# [Proj] Local Projection Selection

Gerald I. Evenden gerald.evenden at verizon.net
Sun Aug 7 10:39:00 EDT 2005

```My immediate question is why not use a geodesic program rather than
projecting
the data and using a plane system with its inherent distortions?

If one insists on using a projection, then Stereographic is probably
most appropriate.
Determining the center optimal central point is a problem as a
inverse/forward
geodesic computation would be needed to determine optimal midpoint
between the
points.

I can see means of minimizing the use of floating point but elimination
would be quite
difficult.

If the geodesic formula is all you need then I suggest trying to take
the geometric
mean of the ellipsoid radii at one point and using the spherical
geodesic to compute
the distance/azimuth to the other point.  Test these values with an
ellipsoid geodesic
function such as program geod or Vincenti's method.  One might be able
to get
half meter accuracy at 50km.  If not, then I recommend Vincenti's method
which
will mean a moderate amount of double precision math.

On Sun, 2005-08-07 at 11:02 +0200, Patrick Mézard wrote:

> Hello,
> I have a function performing distance calculations between points
> defined in WGS-84 and perhaps some angular computations too. The input
> dataset is always restricted to an area of at most 20km around a center
> point, and usually 10km. The area center is unspecified, but extreme
> cases (earth poles) are excluded.
>
> For every function call, I would like to reproject the input dataset
> into a euclidian space where I could perform metric computations.
> Accuracy is important but not critical (I could cope with offsets of
> 2/3m), computation speed is important too since the function is
> implemented on mobile devices where floating-point computations are
> rather slow.
>
> Could you give me advices to select a projection in PROJ.4 I could
> configure with a center point (x,y), matching the constraints above.
>
> Thank you for any hint.
>
> Patrick Mézard

--
_____________________________________________________________
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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