Dry Earth - Where Would Mean Elevation Lie?

[kiwigavin]

<p>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:</p><p>&nbsp;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?</p><p>&nbsp;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).</p><p>&nbsp;Thanks,</p><p>&nbsp;KG&nbsp;</p>

 

[derekmcd]

<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:&nbsp;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?&nbsp;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).&nbsp;Thanks,&nbsp;KG&nbsp; <br /> Posted by kiwigavin</DIV></p><p>I'm assuming your asking if the mass of the earth was redistributed so as the surface was perfectly flat while maintaining the oblate spheroid shape of the earth?</p><p>Good question... <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /> </p><p>I found some references that put the average elevation above mean sea level is 800 meters and the average elevation below is 3500 meters.&nbsp; So your kid is definitely correct... pretty sharp observation for a 10 year old.&nbsp;</p><p>From the book Geological Science by <span class="addmd">Andrew McLeish:</span></p><p><strong>LINK to page 6</strong>&nbsp;</p><p>I wasn't sure how accurate that was, so I found another link <strong>HERE</strong> that seemed to agree with it.&nbsp;</p><p>&nbsp;</p><p>I couldn't find a specific number for what the overall average it, but I did find this:</p><p>http://svs.gsfc.nasa.gov/vis/a000000/a002800/a002866/index.html</p><p>From the site:</p><p><font color="#0000ff"><em>"This color map is the same one used to map Mars in Animation #2868. Instead of using sea level as zero (ie, yellow) we use Earth's mean elevation which is approximately 1 km. below sea level." </em></font></p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

 

[keermalec]

<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:&nbsp;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?&nbsp;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).&nbsp;Thanks,&nbsp;KG&nbsp; <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>&nbsp;</p><p>&nbsp;</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>

 

[kiwigavin]

Chaps, &nbsp;Thanks for the excellent replies. I will leave him to mull over the links and the idea of&nbsp;the altitude of highest gravity.<br />

 

[derekmcd]

<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>Is he asking about the mean elevation, or the reference level "zero" altitude.The mean elevation would be exactly where it is today, wherever that may be.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.&nbsp;&nbsp; <br /> Posted by keermalec</DIV></p><p>Are you describing the Geoid (or Aroid in mars' case)?&nbsp; Your description is a bit confusing.&nbsp; I think the geoid uses an averaged out gravitational potential&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>