[Proj] Local Projection Selection

Gerald I. Evenden gerald.evenden at verizon.net
Sun Aug 7 14:43:13 EDT 2005

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
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
Jerry and the Low Riders: Daisy Mae and Joshua
"The most certain test by which we judge whether a country is
really free is the amount of security enjoyed by minorities"
---Lord Acton, 1907 
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