why the altitude being measured with respect to sea level?
366727
True altitude is the elevation above mean sea level. In UK aviation radio telephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level; below the Transition Level this is referred to over the radio as altitude.\n\nAbsolute altitude or height is the elevation above a ground reference point, commonly the terrain elevation. In UK aviation radio telephony usage, the vertical distance of a level, a point or an object considered as a point, measured from a specified datum; below the Transition Level this is referred to over the radio as height, where the specified datum is the airfield elevation. \n\nPressure altitude is the elevation above a standard datum plane (typically, 1013.2 millibars). Above a transition altitude, which varies by nation, pressure altitude is used to set the altimeter. Pressure altitude divided by 100 is referred to as the flight level. \n\nDensity altitude is the altitude in the International Standard Atmosphere (ISA) at which the air density is equal to local conditions. Aircraft performance depends on density altitude, which is affected by barometric pressure and temperature. On a very hot day, density altitude at an airport may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.
906179
According to the USGS web site,\n\nThe elevation above mean sea level for Elderton, PA is 1270 feet or 378 meters.
817654
Gravity...or if you mean why doesn't it come over the land...then that is because the land is higher then the sea, hence there is not enough water in the sea to raise it above the land.
829824
If this all was true, the waters off the EAST coast would be higher, since Earth spins toward the east. \nHowever; even the ocean has areas of higher/lower elevation, according to Topex Dynamic Ocean Topography.\nhttp://seawifs.gsfc.nasa.gov/OCEAN_PLANET/MOVIES/Topex_Dynamic_Ocean_Topography.mpg\nBut there, around NZ, the ocean does not show higher elevation; it's at a normal  level.
440513
Yes it is wrong.  It might be a good approximation for lower altitudes, but is not truly correct.  In the same way, Newton was wrong with his equations, but they still make good approximations over reasonable ranges of values.
432484
Atmospheric pressure is a direct result of the weight of the air. This means that air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time. Atmospheric pressure drops by ~50% at an altitude of about 5 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5 km). The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 pounds per square inch).
99997
Increased lift at low altitude due to ground effect, yah? \n\nI'm curious.  You apparently know the answer, so why ask the question?
132266
Ignore harsha.  A 1 atm decrease in pressure would mean zero pressure at 100 meters above sea level, which is absurd.\n\nPressure decreases as you rise because the total mass of the atmosphere weighing down on you decreases.  High pressure means you need more energy to make water boil, so at sea level the boiling point will be higher than it will be on a mountain top.
234337
178feet above sea level.
89101
mmHg\nPSI + 14.7 (at sea level)
1026484
to arrive at sea level, drive to the ocean.  then calibrate your altimeter (everyone has one.)  as you ascend, your altimeter will record the change in elevation.  you can buy one on the net for under $25.\n\nyou could also look at maps from the USGS.  they'll tell you your elevation.  globalmapper is a nifty software utility you can download for free that will help you find elevation mapping resources.\n\ni suspect GPS units will also tell you your elevation, but i'm not familiar with that.
122036
A higher elevation means you are farther from the center of the earth. This means that you also travel a bit farther in the same 24 hour time period as somebody at sea level.  Thus, you cover more distance in the same amount of time and are traveling faster.
1170472
You've got a couple of strange answers so far.  Look, the formula written above is correct, the mass of the earth is basically the same, but the mass of your surroundings isn't the same.  The pull of gravity is less the higher your elevation, greater the lower the altitude.  If you're on land with higher density, you'll have a greater pull then on land with a lower density.  Pretty much everywhere on land gravity isn't even, out in the middle of the ocean, it's very close to even.  But, keep in mind, these differences are pretty much negligible.  The only way to be able to measure the difference is with scientific equipment.  Actually, it's common practice to measure gravitational anomalies under ice sheets to gleam information about the formations underneath.
968537
By the use of a specialized instrument called an altimeter.
360638
I'm thinking that you meant to type pressure instead of height. because millibars do not measure height. they measure pressure. and altitude and height are pretty much the same thing.\n\nthe relationship between altitude and pressure can be modeled by this equation\n\nlog(.01*P)~5-(h/50853.0185)\n\nwhen P=pressure in millibars and h=altitude in feet
419077
Hi.  This is pretty hard to measure, but the general range is that sea levels have risen by 4-10 inches, and I believe it's more likely to be at the lower end of that range. Recently, satellites have been launched to help us better measure this, so the data should be better over the next decade or so.\n\nJust as some points of interest, melting ice caps aren't the only source of rising sea levels.  Technically, ice caps are ice that's floating on the water, so the ocean doesn't rise when they melt. (Try floating an ice cube in a glass of water and letting it melt--the water level stays the same.) It's when glaciers and ice sheets (ice that's on land) melt that oceans rise.  Also, about 1/4 of the sea level rise is because water expands when it gets hotter, and the oceans are warming slightly.\n\nPredictions are even more uncertain, but the general consensus is that sea levels will rise by an additional 10-20 inches by 2100.  One big question mark is the west Antarctic ice sheet, which may come loose and plunge into the ocean, which could cause a rapid rise of several feet.
39868
The question is:  What should I set on the sub-scale of my altimeter so that the instrument would indicate my elevation if I were on the ground at your station?\n\nANSWER:  QNH