[MetaCRS] Re: Adding Projection Classes to Proj4js

[Richard Marsden]

Further to last night's email, I now have an implementation of the cea
"Cylindrical Equal Area" projection working. However it currently assumes a
sphere.

I believe I have the forward transform working for an ellipsoid (see below).
Does this look correct?  I try to calculate the Earth's radius at the
coord's latitude and use this for calculating the y ordinate.    This is not
applied to the x ordinate because this would break the "cylindrical" nature
of the projection.

(I notice that many of the projections marked as untested, assume a
spherical Earth)

Personally I don't need the inverse transform, but I'm also not sure how to
apply the ellipsoid correction to the inverse transform.

I know the efficiency of the following code can be improved - eg.
cos(lat_ts) could be cached in the initialisation function.


Richard
-------------------------------


  /* Cylindrical Equal Area forward equations--mapping lat,long to x,y
    ------------------------------------------------------------*/
  forward: function(p) {
    var lon=p.x;
    var lat=p.y;
    /* Forward equations
      -----------------*/
    dlon = Proj4js.common.adjust_lon(lon - this.long0);
    var x,y;

    if (this.sphere)
    {
       x = this.x0 + this.a * dlon * Math.cos(this.lat_ts);
       y = this.y0 + this.a * Math.sin(lat) / Math.cos(this.lat_ts);
    }
    else
    { // Radius at this latitude
      var Sin_Lat = Math.sin(lat);
      var Rn = this.a * (Math.sqrt(1.0e0 - this.es * Sin_Lat*Sin_Lat));

      x = this.x0 + this.a * dlon * Math.cos(this.lat_ts);
      y = this.y0 + Rn * Math.sin(lat) / Math.cos(this.lat_ts);
    }

    p.x=x;
    p.y=y;
    return p;
  },//ceaFwd()
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