Re: [idvusers] Lambert Conic Projection query

• To: Stuart Maclean <stuart@xxxxxxxxxxxxxxxxxx>
• Subject: Re: [idvusers] Lambert Conic Projection query
• From: Don Murray <dmurray@xxxxxxxxxxxxxxxx>
• Date: Wed, 19 Dec 2007 06:36:03 -0700

Hi Stuart-

Stuart Maclean wrote:

I am not sure of the correct home for the ucar.unidata.geoloc.*
package, is it IDV, Netcdf, or some other jar?

The IDV and netCDF Java share some common code and now that
package is part of the netCDF-Java source.

Anyway, in idv 2.2 there exists
ucar.unidata.geoloc.projection.LambertConformal.  I am
copying/transcribing this class into a GWT (Java -> Javascript)
class.  I note this snippet in the projToLatLon method:

double yd = (rhop - fromY); double theta = Math.atan2(fromX, yd); double r = Math.sqrt(fromX * fromX + yd * yd); if (n < 0.0) {

In the call to Math.atan2, we clearly have some x distance as first
parameter, with y second.

Yet the api for java.lang.Math.atan2 suggests that the parameter
order is y, x.  Can anyone explain why the apparent mismatch? I have
used this class in apps before and it seems to work fine.

See:

http://mathworld.wolfram.com/LambertConformalConicProjection.html

for an explanation of the algorithm.  In particular, this
is from:

theta   =       tan^(-1)(x/(rho_0-y))

which ends up as atan(x,y)

Don
*************************************************************
Don Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
*************************************************************

• References:
  • [idvusers] Lambert Conic Projection query
    • From: Stuart Maclean