Aerial Photography

Aerial photography provides the foundation for Horizons' mapping operations. Aerial photo acquisition is provided by Horizons' fleet of six Cessna aircraft equipped with state-of-the-art mapping technology. Equipment includes high-precision Zeiss aerial cameras, FMC (forward motion compensation), gyro-stabilized mounts, computer controlled navigation, and Airborne GPS.

Aircraft, Crews and Equipment

Film Types
Horizons is experienced in the use of the various aerial films manufactured by both Kodak and AGFA. Aerial films are matched to the needs of each project based upon flying height, amount of haze and shadow and client requirements. Films vary in their granularity and resolving power. Some films are intended for photography taken from higher altitudes.

The least expensive aerial film is panchromatic, or black and white film. Panchromatic film offers excellent resolution and ease-of-use in mapping operations. Ease-of-use often equates to cost savings.

Color aerial film depicts the imagery in natural color. This is the most commonly selected aerial film for today's mapping projects. Digital color imagery requires three times the amount of storage space as grayscale imagery produced from the panchromatic film.

Color infrared film is infrared-sensitive, false-color reversal film. Color infrared films are intended for aerial photographic applications such as vegetation surveys, camouflage detection and earth resources monitoring. This type of film is often referred to as "false-color" film because the greens are recorded as reds.

Films can be negative or positive. Positive films are known for excellent color rendition and are used for projection, direct viewing or the production of color prints. Negative films are most commonly used for photogrammetry offering wide exposure latitude for maximum exposure flexibility and properties that are designed for high-temperature processing and flexibility in scanning and reproductions.

Whatever the application, Horizons can find the right film for the job.

Digital Photography Services

Flight Heights

Above Mean Terrain (amt)
Flight height is targeted to match the accuracy and resolution requirements of the mapping project. The photo scale of the aerial photography is directly dependent upon the height of the plane for each exposure. Pilots and aerial photographers attempt to keep the aircraft at a set distance from the terrain in order to keep the scale of the photography consistent. However, each exposure of photography will vary slightly in photo scale. Often flight maps will contain text such as 6000'amt, which indicates that the plane will fly at approximately 6000 feet above the mean elevation of the terrain.

Magnification Ratio
The magnification ratio is a term used to compare the photo scale with the mapping scale. For example, a project with a photo scale of 1"=600' (1:7200) and a mapping scale of 1"=100' (1:1200) is said to have a magnification ratio of 6X. This is one of the factors considered when designing the flying height for a project.

C-Factor
Another consideration taken into account when establishing flight height is the C-factor. This term is used to state the ratio between the flight height and the targeted contour interval. The C-factor ratio is typically held within a range of 1500 to 2200. For example, a project flown at 3600 feet above the terrain and processed to a 2-foot contour interval is said to have a C-factor of 1800 (3600/2). Consideration is given to the instrument to be used for the map compilation; some stereo instruments have higher resolving power and allow for larger C-factors.

Stereo Aerial Photography
Aerial photography designed for photogrammetry requires a specific overlap between adjacent exposures. Along each flight line of photography the exposures require an overlap between 60 to 65%. This overlap allows the pairs of images to be viewed in stereo on specialized equipment.

Overlapping pairs of images are measured and linked together mathematically in a process referred to as aerotriangulation. This process produces a controlled block of photos that can be interpreted in a stereoplotter as a three-dimensional view of the information in the photographs. This is the process that allows 3D compilation of map information.

Adjacent lines of aerial photography are overlapped not for stereo viewing but rather to ensure no gaps in coverage. The amount of side overlap depends upon the terrain and the flight height. Typical side overlap is 30 to 35%.

Oblique / Vertical Photography

Vertical aerial photography is the process of obtaining photos with the plane parallel to the ground and the camera pointing directly out of the belly of the plane. In vertical aerial photography the goal is have the camera take a photo straight down to the earth's surface without roll or tilt. Overlapped vertical aerial photography when supplemented with control information provides the foundation of stereo mapping operations.

Oblique aerial photography is derived from tilting the plane (or camera) so that a portion of the horizon is visible in the photography. This type of photography is typically used for interest shots and not for mapping.