M25 Speed cameras
 

FYI

Don't let the *******s get you.

All

I have just been informed by my dad who works for the Metropolitan Police
Service that as and from the 15th February 08 all the Speed Cameras in the
Variable Speed Limit section of the M25 near Heathrow Airport (JCT 10-15)
are going Digital and the activation limit is being lowered.

They are currently the older film type cameras and Set at 90mph when the
Variable Limit Signs are not in operation.

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) when the signs are
not in use i.e. national speed limit. I can't confirm the activation limit
when the signs are set (variable limits showing) but believe it would be
the same 10% + 2mph of the speed shown by the sign at that time.

Please pass this on to anyone you know.

I hope this saves a few of you getting points on your licence!!!

M25 Speed cameras
 

In message , at 11:52:45 on
Tue, 12 Feb 2008, Anon 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.
--
Roland Perry

M25 Speed cameras
 

On 12 Feb, 12:14, Roland Perry wrote:
In message , at 11:52:45 on
Tue, 12 Feb 2008, Anon 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.
--
Roland Perry

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" ?!

M25 Speed cameras
 

On 12 Feb, 12:26, Mizter T wrote:
On 12 Feb, 12:14, Roland Perry wrote:

In message , at 11:52:45 on
Tue, 12 Feb 2008, Anon 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.
--
Roland Perry

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" ?!

The diference between what the speedomoter reads and the actual speed.
I have a Origin B2 in my car which has GPS speed indication. The
difference between it and my cars speedomoter is roughly 3mpg ...
granted, my car is relaively new and the Germans know precision but
all the same I think 15mph variance is pretty unlikely even with an
Austin Healy rustbucket (but it probably couldn't get to 80 in the
first place!)

M25 Speed cameras
 

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

M25 Speed cameras
 

Anon wrote:

FYI

Don't let the *******s get you.

All

I have just been informed by my dad who works for the Metropolitan Police
Service that as and from the 15th February 08 all the Speed Cameras in the
Variable Speed Limit section of the M25 near Heathrow Airport (JCT 10-15)
are going Digital and the activation limit is being lowered.

They are currently the older film type cameras and Set at 90mph when the
Variable Limit Signs are not in operation.

The new digital cameras are going to be set at the normal 10% of the speed
limit + 2mph (80mph + and your taking a risk basically) when the signs are
not in use i.e. national speed limit. I can't confirm the activation limit
when the signs are set (variable limits showing) but believe it would be
the same 10% + 2mph of the speed shown by the sign at that time.


AIUI the 10% + 2mph formula used to be the official guidance from the
Association of Chief Police Officers (ACPO) to police forces, but I
was under the impression that it was dropped, officially at least,
when Transport 2000 threatened legal action back in 2000 - as detailed
by these two BBC News online stories:

Transport 2000 granted permission for judicial review of ACPO policy:
http://news.bbc.co.uk/1/hi/uk/763331.stm

ACPO climbs down after threat of judicial review:
http://news.bbc.co.uk/1/hi/uk/848464.stm

(The first story speaks of a 5mph 'buffer', but the second correctly
states that the 'buffer' was in fact 10% + 2mph.)


I wouldn't be at all surprised to find that police forces still use
the 10% + 2mph formula when it comes to setting the trigger speeds for
speed cameras.

M25 Speed cameras
 

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?

M25 Speed cameras
 

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

M25 Speed cameras
 

Anon wrote:

FYI
All
I have just been informed by my dad who works for the Metropolitan Police
Service....

This was posted on nanother group last week, and the result was that it
is thought to be a usenet/email hoax.

Please pass this on to anyone you know.

Which proves the above.
Alan.


--
To reply by e-mail, change the ' + ' to 'plus'.

M25 Speed cameras
 

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?

M25 Speed cameras
 

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!

