Accuracy of coords on Google maps

[+loobcom]

Hi,

 

I am a newbie and enjoying GC very much.

 

To get my coordinates I have been using the Iphone, the Garmin checking them against the gadget at

 

www.getlatlon.com which is based on Google Maps.

 

My question is, how reliable is Google Maps, especially around the Prime Meridian as most of the stuff I do is in East London?

 

Loobcom

[+jabberwock82]

I'm not a surveyor, and I'm from the central US, but I use Google Maps quite a bit when I have a DNF (which is all too common ), so I can see if maybe my GPSr was off. When it's a open area with good landmarks on the satellite image (sidewalks, lightposts, etc.), Google Maps agrees with my GPSr to within a couple of feet.

 

--

John

[+genegene]

I have checked the accuracy of google maps to some of my caches and dome are very close and others are about 80ft away.

[+edscott]

I'm not a surveyor, and I'm from the central US, but I use Google Maps quite a bit when I have a DNF (which is all too common ), so I can see if maybe my GPSr was off. When it's a open area with good landmarks on the satellite image (sidewalks, lightposts, etc.), Google Maps agrees with my GPSr to within a couple of feet.

 

--

John

 

And when you are in a poor reception area it's your GPS that gets confused, not the photo. So in the woods the photos are still just as accurate. The biggest problem with Google is they seem to be replacing many of their older photos with ones taken over summer vegetation. Those winter shots that showed every fallen tree made off trail woods navigation relatively easy.

[+StarBrand]

Most all maps have a published accuracy of 48 feet or more at well known locations. Your GPS is more accurate than that. In some spots the google maps are spot on. In other places they are 80 to 100 feet off. Trouble is, you just don't know for sure for where you are looking. I don't trust google sat images for anywhere near me.

[+fizzymagic]

My question is, how reliable is Google Maps, especially around the Prime Meridian as most of the stuff I do is in East London?

Well, here is a log of a Prime Meridian marker with a waypoint marked. As would be expected, the GPS does not read longitude zero at these markers, because the WGS84 datum uses a different prime meridian that is 102.5 m east of the 1884 line.

 

But using Google Maps with satellite view, I get the marker at about:

 

N 51 30.453, W 0 00.098,

 

which is 113 m west of the WGS-84 Prime Meridian.

 

I found another meridian rosette in Greenwich at:

 

N 51 28.879, W 0 00.093

 

which is 107.2 m from the WGS-84 meridian.

 

It's hard to see in the shadows, but the point in front of the Greenwich Observatory is at about:

 

N 51 28.674, W 0 00.090

 

which is 104 meters.

 

So, given those three points in Google Maps, I would say that they are accurate to about 10 m in the London area. Not good enough to use for cache coordinates of a hide, but plenty accurate for helping you find caches.

[+SilentWolf]

I haven't relied on google maps at all - and wouldn't - My home coordinates are at least 100' off what their map shows. I recently was marking coordinates for some people doing research in graveyards so they could have better "descriptions" where certain headstones are, and I found they were also about 100' off the satelite views.

[+Tobias & Petronella]

Here is an example of it being way off. GC1GT8T Google maps puts it in someones front yard and the true GZ is 170 feet north.

[Pajaholic]

It depends on where you are as to how accurate Google Maps (GM) is. In my area, it's within five or six feet and for every one of my ten finds so far it's been within that distance. However, you can check for yourself if you know the WGS84 co-ordinates of a few locations in your area. If you don't have the WGS84 co-ordinates but have the OS grid reference you can convert it to WGS84 with GSAK or an online tool.

 

FWIW, I used OS trig points to check GM accuracy in my area and got the co-ordinates from here. However, note the site owner's caveat:

The OS grid references in the database were generated by converting the WGS84 coordinates to OSGB36 and then generating the grid reference. The algorithm used was that defined in A Guide to Coordinate Systems in Great Britain published by the Ordnance Survey. This algorithm is known to introduce errors of up to 5 metres and my implementation may not be totally debugged, which basically means do not trust the last digit of a grid reference given here. Actually, the WGS84 coordinates may not be trustworthy either.

So it's probably best to use the lat/long co-ords from that site and to verify that GM is or is not accurate in your area, you probably should get as accurate a fix as you can with your GPSr for at least a couple. You should also note that the OS say trig points should not be used as GPS control stations - but they're talking about ETRS89, which is used for precise surveys to the nearest couple of inches and so I suspect that trig points are good enough for our purposes!

 

For example, according to that site, Hilly Fields trig point is at N 51 27.585 W 0 1.433. Enter those co-ords into GM and the resulting marker is about 6ft off the baseplate of the trig point in the photo.

 

HTH,

 

Geoff