Trigonometric indirect topography meveming make it possible to determine the height difference ΔH entre la station T d’un théodolite et un point P. This is done by measuring the inclined distance alog the line of sight Di and zenith angle denoted V in the figure opposite. From the diagram, we can write that:
ΔHTP is the height difference from T to P.
ht is the station height (or trunnion height).
hv is the height of the indicator or more generally the height targeted above the point sought (you can also put a surveying staff at P).
We deduce the horizontal distance Dh
We deduce the distance according to the slope Dp :
There are 2 cases which depend on whether or not it is possible to measure the distance Dp.
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