# [Proj] Local Projection Selection

Patrick Mézard pmezard at gmail.com
Sun Aug 7 15:47:38 EDT 2005

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

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