[Proj] Google Earth Accuracy

[Jan Hartmann]

I can confirm Irwin Scollar's conclusions for the Netherlands: we have 
here a list of about 5000 "official"  points, both in the national grid 
(RD, Rijksdriehoeksnet) and in WGS84 as converted by the official 
transformation program. Quite a few (church towers and the like) are 
easily localisable in Google. From what I have seen, there are 
discrepancies with Google, but they are in an order of magnitude of 
meters. On anything with a scale smaller than 1:10000 there is no real 
problem, but on cadastral levels (I have been working with scales 1:2500 
and larger), you need to be careful when using Google, e.g. in 
archaeological settings. As in the UK, GE data have been derived from 
aerial photographs, manufactured a few years ago on behalf of Google 
itself. I would expect the situation for Germany, where they produced 
the aerial photgraphs the same way, to be the same, although I don't 
have any precise data for that country.

Jan


Irwin Scollar wrote:
 > 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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 >