OT How does GMaps know elevation?

Obviously yes. And while I haven't looked, I'd bet there are multiple sources on the web for elevation of any point above sea level.

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.

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

Also, Google has someone at the beach using Gmaps, so they have that as a reference point to establish sea level.

If you click on the GMaps Menu icon (3 line "hamburger" on top left) there is a choice called "Terrain"

While not a true USGS topo map, you get the idea.

USGS and DMA both generate elevation datasets which feed into the various mapping tools.

Nonsense.

I wonder if they use an existing database or just GPS:

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.

...

Modern GPS receivers use all the satellites in view and compute the best solution from this. ... So the GPS altitude is the GEOMETRIC altitude above Mean Sea Level accurate to 10 to 20 meters.

So, GPS isn't accurate for distances that might matter.

I see one random poster claiming Google uses:

My guess is the Google Car that shoots all of the street view pictures is using GPS to link the pictures to the map and they get the elevation data at the same time.

Just a guess that Google Maps could use elevation data from Google Earth, (or its sources), which, according to this paper from 2017, makes GE generally accurate:

That's a thought.

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?

Wow. I saw that years ago but I guess I forgot.

And that was just what I meant, a topographical map already ;inked to to the street map.

I guess that answers the question.

Gfretwell said:

That would make sense, and would be a lot easier than a lot of the things they do.

Did you all know that besides the Google Car, there is now the Google Man, who wears the camera a couple feet above his head, and they have streetview pictures of some places one can only walk.

I think once I saw google pictures from a bicycle.

Thanks, all.

Probably LIDAR.

But that wont work where the google car hasn't ever been and google maps still gets the elevation correct there. Like

Yep. The basic numbers come from various elevation and terrain models and databases. For example:

For radio propagation studies, I use Radio Mobile software:

I usually take the Radio Mobile maps, which show elevation,

The problem in not the lack of elevation maps, but rather an overchoice of which one to use.

Another great discovery when clicking on the hamburger.

It has a line called streetview.

clicking that turns on all the streetview lines and dots, so that you don't have to move Pegman each time you want to see one. You don't have to use Pegman at all, just click on the location, and after you close that window, the map with all streetview lines and dots will still be there.

Did you all know that when a streetview on a line is open and there is an arrow superimposed on the pavement, you can click on the arrow and the next circular picture will be displayed, and maybe you can skip ahead 2 or 3 pictures if you position the arrow farther away. This is good when you're looking for the perfect view or something specific.

I thought about that and thought they might interpolate from other places the car has been, but you found a place pretty far from google cars and google men, and it gives elevation. It says "mostly flat" but if you click on the arrow, it gives a number.

I found one place where it didn't give elevations for walking but did for bicycling. Very strange.

Google maps is not related to GPS! Google maps is a MAP. A static piece of data that you can navigate.

When you have a map on paper (no idea how old you are and if you have ever seen one), you don't ask "how can the map know this road is here when it doesn't use GPS" either.

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.

Obviously Google Maps uses GPS, that's how it shows your location on the map. Whether it uses that to show your actual altitude, idk. A simple test would be to see what happens when you go up a tall building.