[OSRS-PROJ] da, df

[Gerald I. Evenden]

I didn't mean to be obscure and was only commenting on the d f/a parameters
and the factor that it appears that the *reason* that they used this novel form
was that fewer bytes of storage were required to save the list of ellipsoid parameters
in this form as compared to the full specification of each f and a.

20-30 years ago, when computer memory was a scarce resource we would go
to great lengths such as this differential method but I would not bother now.

If there is any criticism, it is solely directed against the coders of the  GPS recievers.

"Felipe G. Nievinski" wrote:

> On 04/08/01 at 20:19 Gerald I. Evenden wrote:
> >Remarkable.  The lengths some will go to to save such a trivial number of bytes.
> >
> >I used to do this back in the 60's when memory was dear, but now???
> >
>
> Some days ago there was a doubt in this list about what the da and df parameters mean. I found an explanation (from the U.S. NIMA) and sent it hoping it could help someone.
>
> Gerald, I'm sorry, English is not my mother tongue, and I just did not understand what you meant. :((
>
> Felipe.
>
> >"Felipe G. Nievinski" wrote:
> >
> >> FYI (about the DA and DF parameters)
> >>
> >> The DA and DF are the delta values between the ellipsoid used by WGS-84 and another given ellipsoid:
> >> da = [a(WGS 84 ellipsoid) - a(second ellipsoid)] | a = semi-major (equatorial) axis
> >> df = [f(WGS 84 ellipsoid) - f(second ellisoid)*10000] | f = flattening
> >>
> >> They are commonly used by NAVSTAR-GPS receivers, which shall already know the WGS-84 model, and therefore needs less space to store just the delta values for the others Earth models than the a and f values themselves.
> >> For example, the Finnish ellipsoid can be defined by:
> >> da = -251 m
> >> df = -0.14192702
> >
> >    ...
> >
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