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

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.

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

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)

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

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?

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.

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

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

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