What is Aerial Imagery?
Aerial photography, or imagery, as it is now known, is the method of acquiring images from the air. Vertical photographs are taken with the camera pointing straight down, as opposed to oblique photographs which are taken at an angle. The first recorded aerial photograph was oblique and taken of a French village in the late 19th century. The photographer was Gaspar Felix Tournachon, who patented the concept of using aerial photographs to compile maps.
Why is it Used?
Since its inception, aerial imagery quickly proved much more effective than the time-consuming ground surveys that had then been the more commonly used method of the national mapping organizations that developed throughout the 19th century.
There is more to aerial imagery than using a fixed wing aircraft, rotary craft or drone that flies up to take these images. There are multiple factors to an aerial survey, all of which must be thought about to make sure the data is suitable to extrapolate whatever information is needed for your final product. When surveying on the ground, it is often difficult to see or access all features. A trivial bump at ground level can become more significant in a wider context, or a particular area which is difficult or impossible to reach on foot, can be seen and mapped by the aerial imagery.
When to Capture Aerial Imagery:
Aerial imagery should be acquired only when well-defined images can be obtained, typically in clear weather conditions.
- The imagery needs sunshine, but if the shadows are too long there is potential to obscure ground features. This would hinder the ability to collect features or hide elements that would degrade the quality of the final image product.
- Ideally, the sun angle should be more than thirty degrees (30º) above the horizon but less than ninety degrees (90º) as some shadow is required to provide depth.
Conditions that Make for Poor Aerial Imagery:
The imagery should not be undertaken when the ground is obscured by haze, snow, flooding conditions, or when clouds or cloud shadows would appear on more than an acceptable pre-determined amount of the area.
Aerial Imagery in Topographic Mapping:
The use of aerial imagery in producing topographic maps delves into the realm of photogrammetry. Photogrammetry by definition is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant imagery and other phenomena or more simply put the use of photography in surveying and mapping to measure distances between objects.