[Proj] Fw: Re: GeographicLib geoid calculations

[support.mn at elisanet.fi]

Hello,

maybe these "geoid masters" might have a small, accurate and fast method
to solve the geoid anywhere on the earth? ;)

-->

http://gge.unb.ca/PhotoAlbum/1996/GeoidMasters/GeoidMasters1.gif

regards: Janne.

--- Alkuperäinen viesti ---
Aihe: Re: GeographicLib geoid calculations
Päiväys: 18.10.2012 12:51
Lähettäjä: support.mn at elisanet.fi
Charles Karney [charles.karney at sri.com] kirjoitti: 
> The errors for various geoid models and various grid sizes are listed in
> 
>    http://geographiclib.sf.net/html/geoid.html#geoidinterp
> 
> For example for the 15' EGM96 grid using cubic interpolation the max and
> RMS errors are 169mm and 7mm.  However, I would normally recommend the
> 5' EGM95 grid or the 2.5' or 1' EGM2008 grid.
> 

Looks like the bilinear interpolation destroys the accuracy?

> GeographicLib uses 2 bytes per grid point to store the data (vs 4 bytes
> per grid point for most other implementation) and, by default, does not
> read the entire table into memory.
> 

As usual if you need a version that is both fast and small .. you never find
it. It is either fast or small but not  both.

> (Incidentally, GeographlicLib lets you work directly with the spherical
> harmonic expansions of the geoid models using the Gravity class.  So you
> can construct grids for any of EGM84, EGM96, or EGM2008 at any
> resolution.)
> 

The geoid might be an interesting academic study but the ellipsoid is 
a much more stable zero reference and since the earth is not stable
by its nature (plate dynamics) it is obvious that the geoid will never
be fully fixed in time.

(  http://en.wikipedia.org/wiki/Geoid )

"Recent satellite missions, such as GOCE and GRACE, have enabled the study of time-variable geoid signals. The first products based on GOCE satellite data became available online in June 2010, through the European Space Agency (ESA)’s Earth observation user services tools.[11][12] ESA launched the satellite in March 2009 on a mission to map Earth's gravity with unprecedented accuracy and spatial resolution. On 31 March 2011, the new geoid model was unveiled at the Fourth International GOCE User Workshop hosted at the Technische Universität München in Munich, Germany.[13] Studies using the time-variable geoid computed from GRACE data have provided information on global hydrologic cycles,[14] mass balances of ice sheets,[15] and postglacial rebound.[16] From postglacial rebound measurements, time-variable GRACE data can be used to deduce the viscosity of Earth's mantle.[17]"

( http://www.google.com/url?sa=t&rct=j&q=%20time-variable%20geoid%20computed%20from%20grace%20dat&source=web&cd=2&ved=0CCwQFjAB&url=http%3A%2F%2Fwww.fig.net%2Fpub%2Ffig2012%2Fpapers%2Fts04b%2FTS04B_avsar_ustun_5724.pdf&ei=GtB_UIqFD9Db4QSH8YCgDA&usg=AFQjCNFJgtaLbqSRIOCOOh06LKPzjfIR9A&cad=rja )

This GRACE data might be interesting?

Regards: Janne.