title: how map 2d grid points (x,y) onto sphere as 3d points (x,y,z)
body: <p>I have a set of 2d grid points (x,y) that I want to map/project onto a sphere as 3d points (x,y,z).</p>
  
  <p>I realize there will be some warping towards the poles as abs(y) increases but my grid patch will only cover a portion of the sphere near the equator so severe warping will be avoided.</p>
  
  <p>I'm having trouble finding the right equations for that.</p>