[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
the data and using a plane system with its inherent distortions?
If one insists on using a projection, then Stereographic is probably
Determining the center optimal central point is a problem as a
geodesic computation would be needed to determine optimal midpoint
I can see means of minimizing the use of floating point but elimination
would be quite
If the geodesic formula is all you need then I suggest trying to take
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
function such as program geod or Vincenti's method. One might be able
half meter accuracy at 50km. If not, then I recommend Vincenti's method
will mean a moderate amount of double precision math.
On Sun, 2005-08-07 at 11:02 +0200, Patrick Mézard wrote:
> 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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