[talk-au] NSW/VIC Border and Great circles.

Nick Hocking nick.hocking at gmail.com
Wed Apr 9 12:17:24 BST 2008


Last weekend I did a bit of mapping down near Bombala and Delegate. I
discovered that when I entered the tracks into JOSM that the points where
the OSM nsw/vic border intersected two roads that I mapped were in
disagreement with what I observed on the ground by a couple of hundred
metres.

I notice that the OSM coordinates for the end points of this "straight line"
are a bit southish of those used by Google and published by Geoscience
Australia.   Correcting these end points would only make the problem worse
by about another 200 metres.

I visited the site

http://www.livephysics.com/ptools/great-circle-path.php

where you can draw Great Circles on goolemaps and when I asked it to use the
google endpoints. it drew a Great Circle segment that exactly matched the
google rendered border and which exactly matched my observations on the
roads.

I appears that OSM is drawing "straight flat" lines where it (in this case)
needs a "straight curved" line :-)

I plan to revisit the area and to plot as many points on this "Black-Allan"
line as I can comfortably reach in a nice air-conditioned car.
Then joining these dots up with the OSM published endpoints (which may need
a bit of tweaking) we should get a better approximation of the actual
border.

If you draw a Great Circle from the northwest of  Western Australia down to
the south east of New South Wales you will notice that on google it will
appear to curve down to the south in the middle.  This type of curve may
well fix my observed errors.


PS the western endpoint (closest source of the Murray to Cale Howe) is
sometime referred to as Townsend Corner or Indi Springs and the Eastern end
(at Cape Howe) is known as Wauka or Conference Point. For some interesting
reading, google for "black allan line" and have a look at the PFD "Surveying
the Black - Allan Line"

Cheers
Nick
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