Field procedure of aerial photogrammetry, relief displacement

Field procedure of aerial photogrammetry

1. Planning: decide the purpose, scale and accuracy; choose the camera, flying height and overlaps (60 % end lap, 25–30 % side lap); prepare the flight plan.
2. Ground control: establish horizontal and vertical control points (by traversing, levelling or GPS) that can be identified on the photos, and signalise them with targets before the flight.
3. Photography: fly the strips at the planned height and speed in suitable weather; the camera exposes at fixed intervals.
4. Processing: develop the film (or download digital images), prepare contact prints and a photo index.
5. Photo-control identification and plotting: pass the control to the photos, orient the stereo pairs in a plotter, and draw the map or compute coordinates and contours.
6. Field completion and checking: annotate names and features, check the map in the field.

Relief displacement

It is the radial shift of the image of a point on a vertical photo, due to its height $h$ above the datum. From similar triangles,

$$d = \frac{r\,h}{H}$$

where $r$ is the radial distance of the image from the principal point and $H$ is the flying height above the datum. High points move outward from the principal point, low points move inward, and the displacement is zero at the principal point. Because of this, a photograph of undulating ground is not to scale and cannot be used as a map unless corrected (by rectification or orthophoto production). It also lets heights of objects be found: $h = dH/r$, and it gives the parallax needed for stereoscopic viewing.