Nadir is the point directly below the aircraft. The overlap allows photos to be viewed stereoscopically to show relief. The relief view is used to map contours. If scale is not included, the user can often calculate it from objects of known size. Football fields, baseball fields and section lines make good standards for calculating scale.
Electronic systems use electromechanical means to recieve data.
These systems are used primarily on orbiting platforms. Electronic systems can be either active or passive. Active systems emit their own radiation. SLAR and SIR systems emit radar waves from an antenna and record the reflectance electronically. Passive systems rel on the sun's radiation for illumination.