What is the concept of an aerial photograph?
In general terms, an aerial photograph is any photograph taken from the air. Air images are usually taken vertically from an aircraft with a high-resolution camera. You may look for a variety of things to see what distinguishes one image from another in the same region, including film form, size, and overlap. Stereoscopic coverage, fiducial marks, focal length, roll and frame numbers, and flight lines and index maps are all significant concepts in aerial photography. By discussing these basic technical principles, the following content will help you understand the basics of aerial photography. Click here for fashion photography.
Aerial Photography Fundamentals
Color, ultraviolet, and false-color infrared film are sometimes used for special projects, although most air photo missions use black and white film.
The focal length of a camera lens is the distance between the centre of the lens and the focal plane (i.e. the film). Picture distortion decreases as focal length increases. When the camera is calibrated, the focal length is precisely measured.
The ratio of the distance between two points on a photograph to the distance between the same two points on the ground is called scale (i.e. 1 unit on the photo equals “x” units on the ground). The scale is calculated as follows: If a 1 km stretch of highway is covered by 4 cm on an air photo, the scale is calculated as follows:
There are three ways to express scale:
- Representative Fraction Ratio (Unit Equivalent)
The following is how a photographic scale of 1 millimetre on the image corresponds to 25 metres on the ground:
- a unit Representative Fraction – 1/25 000 Ratio – 1:25 000 Equivalent – 1 mm = 25 m
When discussing scale, two words are frequently mentioned:
Larger-scale images (for example, 1:25 000) cover smaller areas in greater detail. The ground features in a large scale picture are simply larger and more precise. The picture shows a smaller area of field coverage than at smaller scales.
Smaller-scale images (for example, 1:50 000) cover larger areas with less detail. A small scale picture simply denotes that the ground features are smaller and less accurate. On the picture, the area of ground coverage is larger than at larger scales.
The National Air Photo Library offers a range of photographic scales, including 1:3 000 (large scale) of selected areas and 1:50 000 of the entire country (small scale).
Small registration marks exposed on the edges of a photograph are known as fiducial marks. When a camera is calibrated, the distances between fiducial marks are precisely measured, and cartographers use this knowledge to compile a topographic map.
Overlap: calculated as a percentage, overlap is the proportion by which one photograph includes the area occupied by another photograph. The photo survey aims for a 60 percent forward overlap (between images taken along the same flight line) and a 30% lateral overlap (between photos on adjacent flight lines).
When two overlapping images (known as a stereo pair) are viewed through a stereoscope, the effect is a three-dimensional image. Each of the stereo pair’s photographs depicts a slightly different view of the same location, which the brain combines and interprets as a three-dimensional image. Click here for more info.
Roll and Picture Numbers
Each aerial photograph is assigned a specific index number based on the roll and frame of the photograph. Image A23822-35, for example, is the 35th annotated photo on roll A23822. This number can be used to locate a picture in NAPL’s database, as well as metadata such as the date it was shot, the plane’s altitude (above sea level), the camera’s focal length, and the weather conditions.
Flight Lines and Index Maps
At the conclusion of a photo mission, the aerial survey contractor plots the positions of the first, last, and fifth photo centres on a National Topographic System (NTS) chart, along with their roll and frame numbers. Small circles are used to display photo centres, and straight lines are drawn between them to show images on the same flight line.
An air photo index map is a graphical representation that allows you to link images to their geographical location. Small-scale photographs are indexed on NTS map sheets with a scale of 1:250 000, while larger-scale photographs are indexed on NTS maps with a scale of 1:50 000.