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My immediate question is why not use a geodesic program rather than projecting<BR>
the data and using a plane system with its inherent distortions?<BR>
<BR>
If one insists on using a projection, then Stereographic is probably most appropriate.<BR>
Determining the center optimal central point is a problem as a inverse/forward<BR>
geodesic computation would be needed to determine optimal midpoint between the<BR>
points.<BR>
<BR>
I can see means of minimizing the use of floating point but elimination would be quite<BR>
difficult.<BR>
<BR>
If the geodesic formula is all you need then I suggest trying to take the geometric<BR>
mean of the ellipsoid radii at one point and using the spherical geodesic to compute<BR>
the distance/azimuth to the other point. Test these values with an ellipsoid geodesic<BR>
function such as program geod or Vincenti's method. One might be able to get<BR>
half meter accuracy at 50km. If not, then I recommend Vincenti's method which<BR>
will mean a moderate amount of double precision math.<BR>
<BR>
On Sun, 2005-08-07 at 11:02 +0200, Patrick Mézard wrote:
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<FONT COLOR="#000000">Hello,</FONT>
<FONT COLOR="#000000">I have a function performing distance calculations between points </FONT>
<FONT COLOR="#000000">defined in WGS-84 and perhaps some angular computations too. The input </FONT>
<FONT COLOR="#000000">dataset is always restricted to an area of at most 20km around a center </FONT>
<FONT COLOR="#000000">point, and usually 10km. The area center is unspecified, but extreme </FONT>
<FONT COLOR="#000000">cases (earth poles) are excluded.</FONT>
<FONT COLOR="#000000">For every function call, I would like to reproject the input dataset </FONT>
<FONT COLOR="#000000">into a euclidian space where I could perform metric computations. </FONT>
<FONT COLOR="#000000">Accuracy is important but not critical (I could cope with offsets of </FONT>
<FONT COLOR="#000000">2/3m), computation speed is important too since the function is </FONT>
<FONT COLOR="#000000">implemented on mobile devices where floating-point computations are </FONT>
<FONT COLOR="#000000">rather slow.</FONT>
<FONT COLOR="#000000">Could you give me advices to select a projection in PROJ.4 I could </FONT>
<FONT COLOR="#000000">configure with a center point (x,y), matching the constraints above.</FONT>
<FONT COLOR="#000000">Thank you for any hint.</FONT>
<FONT COLOR="#000000">Patrick Mézard</FONT>
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Jerry and the Low Riders: Daisy Mae and Joshua
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