Bilinear interpolation is used so that values will vary smoothly within a
longitude-latitude grid cell. Note that the sign convention for
latitude must match that in
topoInterpolate(longitude, latitude, topo)
Vector of longitudes (in the same sign convention as used in
Vector of latitudes (in the same sign convention as used in
A topo object.
Vector of heights giving the elevation of the earth above means sea level at the indicated location on the earth.