Given a lat / long pair, return the grid coordinates at the
    nearest base value.  e.g., (42.3, -5.4) returns a grid at 0.1
    degree resolution of 0.1 degrees of latitude ~ 7km, so this is
    effectively a 14km lat grid.  This is imprecise for longitude,
    since 1 degree of longitude is 0km at the poles, and up to 111km
    at the equator. But it should be reasonably precise given some
    prior logical block (e.g., country).

def latLongGridPredicate(field, digits=1):
    """
    Given a lat / long pair, return the grid coordinates at the
    nearest base value.  e.g., (42.3, -5.4) returns a grid at 0.1
    degree resolution of 0.1 degrees of latitude ~ 7km, so this is
    effectively a 14km lat grid.  This is imprecise for longitude,
    since 1 degree of longitude is 0km at the poles, and up to 111km
    at the equator. But it should be reasonably precise given some
    prior logical block (e.g., country).
    """
    if any(field):
        return (str([round(dim, digits) for dim in field]),)
    else:
        return ()