[Proj] Google Earth Accuracy

Ethan Alpert ealpert at digitalglobe.com
Sun Nov 30 09:46:47 EST 2008

How come no ones questioning the accuracy of the sensors that collected
the imagery in Google Maps? I happen to know that satellite imagery is
not very accurate unless you have decent ground control and a DEM. Even
then it's not that great. It seems pointless to try to determine
accuracy in Google Maps unless you understand where the imagery came
from and how it was produced.


-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Noel Zinn
Sent: Sunday, November 30, 2008 6:13 AM
To: 'PROJ.4 and general Projections Discussions'
Subject: RE: [Proj] Google Earth Accuracy

Interesting studies, Irwin, and I'd very much appreciate the PDF copy
you offer.  A colleague did something similar worldwide with VOR/DME
and airstrip end centers and found that most agreements were better than
meters, but a few were tens of meters off.  But this is Google Earth,
Earth can be rotated and whose implicit projection is something similar
the orthographic.  The Mercator lives in Google Maps, which, if I'm not
mistaken, is a different product.

Noel Zinn

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Irwin Scollar
Sent: Sunday, November 30, 2008 4:44 AM
To: proj at lists.maptools.org
Subject: [Proj] Google Earth Accuracy

In defence of Google coordinates:

>Cliff Mugnier wrote:

>Ah, there's a BIG difference between the true coordinate system
>geodesy used by national governments and one cooked up by an ignoramus
>Google Maps that did not know what they were doing ... I guess there's
>of that going around, too.
>I suppose even twits help contribute to keep knowledgeable consultants

>Noel Zinn wrote:

>Not that Frank is responsible for the geodesy and cartography in Google
>(or their abuse therein), but the phrase "the resulting lat/long
>are intended to be treated as WGS84 after that" so troubles me that I
>sympathetic to Cliff's sentiments.  So, let's quantify the offense with
>experiment that anyone can duplicate, perhaps in Proj4 (I work in

I think that an appropriate way to examine the errors in Google Earth 
is to use points with known WGS 84 coordinates that can also be seen 
clearly in Google Earth.

In the UK, you can obtain free complete lists of latitudes and 
longitudes for more than 6000 white trigonometric point pillars 
placed by the Ordnance Survey between 1936 and 1962 from:


The lists include coordinates in the British National Grid and their 
values converted to WGS84 using  official Ordnance Survey software.

Unfortunately, many of these points are no longer maintained and many 
are not visible in Google
Earth. However, checks made on a random sample of 12 visible trig 
points in England and Wales showed most to be positioned by GE within 
about 2.0m which is adequate for any work on 1:10,000 scale maps such 
as the digital versions issued by the Ordnance Survey. However, some 
were off by as much as 12 meters. Similar lists of trigonometric 
points are available in some other countries, but usually require 
payment for accessing them.

The UK data probably gives a good idea of general GE accuracy under 
the most favourable conditions. Much of the GE data in the southern 
UK is obtained from vertical aerial cover rather than from satellite 
imagery.  In other parts of the world where GE does not use data of 
this quality, errors are higher, but are usually acceptable if the 
area examined is less than a 2 km square and only one image rather 
than a mosaic of multiple images is the area of interest.

A report on this test in PDF format is available on request.

I also compared the results obtained by a collaborator who used a 
hand-held Garmin GPS receiver (+/- 3 meter accuracy)  at some clearly 
visible GE points in a few places in Egypt and the Sinai desert with 
Digital Globe imagery and found that the errors were acceptable 
compared with the Egyptian Old Datum maps at 1:25000 after datum 
conversion using the old NIMA 3 parameter constants for datum 
transformation between the Helmert 1906 and WGS84 ellipsoids.

Irwin Scollar 

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