code: func newMapGrid(area camtypes.LocationBounds, dim int) *mapGrid {
  	if area.SpansDateLine() {
  		panic("invalid use of newMapGrid: must be called with bounds not overlapping date line")
  	}
  	return &mapGrid{
  		dim:        dim,
  		area:       area,
  		cellWidth:  area.Width() / float64(dim),
  		cellHeight: (area.North - area.South) / float64(dim),
  	}
  }
docs: // newMapGrid returns a grid matcher over an area. The area must not
  // span the date line. The mapGrid maps locations to a grid of (dim *
  // dim) cells.