var xoff = (x * size - xMin) / gZoomFactor;
var yoff = (y * size - yMin) / gZoomFactor;
var tileKey = encodeIndex(x, y, gPos.z);
var xoff = (x * size - xMin) / gZoomFactor;
var yoff = (y * size - yMin) / gZoomFactor;
var tileKey = encodeIndex(x, y, gPos.z);