# [Proj] Google Earth Accuracy

Noel Zinn ndzinn at comcast.net
Sun Nov 30 08:13:07 EST 2008

```Interesting studies, Irwin, and I'd very much appreciate the PDF copy that
you offer.  A colleague did something similar worldwide with VOR/DME beacons
and airstrip end centers and found that most agreements were better than 5
meters, but a few were tens of meters off.  But this is Google Earth, whose
Earth can be rotated and whose implicit projection is something similar to
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 relations
of
>geodesy used by national governments and one cooked up by an ignoramus at
>Google Maps that did not know what they were doing ... I guess there's a
lot
>of that going around, too.
>
>I suppose even twits help contribute to keep knowledgeable consultants in
>business.

>Noel Zinn wrote:

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

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:

http://www.haroldstreet.org.uk/waypoints.php?hill=trigpoints

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Unfortunately, many of these points are no longer maintained and many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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