M25 Speed cameras
 

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

M25 Speed cameras
 

At 13:44:35 on Tue, 12 Feb 2008 Roland Perry opined:-

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.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.
--
Thoss

M25 Speed cameras
 

In message , at 18:20:31 on Tue, 12 Feb
2008, thoss remarked:
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.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.

Absolutely no margin allowed for under-reading. Sources differ slightly
with regard to over-reading, but the limit seems to be +10% + 4kph (from
Hansard in 2001).

As there are apparently anything up to seven amendments *per year* to
the Construction and Use Regulations, I'll leave it to someone else to
find the exact cite.
--
Roland Perry

M25 Speed cameras
 

On Tue, 12 Feb 2008 05:02:55 -0800 (PST), Mizter T
wrote:

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.

Be careful if you do use a sat-nav. If you're going up or downhill,
it will read slower because (so far as the satellites can see) you've
slowed down because your speed has a vertical component as well.

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?

Yes, as it's illegal for one to read even 1mph under, but may read up
to 5% (I think) over. Thus, they all read over.

Neil

--
Neil Williams
Put my first name before the at to reply.

M25 Speed cameras
 

"thoss" wrote in message

At 13:44:35 on Tue, 12 Feb 2008 Roland Perry opined:-

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.

ISTR that the Construction and Use Regulations require speedos to be
accurate to +/-10%. I don't think you can rely on yours over-reading.

I have a hand-held GPS that shows my car's speedo over-reads by about 6%
or 7%. One oddity is that my cruise control allows me to set an exact
speed (digitally), which then doesn't agree with analogue speedo (even
though I assume they are both driven off the same digital source,
probably in the gear-box). My GPS shows that the analogue speedo is more
accurate than the cruise control. So, if I want to drive at exactly
70mph, I would set the cruise control at 75 or 76 mph.

One reason speedos have to over-read is that even the most precise
speedo driven from the drive train will not be exactly right -- it
depends on your tyre pressures and air temperature, as well as tyre
characteristics. If you have been driving at speed for a while, the
tyres will warm up, increasing the pressure in the tyres, and hence
their rolling radius. So, with the exact same speedo reading (and cruise
control setting), the car will actually travel a bit faster.

My car is available with 18", 19" and 20" wheels. All have tyres with
the same nominal external diameter, but the higher profile tyres fitted
to the smaller diameter rims will be less stiff than the ultra-low
profile tyres fitted to the 20" rims. The latter will therefore have a
slightly larger rolling radius, so the car will be travelling a bit
faster with the same speedo reading. The rolling radius will reduce
slightly if the car is heavily loaded.

This is all in addition to any manufacturing tolerances in the speedo
itself. That's why manufacturers typically aim at a +5% reading, so the
actual readings will be in the 0 to +10% error range.

M25 Speed cameras
 

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

M25 Speed cameras
 

On Tue, 12 Feb 2008, Neil Williams wrote:

On Tue, 12 Feb 2008 05:02:55 -0800 (PST), Mizter T
wrote:

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.

Be careful if you do use a sat-nav. If you're going up or downhill,
it will read slower because (so far as the satellites can see) you've
slowed down because your speed has a vertical component as well.

Unless your box is using the height it's inseparably calculated in
determining your speed!

If it was, it could tell you the gradient you're climbing, too.

tom

--
No hay banda

M25 Speed cameras
 

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.

M25 Speed cameras
 

At 23:19:40 on Tue, 12 Feb 2008 Tom Anderson opined:-

On Tue, 12 Feb 2008, Neil Williams wrote:

On Tue, 12 Feb 2008 05:02:55 -0800 (PST), Mizter T
wrote:

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.

Be careful if you do use a sat-nav. If you're going up or downhill,
it will read slower because (so far as the satellites can see) you've
slowed down because your speed has a vertical component as well.

Unless your box is using the height it's inseparably calculated in
determining your speed!

If it was, it could tell you the gradient you're climbing, too.

