<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>This is not about space per se but it is kind of about planetary mapping. Anyway, my 10-year-old mega-boffin is pestering me about this one: If all of our oceans evaporated where would the mean elevation of the "Dry Earth" lie? He's pretty sure it would be below our current mean sea level but how far? Of course, you have to establish a frame of reference. Since the earth is not spherical any measurements from the centre of the earth would distort the figures so let's just frame it relative to msl or say the base of the Marianas Valley (ex-Trench). Thanks, KG <br />Posted by kiwigavin</DIV><br /><br />Is he asking about the mean elevation, or the reference level "zero" altitude.</p><p>The mean elevation would be exactly where it is today, wherever that may be.</p><p>The reference zero altitude however, could no longer be at sea level as that would not exist anymore. It would probably be calculated the way it is today on mars, ie at the altitude of highest gravity. When you go up or down from this point, gravity falls. Up from altitude zero, gravity falls because of the inverse distance square law. Down from altitude zero, gravity rises because there is less Earth mass below.</p><p> </p><p> </p> <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>