4 ms·
" a GPS receiver on a ship may, during the course of a long voyage, indicate height variations, even though the ship will always be at sea level (neglecting the
by axus 3y ago
" a GPS receiver on a ship may, during the course of a long voyage, indicate height variations, even though the ship will always be at sea level (neglecting the effects of tides). That is because GPS satellites, orbiting about the center of gravity of the Earth, can measure heights only relative to a geocentric reference ellipsoid. To obtain one's orthometric height, a raw GPS reading must be corrected. Conversely, height determined by spirit leveling from a tide gauge, as in traditional land surveying, is closer to orthometric height. Modern GPS receivers have a grid implemented in their software by which they obtain, from the current position, the height of the geoid (e.g. the EGM-96 geoid) over the World Geodetic System (WGS) ellipsoid. They are then able to correct the height above the WGS ellipsoid to the height above the EGM96 geoid. When height is not zero on a ship, the discrepancy is due to other factors such as ocean tides, atmospheric pressure (meteorological effects), local sea surface topography and measurement uncertainties."
- hinkley 3y agoI would think if one needed to measure the altitude of their ship, things have already gone very, very sideways at this point.
- defrost 3y agoHmmm. It might be worth thinking a bit harder then. Can you think of any reason why the current displacement above or below the regional mean sea level might be of crucial and pressing interest to maritime navigation?
- hinkley 3y agoIf you can't tell your ship is sinking because of water alarms going off in your hold or the boat is visibly sinking, then what is GPS going to do for you, exactly? Either everyone on the ship is dead, and the entire electrical system except the GPS is shot, or aliens are lifting your ship with a tractor beam. All situations where things have gone very, very sideways. Wouldn't you say?
- defrost 3y agoSigh. Have you ever been in a boat or gone fishing? Think harder .. why would there be interest in the surface level of water rising and falling (and taking all boats and bouys with it)? This is not a difficult question.
- hinkley 3y agoI’m not doing your homework assignment. Get to the point.
- defrost 3y agoYou're apparently fixated on ships sinking or rising into the air when you discuss "altitude". You've apparently not thought much on geoids, ellipsoids, the meaning of "sea level", geodesy, etc. You might want to reflect upon what "altitude" means and upon why it's been commonplace for centuries to measure the 'altitude' of things that float. Should you do so you'll see your comments above in a different light.
- hinkley 3y agoYou're still bloviating. How many other people have read this and not learned anything? If you're going to teach people, or disagree with them, at least state your premise instead of trying to sound like Yoda.