When an Aerial Photograph Isn’t a Map

Someone looks at a high-resolution aerial photograph of a farm, puts a ruler on the screen, measures the distance from the homestead to the river, checks the size of the grazing camps, and makes an offer. It happens constantly, and the measurements are wrong.

This discussion works through why. A camera in the air views only the single point directly beneath it from straight overhead — everything else is seen at an angle. Silos lean outward. Scale shrinks toward the edges of the frame, so a centimetre measured in one corner is not the same distance as a centimetre measured in the middle. On uneven ground, high land is displaced outward and valley floors inward.

It then covers what orthorectification does about all of that, what accuracy is realistic, and the practical uses on a working property — arable hectares against the title deed figure, fence lines against boundaries, dam capacity, bush encroachment.

It is also honest about the limits: water reconstructs poorly, dense canopy hides the ground, and an orthomosaic does not replace a professional land surveyor for legal boundary determination.