[Proj] Local Projection Selection

[Patrick Mézard]

Gerald I. Evenden wrote:

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


Ok, and how do I compute the "real" scale factor? By comparing the 
"projected" and geodesic distance of two points ? It would make sense 
and given the geodesic functions in PROJ.4, I could generate the k_0 
from any defined limit radius with some kind of minimization heuristic. 
But you are right, I will stay with k_0=1.0 for now.

>>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.

Great, thank you.

Patrick Mézard