OT How does GMaps know elevation?

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When you use google maps to choose a route between two locations, and
choose walking or bicycling, it also shows starting and ending elevation
and a graph that shows changes, so you can see how many and how big the
hills are. Also useful, of course, just for finding elevation without
any walking.

But how does it know elevation.  If a GPS gets signals from 3
satellites, it can calculate location, and if it gets signals from 4 of
them, it can calculate elevation.  But the calculation is made in the
GPS device, such as a smartphone.  Google doesn't have GPS devices at
any random location the user picks, and it has no way to read satellite
signals.    

Is there some topographical map underlying the google maps, even though
there is no way to display that map wwithin google?

Re: OT How does GMaps know elevation?
Satellite.  

Re: OT How does GMaps know elevation?
On 30/07/2019 19:43, micky wrote:
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Check out a convenient Google height and you'll find its a load of junk.
In the UK luckily there are still a lot of surviving Ordnance Survey  
benchmarks around and you can optically survey to them and easily get to  
mm accurace if no more than 1-200m away (wrt the old map data containing  
benchmark heights).
It is possible of course that the underlying land has risen or fallen  
from post glacial isostatic rebound or water/oil extraction etc.
But even with mobile phone link dGPS (aGPS) surveying the  
resolution/repeatability is no better than 15mm. And can easily be a lot  
worse and without a second method (eg benchmarks) you won't know.
Outside the UK I've no idea if there are other networks of benchmarks.
I would like to find an "ap" where , as the GPS orbits are known , that  
it would be possible to find times in the near future where 4 satellites  
would be near enough overhead and none at lower elevation.
I wonder what sort of height determination you could get with that for  
height determination.









--  
Monthly public talks on science topics, Hampshire , England
<http://diverse.4mg.com/scicaf.htm

Re: OT How does GMaps know elevation?
micky wrote:

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Every user carrying a GPS enabled android device is crowd-sourcing data  
for google, if someone's been there, google will know the altitude.

Re: OT How does GMaps know elevation?
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Nonsense.

Re: OT How does GMaps know elevation?
In alt.home.repair, on Tue, 30 Jul 2019 20:19:06 GMT,
snipped-for-privacy@slp53.sl.home (Scott Lurndal) wrote:

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That's a thought.  

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That's a thought, too!

BTW, I tried to use google walking to see how much elevation change
there was to the top of the next hill.  I had thought the top of the
hill was the big street nearby, going along a crest (although crest
sounds more like a mountain than a hill.)  

I found out, if it's at all correct, that's it's about halfway between
that big street and the next one aand that my elevation is about 400
feet.  Typically, google directions gives three routes, and when I
changed to one of the others, it changed my elevation to 403 feet, even
though it was the same place.  

It turns out I'm about 65 feet below the top of the ridge of hills a
half mile away.   I think I don't get as much breeze as I would if I
were higher up. I'm not sure because I can't be in two places at once
and I spend comparitively little time farther up the hill.  I like
living next to a stream, but not getting much breeze seems to be the
other side of the coin.    

Does it seem correct to you that a mere 65' valley would be enough to
lessen the breeze I get?  

Re: OT How does GMaps know elevation?
On 7/30/2019 1:19 PM, Scott Lurndal wrote:
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How so? Wikipedia says this:

"Global Positioning System (GPS) receivers can also determine altitude  
by trilateration with four or more satellites. In aircraft, altitude  
determined using autonomous GPS is not reliable enough to supersede the  
pressure altimeter without using some method of augmentation"

https://en.wikipedia.org/wiki/Altimeter

Re: OT How does GMaps know elevation?
On 08/05/2019 11:20 PM, Mike S wrote:
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Barometric altimeters aren't that great either. I haven't flown in years  
so the procedure might have changed but airports would have their  
elevation prominently marked. Part of the preflight was setting the  
altimeter to match.

Re: OT How does GMaps know elevation?


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That?s because air pressure changes. Laser height measuring from
satellites and planes for mapping works fine and is a lot more
accurate than GPS.  


Re: OT How does GMaps know elevation?

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Why is laser measuring much more accurate than GPS? They both use
light and clocks.  
Eric

Re: OT How does GMaps know elevation?
snipped-for-privacy@whidbey.com writes:
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GPS doesn't use lasers.  The receiver triangulates signals from multiple
satellites to determine x, y and z coordinates of the receiver.

The z coordinate (altitude) is not very accurate

http://gpsinformation.net/main/altitude.htm

Re: OT How does GMaps know elevation?
On Tue, 06 Aug 2019 19:30:56 GMT, snipped-for-privacy@slp53.sl.home (Scott Lurndal)
wrote:

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I know GPS doesn't use lasers. It uses radio waves which are light and
move the same speed as lasers. What is it about using lasers that
makes them more accurate?
Eric

Re: OT How does GMaps know elevation?


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It's a direct ping from the satellite or plane with the laser.
With GPS its continuous radio waves with varying propagation
effects. That's why you need more than one visible satellite
with GPS.  


Re: OT How does GMaps know elevation?
snipped-for-privacy@whidbey.com says...
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Wavelength.

Mike.

Re: OT How does GMaps know elevation?


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Nope, it's a laser pulse from the plane or satellite, travel time measured.

GPS is continuous transmission with digital encoding on that.  


Re: OT How does GMaps know elevation?


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Because the laser is the most accurate way to measure things.

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GPS doesn't. It used radio waves.

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Yes.  


Re: OT How does GMaps know elevation?
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Did your mummy say "Because I said so!" a lot?

Mike.

Re: OT How does GMaps know elevation?
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Nope, both mum and dad quickly worked out that doesn't work with me.  

I still remember the mega stoush that happened when I refused
to eat something like spinach. She never tried that again.  

Re: OT How does GMaps know elevation?
On 08/06/2019 01:31 PM, snipped-for-privacy@whidbey.com wrote:
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A GPS receiver extrapolates from the satellite constellation. The closer  
to the horizon the birds are and the more angular distance separating  
them, the better it works. In cities or mountainous terrain the view of  
the sky can be limited so it has to work with those overhead. It has  
nothing to do with light unless you have an extremely loose definition  
of the spectrum.

With a laser you're bouncing the beam off an object.

Re: OT How does GMaps know elevation?
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Google maps doesn't feed back user information into the maps, they
use existing survey data and also do their own surveys and photography.

The only feedback you see in the maps is the traffic info, obtained
by observing speed of users along roads.

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