On 12 Feb, 12:45, Roland Perry wrote:
In message
, at
04:26:08 on Tue, 12 Feb 2008, Mizter T remarked:

The new digital cameras are going to be set at the normal 10% 0f the speed
limit + 2mph (80mph + and your taking a risk basically)

A true 80mph will typically be over 95mph indicated, so I have no
sympathy for people caught by these cameras.

Perhaps I'm being thick Roland but I don't really understand the point
you're making - how is "a true 80mph" in fact "typically [...] over
95mph indicated" ?!

Sorry, a typo. I meant 85mph, of course.
--
Roland Perry


OK, stupidly I hadn't figured that out!

I've never really used a sat-nav system to compare true speeds with
indicated speeds, so I've never really been aware of the difference
between the 'true' speed I've been travelling at as opposed to the
'indicated' speed.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

In message
, at
05:02:55 on Tue, 12 Feb 2008, Mizter T remarked:
I've never really used a sat-nav system

Do they display a GPS speed? I use a rather older handheld GPS system
(that doesn't have maps).

to compare true speeds with indicated speeds, so I've never really been
aware of the difference between the 'true' speed I've been travelling
at as opposed to the 'indicated' speed.

My current car indicates 60mph at a true 56mph. And similarly pro-rata
at higher speeds.

I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.
--
Roland Perry

On 12 Feb, 13:44, Roland Perry wrote:
In message
, at
05:02:55 on Tue, 12 Feb 2008, Mizter T remarked:

I've never really used a sat-nav system

Do they display a GPS speed? I use a rather older handheld GPS system
(that doesn't have maps).


I think some do.


to compare true speeds with indicated speeds, so I've never really been
aware of the difference between the 'true' speed I've been travelling
at as opposed to the 'indicated' speed.

My current car indicates 60mph at a true 56mph. And similarly pro-rata
at higher speeds.


Interesting, that's a significant enough difference.


I guess that implementing a very accurate vehicle speedometer system
is hard to do, so I wonder if most vehicle (and in particular car)
manufacturers design their speedometers "on the safe side", so as to
indicate a faster speed than the true speed - based on the rationale
that it's better for drivers to think they're going faster than they
are, rather than think they are going slower than they are?

They are legally required to. Any error *has* to mean the speedo is
over-reading.


I didn't realise it was a legal requirement, but of course it's
perfectly logical that it is. To be honest I hadn't ever spent much
time thinking about it!

Roland Perry wrote:
In message
,
at 05:02:55 on Tue, 12 Feb 2008, Mizter T
remarked:
I've never really used a sat-nav system

Do they display a GPS speed?

TomTom does. It gives lat long as well.

It doesn't give height, though. Is it hard for GPS to be used to determine
height above sea level?

In message , at 14:10:50 on Tue,
12 Feb 2008, John Rowland
remarked:
I've never really used a sat-nav system

Do they display a GPS speed?

TomTom does. It gives lat long as well.

It doesn't give height, though. Is it hard for GPS to be used to determine
height above sea level?

Height is much less accurate than position. It's also of little use to a
driver trying to find his way (rather than, say, a hiker on foot with a
map and contour lines, or an aircraft pilot).
--
Roland Perry

On 12 Feb, 14:10, "John Rowland"
wrote:
It doesn't give height, though. Is it hard for GPS to be used to determine
height above sea level?

A GPS receiver works by narrowing down your position in three-
dimensional space, so figuring out your altitude and figuring out your
location are inseparable.

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

Mr Thant wrote:
On 12 Feb, 14:10, "John Rowland"
wrote:
It doesn't give height, though. Is it hard for GPS to be used to
determine height above sea level?

A GPS receiver works by narrowing down your position in three-
dimensional space, so figuring out your altitude and figuring out your
location are inseparable.

If only three satellites are visible, the locus of possible locations is a
straight line in space which intersects the earth's surface at two points.
Knowing which satellites are visible enables you to eliminate one of these
points, but it doesn't give you the height. Even if quite a few satellites
are visible which were all roughly in a plane, there would be low precision
on the height, and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

On 13 Feb, 00:21, "John Rowland"
wrote:
If only three satellites are visible, the locus of possible locations is a
straight line in space which intersects the earth's surface at two points.
Knowing which satellites are visible enables you to eliminate one of these
points, but it doesn't give you the height.

Unless the line is vertical you need to know (or guess, I suppose) the
altitude to provide an accurate lat/long.

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

In uk.transport.london message ,
Wed, 13 Feb 2008 00:21:44, John Rowland
n.co.uk posted:
and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

Not so. Consider two satellites at the same height above a flat earth,
for high and low satellites, and emitting pulses simultaneously.


S1 S2









s1 s2
_____JR______________ - You X -


The delay between hearing S1 & S2 is clearly less than that between
hearing s1 & s2. JR can therefore tell, if the X-positions of the
satellites are known, how far below the satellite line he is.

--
(c) John Stockton, nr London UK.
Web URL:http://www.merlyn.demon.co.uk/ - FAQish topics, acronyms, & links.
Correct = 4-line sig. separator as above, a line precisely "-- " (SoRFC1036)
Do not Mail News to me. Before a reply, quote with "" or " " (SoRFC1036)

On Thu, 14 Feb 2008, Dr J R Stockton wrote:

In uk.transport.london message ,
Wed, 13 Feb 2008 00:21:44, John Rowland
n.co.uk posted:
and if all the satellites were exactly in a plane, there
would be no information to calculate the height.

Not so. Consider two satellites at the same height above a flat earth,
for high and low satellites, and emitting pulses simultaneously.


S1 S2









s1 s2
_____JR______________ - You X -


The delay between hearing S1 & S2 is clearly less than that between
hearing s1 & s2. JR can therefore tell, if the X-positions of the
satellites are known, how far below the satellite line he is.

True. Although now John doesn't have any reason to shout "I want these
motherf****ing satellites OFF the motherf****ing plane!", which is a
shame.

Am i right in thinking that you couldn't calculate height if the
satellites were all equidistant from you? But then you wouldn't be able to
calculate position at all. Is there a configuration where you can get a
fix in XY but not Z?

tom

--
Work alone does not suffice: the efforts must be intelligent. -- Charles
B. Rogers