[Proj] Google Earth Accuracy
Jan Hartmann
j.l.h.hartmann at uva.nl
Sun Nov 30 10:07:08 EST 2008
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 Unfortunately, many of these points are no longer maintained and many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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