[OSRS-PROJ] da, df

Felipe G. Nievinski fnievinski at terra.com.br
Sun Aug 5 13:54:04 EDT 2001

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 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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