The parallax equation $$\Delta {\text{p}} = \frac{{{\text{Bm}}\Delta {\text{h}}}}{{{\text{H}} – {\text{h}}}}$$ is applicable to entire overlap of the photographs only if parallax is measured A. Normal to base line B. Parallel to base line C. Both (A) and (B) D. Neither (A) nor (B)

The correct answer is: A. Normal to base line.

Parallax is the apparent displacement of an object relative to a background when viewed from different positions. In the context of aerial photography, parallax is used to measure the height of objects by comparing the position of the object on two photographs taken from different positions. The parallax equation is given by:

$$\Delta {\text{p}} = \frac{{{\text{Bm}}\Delta {\text{h}}}}{{{\text{H}} – {\text{h}}}}$$

where:

• $\Delta p$ is the parallax,
• $Bm$ is the base length,
• $\Delta h$ is the height difference between the two photographs,
• $H$ is the flying height, and
• $h$ is the height of the object.

The parallax equation is only applicable to the entire overlap of the photographs if the parallax is measured normal to the base line. This is because the parallax is only a function of the height difference between the two photographs and the flying height, and it is not affected by the direction of the parallax.

If the parallax is measured parallel to the base line, then the parallax equation will not be applicable to the entire overlap of the photographs. This is because the parallax will be affected by the distance between the two photographs, and the parallax equation does not take into account the distance between the two photographs.