: |
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| Oldboy wrote: | | DennisN wrote: | | 2. Count sheep. |
Used to be cows when going past Milton Keynes. |
When I were a lad, they didn't have Milton Keynes. 
Maybe it's time for somebody to lock this one?
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Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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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 6:21 pm Post subject: |
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Well before you do...
The learned paper I quoted from:
Witte TH, Wilson AM. “Accuracy of non-differential GPS for the determination of speed over ground”, 2004
used GPS devices stuck on cyclists heads as they went round a track in one experiment.
There are even diagrams explaining how they had to compensate for the distance travelled by the satnav was less than the wheels 'cos the rider leant into the curves!
Oh, and the snooker table came about because we needed to compensate for the curvature of the Earth on the base of the triangle, Mike.  |
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GerryC
Pocket GPS Moderator
Joined: Mar 01, 2005
Posts: 1513
Location: West Mids
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| Posted: Wed Jan 02, 2008 6:31 pm Post subject: |
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I feel sorry for the next person to ask this seemingly standard question getting a link to the topic as way of explanation. 
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Gerry
TomTom730T
Cameralert for Android
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fahl
Occasional Visitor
Joined: May 01, 2007
Posts: 6
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| Posted: Thu Jan 03, 2008 10:34 am Post subject: |
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Cant someone do this the easy way,drive past a 30mph speed cam on a known hill at a gps indicated 40mph,(be nice to note the speedo reading at same time)
Then all you need to do is wait a few weeks for that piece of paper with your actual speed on it??? 
If all this is true,those hump back bridges must reallyupset speed readings
& what about speed humps???
And if you inadvertantly drive off a cliff would gps show correct speed straight down???
are you going faster by going round a roundabout or straight over it???
Got to stop,Its time to take my pills  |
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DennisN
Tired Old Man
Joined: Feb 27, 2006
Posts: 14892
Location: Keynsham
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| Posted: Thu Jan 03, 2008 11:32 am Post subject: |
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| GerryC wrote: | | I feel sorry for the next person to ask this seemingly standard question getting a link to the topic as way of explanation. |
Don't worry, Andy_P2002 has sworn a terrible oath never again to tell somebody to use the Search button. 'Course, he won't be breaking his oath if he simply publishes a link. And anyway, he had his fingers crossed behind his back when he did the oathing bit. But maybe he'll just publish the bike picture again.
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Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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