Short note: Photo scale, photogrammetry uses

Scale of a vertical photograph

The scale of a vertical photograph is the ratio of a distance on the photo to the same distance on the ground. By similar triangles in the camera,

$$S = \frac{\text{photo distance}}{\text{ground distance}} = \frac{f}{H-h}$$

where $f$ is the focal length of the camera, $H$ is the flying height above the datum and $h$ is the elevation of the ground above the datum. The scale varies from point to point when the terrain is not flat, and is larger for higher ground. If $h = 0$, $S = f/H$.

Uses of photogrammetry

• Preparation of topographic maps and contour maps, and orthophotos.
• Route location for roads, railways, pipelines and canals.
• Site selection for dams, bridges and airports.
• Measuring volumes (earthwork, stockpiles) and cross-sections.
• Mapping of inaccessible, hazardous or very large areas.
• Land use, forestry, geology and urban planning surveys.
• Cadastral and property mapping; disaster damage assessment.
• Close-range photogrammetry: structural deformation, monuments and archaeology.