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

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.

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.

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.

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.

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

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

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

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

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