[GRASS5] GRASS, PROJ and NAD
hamish_nospam at yahoo.com
Mon Oct 13 06:52:12 EDT 2003
> The 'east' is actually west of 'west', so that the longitudinal extent
> wraps around the planet, for all but the 5km between the two!
> 129582 cols * 1/6 nmile = 21597.
> $ gcb n 44.25N103.51W 44.25N103.44W
> Short Path Bearing is 090 Degrees for 5 Nautical Miles
> Long Path Bearing is 270 Degrees for 21595 Nautical Miles
and that's a bug in gcb.
R(lat) = Rearth*cos(90-lat)
R(44.417) = 6367.4km * cos(90 - 44.417) = 4456.3km
Circumference along the 44.417 parallel is therefore 4456.3km * 2PI
= 28,000km or 15,119 nautical miles.
As to what effect that will have in GRASS on global datasets, such as
the accuracy of data viewed with d.*.viewproj ....? Not much we can do
about it I guess without going to a 3d grid.
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