[time-nuts] question about Thunderbolt geo acuracy

Chris Albertson albertson.chris at gmail.com
Thu May 10 16:09:33 UTC 2012


There is an error in your quoted text.   The author must have though
there was a difference between WGS84 and "true sea level".   No that
is not true.   If you paper map that you bought from US Gological
Survey says "WGS84" on it then THAT is the definition of sea level on
that map.   The altitudes of contour lines and peaks will be in WGS84
and should match what the GPS says.     Many older maps use a
different system so their saw level is defined differently.  Almost
all GPSes have away to select the elipoid.  It defauls to WGS84 but
you need to set it to match your paper map

One problem is the geometry of the satellites in view. Unless the
antenna can see to the horizon the sight lines up to the sats make a
deep "V" and if you can see to the horizon there is 3X or 4X more
atmosphere along the horizontal path

On Thu, May 10, 2012 at 7:40 AM, Arthur Dent <golgarfrincham at yahoo.com> wrote:
> 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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-- 

Chris Albertson
Redondo Beach, California



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