Geometry of Aerial Photography

Taking Vertical AP: Flying Pattern

Photographic Coverage Along A Flight Strip

1. Fiducial mark Fiducial marks Marks on the photograph margins used to locate principal point in photo. Fiducial marks

Fiducial axes

2. Principal Point Geometric center of photograph. Literally the point on the ground in line with axis of camera lens.

3. Nadir - The Nadir is the point vertically beneath the camera  center at the time of exposure. 4. Isocenter - The point on the photo that falls on a line half- way between the principal point and the Nadir point.

5. Side lap An aerial photograph mission will be flown in strips, shutter timing set for 60% endlap (needed for parallax) and strips spaced for 30% sidelap (to avoid missing bits)

6. Forelap Endlap (or forelap) is the important bit It ensures every point on the ground appears in at least two photographs Distance between principal point of adjacent photographs is known as the “air base”

Factor affecting aerial photography The orientation of aircraft is influenced by wind conditions. Which causes an aircraft to orient in three different rotation angle namely roll, pitch, yaw. FIG. DIFFERENT ROTATION ANGLES- ROLL ,PITCH ,YAW

Crab & Drift

SATELLITE IMAGES VS AERIAL PHOTOGRAPHS Satellite imagery Aerial photography Collecting large amounts of data in relatively small amounts of time. Hundred kilometers above the earth's surface. Lower resolution than aerial photography. Thin cloud can still have a large effect on the quality of satellite imagery. Many modern satellites can collect a variety of data. Data is usually gathered by a digital CCD camera. The advantage of the CCD is that we measure quantitatively the radiation reaching the sensor. Image is created line after line; there for, the geometrical correction is much more complex, with each line (or even pixel) needing to be treated as a central projection Satellite images also from satellites. Sensors can be designed to measure radiation all along the Electromagnetic spectrum. Images are available since the early 1970’s. Images are more difficult to process, and require trained personnel. Slow and time consuming process. Closer to the subject, in this case the land. Resolution of between 50cm to 12.5cm per pixel. can be affected by adverse weather conditions Some aircraft can mount multiple cameras or sensors and some of the latest aerial cameras capture different types of data simultaneously Photos are taken by an analog instrument: a film of a (photogrammetric) camera then scanned to be transformed to digital media. Photograph is a central projection, with the whole picture taken at one instance. Aerial photos are usually taken from planes. Aerial photos usually gather data only in the visible spectrum. In Photogrammetry the main efforts are dedicated for the accurate creation of a 3d model, photos provide a longer time span for landscape change detection. Aerial photographs can be interpreted more easily.