Written by Globphy in Article
Apr 21 st, 2020
Film: Many air photo missions fly using black and white cameras, but occasionally colour, infrared, and false-color infrared cameras are used for special projects.
Focal length: The gap from the camera lens centre to the focal plane (i.e., film). If the focal length increases, distortion of the image decreases. When the camera is calibrated the focal length is accurately measured.
Scale: The distance relation between two points on a map and the average distance between the same two points on the ground (i.e. 1 unit on a map equals “x” units on the ground). If an air photograph shows a 1 km strip of highway spanning 4 cm, the scale is measured as:
Two terms that are normally mentioned when discussing scale are:
Large Scale: Larger-scale images (e.g. 1:25 000) provide greater detail on small regions. A large-scale photograph essentially means the ground elements are in a broader, more complex dimension. The area of field covering shown on the picture is less than that of smaller sizes.
Small Scale: Smaller images (e.g. 1:50,000) provide even less detail in wide fields. A shot on a small scale essentially means the ground characteristics are at a smaller, less precise level. The area of field coverage seen on the picture is greater than in larger sizes.
Fiducial marks: Small identification markings seen on the edges of the frame. When a camera has calibrated the distances between fiducial marks are precisely measured, and this detail is used by cartographers when making a topographic map.
Overlap: is the amount by which one photograph includes the area covered by another photograph, and is expressed as a percentage. The photo survey is designed to acquire 60% forward overlap (between photos along the same flight line) and 30% lateral overlap (between photos on adjacent flight lines).
Stereoscopic Coverage: A three-dimensional vision that occurs when a stereoscope is used to display two overlapping images (called a stereo pair). Each stereo pair image offers a subtly different perspective of the same region which is merged and viewed as a 3-D perspective by the brain.
Roll and Photo Numbers: each aerial photo is assigned a unique index number according to the photo’s roll and frame. For example, photo A23822-35 is the 35th annotated photo on roll A23822. This identifying number allows you to find the photo in NAPL’s archive, along with metadata information such as the date it was taken, the plane’s altitude (above sea level), the focal length of the camera, and the weather conditions.
Flight Lines and Index Maps: at the end of a photo mission, the aerial survey contractor plots the location of the first, last, and every fifth photo center, along with its role and frame number, on a National Topographic System (NTS) map. Photo centers are represented by small circles, and straight lines are drawn connecting the circles to show photos on the same flight line.
Concepts of Aerial Photography. (2016, September 1). Retrieved from https://www.nrcan.gc.ca/earth-sciences/geomatics/satellite-imagery-and-air-photos/national-air-photo-library/about-aerial-photography/concepts-aerial-photography/9687
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