Re: Altitude in meters above WGS-84 Geoid ?

Jim Varney (
Mon, 2 Mar 1998 08:23:16 -0800 (PST)

---Ruben Velasco <> wrote:

> When you enter your local altitude to a tracking software like Iridflar,
> Stsplus, Trakstar...  should you enter the metres above the sea level or
> above the WGS-84 model?  For example, I'm writing this from a place
> "oficially" at 9 meters "Above Sea Level".  But the sea level stardard
> we use here is actually several metres higher than, say, that of the
> Indian Ocean near Sri Lanka.  In other words, the Earth is not a perfect
> spheroid, etc, etc (you already know the story :)

Technically, you are right, you would want to enter your altitude relative
to the spheroid model being used by the program.  The whole purpose of
entering altitude is to refine the vector being calculated from the earth's
center to the observer and thereby calculate sight lines from the observer
to the satellite.

Programs ask for altitude/elevation/height (I'll use all three so I won't
restart a thread about the 'proper' use of those terms) above mean sea level
is that the common number end users have access to.  You're also right that
the difference between the sea level datum and the spheroid datum makes no
practical difference: your 30 meter 'error' relative to a 6378 km earth
radius is one part in 200,000.
Jim Varney   121.398W  38.458N 8m   Sacramento, CA
  (Sorry.  I am not the "Voice of Slinky Dog."),

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