Terrestrial photogrammetry without ground control points

Terrestrial photogrammetry should be the survey technique of choice when updating large scale urban maps and GIS databases, where 3D data and attribute data are required. Its main drawback is the need for Ground Control Points (GCP) to reference the survey. To make image georeferencing easier and to provide control information, the use of a simple system, made of a photogrammetric camera fastened to a GPS antenna, is proposed. A photogrammetric block, composed by at least three images, is taken around the object with the receiver measuring in kinematic mode. Tie points are automatically extracted by Structure from Motion (SfM) algorithms or measured manually; block orientation is performed by GPS assisted Aerial Triangulation. Advantages as well as limitations of the system are discussed, with particular attention to GPS availability or ill-conditioned block configurations. The issue of system calibration (i.e. measurement of eccentricity between camera and antenna) is also addressed. Several test cases are presented, in which absolute accuracies, verified on check points independently surveyed range from 4 to 7 cm.