Since we're here talking sat-navs and speed limits, and since sat-navs
have maps, measure speeds and know about speed limits, are there any
which issue a warning when the local limit is being exceeded?
--
Thoss

M25 Speed cameras
 

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

M25 Speed cameras
 

On 12 Feb, 14:10, "John Rowland"
wrote:
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?

Mine does (BMW sat-nav), along with north/south etc. Not had much use
for it, but I guess it could be handy for figuring out if you're on a
flyover, or the road undreneath for navigation purposes.

M25 Speed cameras
 

thoss wrote:

Since we're here talking sat-navs and speed limits, and since sat-navs
have maps, measure speeds and know about speed limits, are there any
which issue a warning when the local limit is being exceeded?

Tomtom.

M25 Speed cameras
 

David F wrote:
On 12 Feb, 14:10, "John Rowland"
wrote:

Is it hard for GPS to be used to
determine height above sea level?

Mine does (BMW sat-nav), along with north/south etc. Not had much use
for it, but I guess it could be handy for figuring out if you're on a
flyover, or the road undreneath for navigation purposes.

And does it use the height information in conjunction with flyover height
information to work out which road you're on?

My Tomtom has a weakness here. You're heading from Central London to Hanwell
and it gives the correct route, turning off the A4 at one of the junctions
under the M4 elevated section. Then when you get beneath the M4, it suddenly
decides you have wrongly gone on the M4, and gives you a new much longer
route turning off the M4 miles away, with no clue about which A4 junction
you're supposed to use.

M25 Speed cameras
 

John Rowland ("John Rowland" )
gurgled happily, sounding much like they were saying:

Mine does (BMW sat-nav), along with north/south etc. Not had much use
for it, but I guess it could be handy for figuring out if you're on a
flyover, or the road undreneath for navigation purposes.

And does it use the height information in conjunction with flyover
height information to work out which road you're on?

It's nowhere near accurate enough.

My Tomtom has a weakness here. You're heading from Central London to
Hanwell and it gives the correct route, turning off the A4 at one of the
junctions under the M4 elevated section. Then when you get beneath the
M4, it suddenly decides you have wrongly gone on the M4, and gives you a
new much longer route turning off the M4 miles away, with no clue about
which A4 junction you're supposed to use.

chuckle
And this, children, is just one of many reasons why GPS speed limiters
won't work.

M25 Speed cameras
 

IIRC the use of GPS to determine heights is a complex topic. You need
to determine the spheroid and geoid separation in relation to the grid
used and so on. Remember that many countires use by the different
versions of these for their mapping and with different origins.

Additionally as identified above there are satellite fix issues.

I suspect then, as a relative method it would be reliable but more
difficult to be reliable as an absolute method.

OC

M25 Speed cameras
 

On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need to
determine the spheroid and geoid separation in relation to the grid used
and so on. Remember that many countires use by the different versions of
these for their mapping and with different origins.

If you want to know the height above local sea level, then yes, you need a
map of the geoid. But nobody uses that. In the UK, we use height above the
OSGB36 datum, which can be computed from the WGS84-based GPS height fairly
easily (not trivially, but a computer can do it without breaking a sweat).
Ditto for any other reference frame.

Additionally as identified above there are satellite fix issues.

I think this is the killer. There's just so much inaccuracy in a typical
height measurement that it's not very useful.

I suspect then, as a relative method it would be reliable but more
difficult to be reliable as an absolute method.

Interesting point.

tom

--
Me ant a frend try'd to WALK the hole unterrgrand but was putting off -
sometime we saw a trane! -- Viddler Sellboe

M25 Speed cameras
 

On Wed, 13 Feb 2008, Tom Anderson wrote:

On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need to
determine the spheroid and geoid separation in relation to the grid used
and so on. Remember that many countires use by the different versions of
these for their mapping and with different origins.

If you want to know the height above local sea level, then yes, you need a
map of the geoid. But nobody uses that. In the UK, we use height above the
OSGB36 datum,

Hang on, no, that's rubbish. We do use the local sea level, aka Ordnance
Datum Newlyn.

