Suunto TANDEM Guide De L'utilisateur page 16

All readings on the percentage scale are based on the horizontal distance. This
means that if the distance on sloping terrain is measured along the ground an error is
introduced, and this must be corrected for accurate results. The error is insignificant
for most purposes at small ground slope angles but increases progressively as the
angle increases.
The trigonometrical correlation is H = h x cos a
where
H = the true or corrected height,
h = the observed height and
a = the ground slope angle.
With the aid of the above equation the correction can also be made in the distance,
where
h = the distance measured along the ground
H = the horizontal distance sought. If the corrected distance is used no correction in
the height observed is needed.
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