[Proj] Local Projection Selection

[Gerald I. Evenden]

On Sun, 2005-08-07 at 18:02 +0200, Patrick Mézard wrote:
	...

> I suppose I can ignore the False Easting and Northing. Latitude and 

Yes, indeed

> longitude of natural origin would be the projection center point. 
> Remains the scale factor. I could let it at 1.0. But maybe there is a 
> way to generate it programmatically given a center point, and ellipsoid 

Assuming that most of the points are within so many meters (ie. 50m) of
the central point
(lat_0, lon_0) then select a k_0 factor that would make the scale factor
1
half way between the center point and projection limit.  The center's
true scale will be
somewhat larger than 1. and the rim slightly smaller.  Start with about
+k_0=.99999 .
Use the -V option to experiment.

If you are keeping within 50km. scale errors should probably kept to
within much better than 1 part in
100000 or close to your 2/3 meter.

> and a working area radius. It probably does not worth the pain however. 
> Are the output coordinate units naturally in meters or have I missed 
> other parameters ?



The Cartesian coordinates are the same as the ellipsoid major axis
units.  If 'a' is in
meters then the x-y values are in meters.  There is also a scaling
parameter.
	...
-- 
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