[time-nuts] question about Thunderbolt geo acuracy

Arthur Dent golgarfrincham at yahoo.com
Thu May 10 14:40:48 UTC 2012


I've found significant altitude errors using a GPS and the following quotes 
found on the internet will explain why. From my experience of hiking 
in the mountains of New Hampshire an aneroid altimeter will vary with 
atmospheric pressure about 200 feet for a change of 0.2" of mercury 
so you have to continually set it at known waypoints, just like setting a 
frequency standard against a known reference, and then it will be 
'accurate' for some length of time, and then you set it again. GPS altitude 
will be off but it should be fairly consistent in spite of the changing 
atmospheric pressure. The earth neither spins at a constant rate nor is 
it a perfect sphere. Maybe we need to trade it in for a newer model. ;-) 

"GPS altitude measures the users' distance from the center of the SVs 
orbits. These measurements are referenced to geodetic altitude or 
ellipsoidal altitude in some GPS equipment. Garmin and most equipment 
manufacturers utilize a mathematical model in the GPS software which 
roughly approximates the geodetic model of the earth and reference 
altitude to this model. As with any model, there will be errors as the 
earth is not a simple mathematical shape to represent.  What this 
means is that if you are walking on the seashore,  and see your altitude 
as -15 meters,  you should not be concerned.  First,  the geodetic model 
of the earth can have much more than this amount of error at any specific 
point and second,  you have the GPS error itself to add in.  As a result of 
this combined error,  I am not surprised to be at the seashore and see -40 
meter errors in some spots."

"We have to make some assumptions about the shape of the earth. WGS84 
has defined that shape to be an ellipsoid, with a major and minor axis. The 
particular dimensions chosen are only an approximation to the real shape. 
Ideally, such an ellipsoid would correspond precisely to "sealevel" everywhere 
in the world. As it turns out, there are very few places where the WGS84 
ellipsoid definition coincides with sealevel. On average, the discrepancy is 
zero, but that doesn't help much when you're standing at the water's edge of 
an ocean beach and your GPS is reading -100ft below sealevel. The deviation 
can be as large as 300ft in some isolated locations. When the National Marine 
Electronics Association came up with the NMEA standard, they decreed that 
altitudes reported via NMEA protocol, shall be relative to mean (average) sea 
level. This posed a problem for GPS manufacturers. How to report altitudes 
relative to mean sea level, when they were only calculating altitude relative to 
the WGS84 ellipsoid. Ignoring the discrepancy wasn't likely to make GPS users 
very happy. As it happens, there is actually a model of the difference between 
the WGS84 ellipsoid and mean sea level. This involves harmonic expansions 
at the 360th order. It's a very good model, but rather unusable in a handheld 
device. It was determined that this model could be made into a fairly simple 
lookup table included in the GPS receiver. The table is usually fairly coarse 
lat/lon wise, but the ellipsoid to mean sea level variation, known as geoidal 
separation, varies slowly as you move in lat/lon." 

-Arthur 


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