So yes, you're right.

Clearly, the solution is just to switch to using geometric rather than
gravitational heights.

The use of an irregular height datum kind of freaks me out. It's fine for
going up and down without moving across the planet, but it means that you
can't relate a height in one place to a height in another, in terms of
position in space, without knowing the shape of the datum. It means our
coordinate system isn't really a coordinate system.

But if you used geometric coordinates, then you'd find that sometimes,
walking along a contour was walking up or down the gravity well. And the
maths for working out distance is still hard, because it's all on the
surface of a spheroid!

Cartography is hard.

tom

--
Me ant a frend try'd to WALK the hole unterrgrand but was putting off -
sometime we saw a trane! -- Viddler Sellboe

M25 Speed cameras
 

David F wrote:

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" ?!

The diference between what the speedomoter reads and the actual speed.
I have a Origin B2 in my car which has GPS speed indication. The
difference between it and my cars speedomoter is roughly 3mpg ...

You appear to be comparing apples with oranges! :-)

Peter Beale

M25 Speed cameras
 

At 11:33:59 on Wed, 13 Feb 2008 John Rowland opined:-

thoss wrote:

Since we're here talking sat-navs and speed limits, and since sat-navs
have maps, measure speeds and know about speed limits, are there any
which issue a warning when the local limit is being exceeded?

Tomtom.


Thanks. I'll check them out.
--
Thoss

M25 Speed cameras
 

On Wed, Feb 13, 2008 at 09:39:38AM +0000, thoss wrote:

Since we're here talking sat-navs and speed limits, and since sat-navs
have maps, measure speeds and know about speed limits, are there any
which issue a warning when the local limit is being exceeded?

Mine turns the speed display from white to red when I exceed it
(possibly when I exceed it by a certain amount, not sure). Trouble is,
it's *far* too small to even see what colour the text is without taking
my eyes off the road for longer than I want to. I've not found any way
of getting an audible warning out of it.

It does give audible warnings when I approach speed cameras. It warns
me even when I'm stuck in traffic and being overtaken by arthritic old
ladies in zimmer frames.

I use Tomtom on a Palm Treo 680, I presume that other Tomtoms are the
same.

--
David Cantrell | Official London Perl Mongers Bad Influence

Featu an incorrectly implemented bug

M25 Speed cameras
 

David Cantrell wrote:


I use Tomtom on a Palm Treo 680.
Mine turns the speed display from white to red when I exceed it
(possibly when I exceed it by a certain amount, not sure). Trouble
is, it's *far* too small to even see what colour the text is without
taking my eyes off the road for longer than I want to. I've not
found any way of getting an audible warning out of it.

Have you downloaded the latest software? My latest download introduced an
option under "Safety preferences" to choose a sound for when you go over the
speed limit. I've never actually tried it, though.

M25 Speed cameras
 

At 11:54:45 on Thu, 14 Feb 2008 David Cantrell opined:-

On Wed, Feb 13, 2008 at 09:39:38AM +0000, thoss wrote:

Since we're here talking sat-navs and speed limits, and since sat-navs
have maps, measure speeds and know about speed limits, are there any
which issue a warning when the local limit is being exceeded?

Mine turns the speed display from white to red when I exceed it
(possibly when I exceed it by a certain amount, not sure). Trouble is,
it's *far* too small to even see what colour the text is without taking
my eyes off the road for longer than I want to. I've not found any way
of getting an audible warning out of it.

It does give audible warnings when I approach speed cameras. It warns
me even when I'm stuck in traffic and being overtaken by arthritic old
ladies in zimmer frames.

I use Tomtom on a Palm Treo 680, I presume that other Tomtoms are the
same.

I was hoping for something with an audible warning.
--
Thoss

M25 Speed cameras
 

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)

M25 Speed cameras
 

Tom Anderson wrote:
On Wed, 13 Feb 2008, Tom Anderson wrote:

On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need
to determine the spheroid and geoid separation in relation to the
grid used and so on. Remember that many countires use by the
different versions of these for their mapping and with different
origins.

