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VK2TV > GOOGLE 09.03.08 03:00l 121 Lines 5919 Bytes #999 (0) @ WW
BID : 35403_VK2TV
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Subj: Google Earth Accuracy
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From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
To : GOOGLE@WW
# Generated by: TstHWin v2.21b - Registered to VK2TV
# On : 3/9/2008 11:42:25
# UTC: 3/9/2008 0:42:25 A
Hi All,
As someone who considers himself to be a mapaholic, I consider I'm well
versed to make some comments on Australian topographic maps and their
accuracy.
I have relied heavily on navigation by topograpic maps and compass in some
of the most remote, devoid of roads areas in Australia. I have used
1:25,000 maps in reasonably well inhabited areas, and both 1:1,000,000 and
1:250,000 maps in remote areas. For much of central Australia, 1:250,000
scale is the best available.
But first we need to consider if accuracy is really the correct word, or do
we mean currency?
Most topographic maps are as accurate as technology of the day permits,
with maps where extensive field checking took place being a little more
accurate than those for which field checking is not as extensive. These
maps are accurate in detail only at the time the latest field check was
done, or aerial (satellite) photograph taken. The time lag between then
and the day the map is printed can be many years.
In conversation 25 years ago with a surveyor for National Mapping Australia,
he claimed that the 1:250,000 maps of Central Australia could be in error
by as much as a kilometre, or more.
As an indication of the currency and accuracy of some the 1:250,000 Central
Australia topographic maps we used during an outback trip take a look at
the two examples below. We used the R 502 series in 1982, these being the most
recent verions at the time.
McDills (SG 53-7) - printed in 1971 but compiled in 1962 from 1950-1960 aerial
photography. Some ground examination in 1970.
Pandie Pandi (SG 54-9) - printed 1969 but compiled in 1963 from 1958 aerial
photography. Minor amendment in 1968. This map has an east-west void 40km x
1km due to a void in the aerial photo.
Despite these "problems" the maps could still be relied on because the
topographic information doesn't change. Much of this area is endless
sandhills. A first glance at the map would make one conclude a graphic
artist went mad drawing the lines that represent the sandhills but the
reality is that every one of those sandhills is shown in the right place.
We were able to count sandhills from a location and know that our
destination of interest would be that many sandhills away. Prior to a 1985
trip through the North Simpson Desert, my friend purchased the latest
aerial photographs, no more than a year old. We could perfectly match the
sandhills with the maps based on photos taken 25 years previously.
In Dr. C.T. Madigan's book recalling his camel expedition across the North
Simpson Desert in 1939, Madigan described features that, in 1985, were still
exactly as he described in 1939.
So, from a navigation point of view, the age of the map's topographic
information may be quite irrelevent. However, we found the remains of 40 to
100 year old fences to be an excellent navigation aid, those fences also
appearing on the maps, at times.
One must obviously apply lots of commonsense when a map based on 20 year
old information, and a compass, are the difference between success or
disaster. GPS in those areas would be a tremendous asset that wasn't
available to us at the time. However, one would be a complete fool to go
into those remote areas without paper maps, a compass, and the knowledge to
use them. I suspect it's only a matter of time before we read of a tragedy
that was the result of a GPS failure. We used a sextant occasionally to
verify position.
In my dealings with Google Earth, I've found the topographic detail to be
at least as good as, and more often, more accurate than paper maps.
The question of accusracy has raised its head a few times with radio site
coordinates taken from 1:25,000 topo maps being in a different position to
those obtained with satellite based (GPS) systems. This showed up in a
particularly noticeable way when I plotted coverage of my UHF CB repeater
and found a huge shadow to the west of the site. The site sits on a very
narrow north-south ridge and the paper map had the site down the eastern
edge of the ridge. Paper based 1:25,000 (and possibly others as well) show
a correction factor to relate to satellite based positions.
Since I have little interest in urbanised areas, any innacuracy in the
detail shown on either paper maps or Google earth is of little consequence
to me. But even in suburbia, even the latest paper map is out of date before
it's been printed.
Landcare authorities in this state rely on images from the US Landsat
satellite to provide "recent" pictures of activity, such as mass land
clearing without authority. I don't know how recent "recent" is but it was
only a matter of weeks between a local clearing his land with a D9 and the
summons arriving. It cost him $100,000 in fines.
Google Earth is a very useful tool but just like spacial information from
any source, we need to learn how to use it, and to appreciate any
shortcoming it may have.
Google Earth is free, and it is usually more accurate than the latest paper
maps that cost around $10 each, for just a small area. A typical 1:25,000
topographic map covers just 7.5 minutes of Latitude and 15 minutes of
Longitude, or about 14km x 24km, at my Latitude. A minute of Longitude is
greatest in distance at the Equator and smallest at the poles (actually
zero!).
To get all of Australia in 1:1,000,000 scale requires 51 maps (I bought them
all when they were only $2.50 each). Google Earth returns much more detail,
it's free, and I don't need a cupboard in which to house it!
The foregoing relates to navigation and topographic features. It was never
intended to relate to changing development activities in an area, even
though some man made features are sometimes used for navigation purposes.
Pilots use them all the time:-)
Cheers ... Ray vk2tv
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