Conclusions
The RDC approach provides an effective way to establish a reference system for measuring vertical geometry and deflection for many bridge engineering applications of close-range photogrammetry. The accuracy provided by the RDC approach is comparable to that of the conventional control-point method, while the former significantly reduces the field work. The RDC approach was verified through comparisons with dial gage and differential-level measurements; the difference was generally within 0.2 mm (0.008 inch), with the maximum less than 1 mm (0.04 inch) from the dial gage measurement in the lab study, and generally within 1 mm (0.04 inch), with the maximum less than 1.5 mm (0.06 inch) from the level readings in the field study. Engineers can easily apply the RDC approach to perform photogrammetric structural measurement without the need of conventional surveying equipment (e.g., total station). It is concluded that the RDC approach can be used as an effective and more efficient photogrammetry network control method in lieu of the control-point method for bridge engineering applications.