Question

A pair of overlapping vertical photographs were taken from a flying height of $1230~\text{m}$ above sea level with a camera having a focal length of $152.4~\text{mm}$. The distance between the consecutive exposure stations is $350~\text{m}$. The parallax bar reading of a point A on the photograph is observed as $10.96~\text{mm}$. The parallax bar constant for this setup is given as $80.71~\text{mm}$. The elevation of point A above sea level is ___________ m (Rounded off to 2 decimal places).

Hint:

For absolute height from a stereo pair: (i) get absolute parallax $p=C+r$ (bar constant $+$ reading), (ii) use $h=H-\dfrac{Bf}{p}$ with $B,H$ in the same length unit and $f,p$ in the same (typically mm).

Answer 1

Key relation (vertical photographs).
Absolute stereoscopic parallax of a point: \[ p=\frac{B\,f}{H-h},\qquad ⇒\qquad h=H-\frac{B\,f}{p}, \] where $B$ is the air base (distance between exposure stations), $f$ the focal length, $H$ the flying height above datum (MSL here), and $h$ the elevation of the ground point.
Step 1: Determine the absolute parallax $p$.
With a parallax wedge/bar, the absolute parallax equals the bar constant plus the observed reading: \[ p=C + r = 80.71~\text{mm} + 10.96~\text{mm}=91.67~\text{mm}. \] Step 2: Compute the elevation.
Use consistent units: let $B$ and $H,h$ be in metres; $f$ and $p$ in millimetres. \[ h = 1230 - \frac{350 \times 152.4}{91.67} = 1230 - 582.0 \approx 648.0~\text{m}. \] \[ \boxed{h \approx 648.00~\text{m}} \]