projection conversion equations

[Martin Weinelt]

On Tuesday 07 June 2005 08:09, Dylan Beaudette wrote:
> Hi,
> 
> I would like to re-implement some functionality in the DHTML mapserver 
> interface for converting between Lat/Lon to an Albers Equal Area 
> Projection.
> 
> Currently, I have the proj library doing this for single coordinate 
> pairs, but I would like to integrate this functionality in the form of 
> a JS routine for on-the-fly conversion: much like the existing UTM 
> <->lat/lon conversion.
> 
> Does anyone have any ideas on where to get started?
> 
> thanks in advance!

Dylan,

start with " Snyder, J.P, 1987: Map Projections - A Working Manual".
There you find the formulas for the projections and you can translate 
this to JS and boil it down. I did two for a project lately which I attach 
to give you an idea: Mercator invers and Lambert Azimuthal Equal Area 
invers:

function refSys(radius) {  // some useful constants
  var t=this;
  t.R=radius;
  t.halfpi=Math.PI/2;
  t.R2=t.R*2;
  t.d2r= Math.PI/180;
}
var sphere=new refSys(6371000);

function invMerc(xn,yn) {
  var eyr,phi,lam;
  eyr = Math.pow(Math.E, (yn * -1)/sphere.R);
  phi = (sphere.halfpi - (2*Math.atan(eyr)))/sphere.d2r;
  lam = (xn/sphere.R)/sphere.d2r; 
  return [phi,lam];
}

function invLaea(xn,yn,asp)  {  // sphere, polar aspect, lon0=0
  var p,c,phi,lam,adj;
  if (asp=='N') yn*=-1;
  p = Math.sqrt(Math.pow(xn,2) + Math.pow(yn,2));
  if (p==0) return([90,0]);
  c = 2* Math.asin(p/sphere.R2); 
  phi = Math.asin(Math.cos(c)) / sphere.d2r;
  (yn<=0&&xn>0)?adj=180:adj=0;  // ad-hoc quadrant adjustment
  (yn<=0&&xn<0)?adj=-180:adj=adj;
  var lam = Math.atan(xn/yn) / sphere.d2r + adj;
  return [phi,lam];
}

Cheers, Martin