If you want to know the height above local sea level, then yes, you
need a map of the geoid. But nobody uses that. In the UK, we use
height above the OSGB36 datum,

Hang on, no, that's rubbish. We do use the local sea level, aka Ordnance
Datum Newlyn.

Well whenever I am using GPS these days [1], I can find my altitude by
reference to my watch and a copy of Reed's Almanac. And that leads me
to the question, what sea level are you taking? Certainly most charts
I've found (Admiralty and Imray) use LAT [2] as their datum for points
below MHWS [3], and MHWS for heights on dry land.

[1] and, no, I don't rely on it, I always have several alterntive
methods of navigating at the same time, just in case.
[2] lowest astronomical tide
[3] mean high water springs

Robin

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R.C. Payne wrote:

Well whenever I am using GPS these days [1], I can find my altitude by
reference to my watch and a copy of Reed's Almanac.

Remind me again...how does that work?

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John Rowland ("John Rowland" )
gurgled happily, sounding much like they were saying:

Well whenever I am using GPS these days [1], I can find my altitude by
reference to my watch and a copy of Reed's Almanac.

Remind me again...how does that work?

Dead easy... Reed's Almanac is a tide table...

I'm not quite sure how it'll tell you whether the transporter that your
boat is on the back of is on the M4 or the A4 underneath it, though...
Oh, wait. If there's a horrible scraping cracking noise, it's the remains
of the top of the mast against the underside of the M4 flyover.

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Tom Anderson wrote "Cartography is hard."

Tom

Sorry, whilst catography is a related topic, this problem normally
comes under geomatics these days (old fashioned land surveying).

OC

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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

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On Thu, 14 Feb 2008, R.C. Payne wrote:

Tom Anderson wrote:
On Wed, 13 Feb 2008, Tom Anderson wrote:

On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need to
determine the spheroid and geoid separation in relation to the grid used
and so on. Remember that many countires use by the different versions of
these for their mapping and with different origins.

If you want to know the height above local sea level, then yes, you need a
map of the geoid. But nobody uses that. In the UK, we use height above the
OSGB36 datum,

Hang on, no, that's rubbish. We do use the local sea level, aka Ordnance
Datum Newlyn.

Well whenever I am using GPS these days [1], I can find my altitude by
reference to my watch and a copy of Reed's Almanac. And that leads me
to the question, what sea level are you taking? Certainly most charts
I've found (Admiralty and Imray) use LAT [2] as their datum for points
below MHWS [3], and MHWS for heights on dry land.

Really? I know about LAT, but i'm surprised to hear that land heights are
measured from MHWS. OS maps use the Newlyn datum, which is the mean sea
level at Newlyn back in 1915 or something; that's carried through the
country by levelling, so the datum is an gravitational isopotential
surface. MHWS is not only a high, not mean, tide, but is something that's
affected by local seabed topography, and so is not an isopotential
surface. That means it won't be parallel to the Newlyn datum, so not only
will Admiralty heights be different to OS heights, but the difference will
vary across the country!

Horses for courses, though. Nautical charts use LAT as a datum because
depths are there so you can work out if you're going to run aground and
that lets them have tide values which are always positive. Plus, it means
that when you see a blue bit on a chart, you know it's always underwater.
You couldn't use LAT for land heights, because it's not defined on land. I
suppose they use MHWS on land because it has a similar property - anything
with a positive height is always above water.

Hang on, how do they determine MHWS on land? Are you sure they don't use
ODN?

It irks me that the Newlyn datum is a mean sea level, and not LAT. But
then i suppose it's natural to define an isopotential surface that way,
because it's the sea level you'd have if you got rid of the moon. Except
it's not, because of topographic effects. I think.

In conclusion, geomatics is hard.

Anyway, my proposal is for *all* heights to be measured as distance from
the centre of mass of the earth. SOLVED!

tom

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