Tau..

-5700 ft.level means 5,700 feet(12.inches per foot) below surface..that is from where you are standing on top of the ground..now in Geraldton that may be 1900 Feet above sea level,but that does not work into the equation..unless you are mining coal in Cape Breton NS and you are 5700 feet below surface ..you may also be 5600 feet below sea level..as these mines extend out under the sea..

The bottom of the sea above the mine shaft may also be expressed in this equation at this point as being 600 feet above mining workings ,while the mining workings are still 5600 feet below sea level(The top of the sea) so you at that point would have 600 feet of water from the water surface to the bottom of the ocean,and have 5000 feet of rock above where you were moiling for coal..(Like that word..???)or if you were fishing sitting on 100 fathoms of water..

Clear as mud...??

Portee