kdxn said:Earth curvature is 78.4mm height difference per 1km at the equator, 78.7mm at the poles. Refraction usually makes this effect less.
Thank you for the correction. So its about two-tenths of an inch in a hundred yards. My dim and distant recollection of basic survey 1981 was out by a factor of five. :-[
That's a bit under a metre over the c.15km extent of Mammoth Cave mentioned up-thread. In a high grade traditional surface survey to plot entrance locations (and hence to establish a framework on which to base the underground survey) that would be worth taking into account I think - but very few cave surveyors possess instruments to work to such accuracy that curvature becomes worth taking into consideration.
Excellent cartoon about "What your favourite map projection says about you": http://xkcd.com/977/