Function sample 

pub fn sample(
    source: &dyn TileSource,
    bbox: &Bbox,
    num_lines: usize,
    elevation_pts: usize,
) -> Array2<f64>

Sample num_lines x elevation_pts elevations over bbox.

Mirrors the upstream sampling exactly (/N, not /(N-1), so the last row/column stops one step short of the far corner):

lat = lat0 + r / num_lines       * (lat1 - lat0)
lon = lon0 + c / elevation_pts   * (lon1 - lon0)

Voids (open ocean, bad samples) are NaN.