7 ft below sea level is is farther below sea level than 4 1/2 ft below sea level. This is because:
7-4 1/2
=7-4.5
=2.5 ft
this means that 7 ft below sea level is 2.5 ft deeper than 4.5 ft deep below sea level. Literally, it is the measure of how deep the length goes below the surface of the sea.
