[Proj] Fw: Re: Fw: Re: GeographicLib geoid calculations

support.mn at elisanet.fi support.mn at elisanet.fi
Thu Oct 18 09:46:28 EST 2012

--- Alkuperäinen viesti ---
Aihe: Re: [Proj] Fw: Re: GeographicLib geoid calculations
Päiväys: 18.10.2012 17:45
Lähettäjä: support.mn at elisanet.fi
Charles Karney [charles.karney at sri.com] kirjoitti: 
> It might be worth making a few other remarks about the geoid:
> (1) The geoid is not mean sea level.  MSL includes local topographic
> effects due to currents and the shape of the sea bed.  Marine charts
> obvious will need to account for these (and they include tidal effects
> with mean low tides or whatever).
> (2) The normal to geoid does not give you the direction of gravity on
> land.  In calculating the geoid separation, a fudge is added to the
> gravity field to account for the mass of the terrain above the geoid
> (the zeta-to-N correction).
> (3) The NGA code which computes the EGM2008 uses different masses for
> the real earth and reference ellipsoid.  This leads to a 1/r effect
> which is ignored.  (There is an additional bug in the code which affects
> the calculation of the deflection from the normal.  I've pointed this
> out to the authors --- and gotten no response!)
> (4) If you're really interested in the gravity field then use the
> EGM2008 gravity model (or more recent models) directly.  If working with
> sensitive equipment,  you might need to account for the mass of the
> atmosphere (included in EGM2008!), the buoyancy of your instruments,
> earth and ocean tides, etc.
> It's best to view the geoid height as "just" a datum shift.  It's
> computed to approximate a surface of constant potential and the whole
> procedure has various warts.  But these defects don't matter as long as
> you make clear what geoid model you used and how you interpolated into
> it.

Thanks for the ideas! Must do some investigations...


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