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In Christen hypsometer, find length bd for a tree of 24 m height when BD (staff length) is 4 m and ab (distance between the flanges) is 33 cm.

Observer eye E sights through a Christen hypsometer with flanges a and b and reading point d, towards a tree AB with a staff of top D held at its base B

  • A.2.50 cm
  • B.3.50 cm
  • C.4.50 cm
  • D.5.50 cm

Answer: D

The length bd is 5.50 cm.

The Christen hypsometer is held so that the top A and base B of the tree line up with its two flanges a and b. The line of sight to the top D of a staff of known length BD then cuts the scale at d. By similar triangles:

\[ \begin{aligned} \frac{bd}{ab} &= \frac{BD}{AB} \\ bd &= \frac{33 \times 4}{24} \\ &= 5.5\ \text{cm} \end{aligned} \]

The scale is graduated this way: the taller the tree, the shorter bd. 2.50, 3.50 and 4.50 cm would correspond to trees of about 53, 38 and 29 m.

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