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I'm using spatialite 3.0.1 with the following test data (in EPSG: 4326):

Name        astext(geometry)    
----------  --------------------
Belitsa     POINT(23.563 41.947)
Malko Tarn  POINT(27.533 41.983)
Kotel       POINT(26.4752 42.938

My client demands I provide the projected st_length distance instead of geodesiclength.

(compare the following queries)

select,, st_length(transform(makeline(a.geometry, b.geometry), 32635)) from ProjCist a, ProjCist b;

Belitsa     Malko Tarn  329056.08890108 


select,, geodesiclength(makeline(a.geometry, b.geometry)) from ProjCist a, ProjCist b;

Belitsa     Malko Tarn  329092.535472995 

The 50 meter difference in length, for our purposes, is hugely significant. Which is more correct? Why?

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If you're working with the local ellipsoid then it's a better distance approximation than the pythagorean distance, reason for that (in my knowledge) is quite simple : ellipsoid is meant to be a better approximation to distances. So unless you're working with a non-local ellipsoid, it's the way to go.

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