## 1 Introduction
[9] introduces a low-cost robotic camera system (RCS) for accurate measurement collection of structural response. The low-cost RCS provides very accurate vertical displacements. The measurement error of the RCS is 1.4%. [PERSON] et al. [10] proposed a bridge monitoring system, which combines camera and laser indicator; the elastic line inclination is measured by analyzing the single frames of an HD video ofthe laser beam imprint projected on a flat target. The inclination of the elastic line at the support was obtained with a precision of 0.01 mrad. [PERSON] et al. [11] measured the overall deformation of the masonry wall subjected to cyclic loads, using the 3D image correlation technology. The displacement, rotation, and interface slip between the reinforced concrete column and masonry were measured. [PERSON] et al. [12] used the close-range photogrammetry technology to monitor the displacement of tunnel caverns. The measured results were compared with the values measured by mechanical convergence meter, and the difference between the two methods is no more than \(\pm\)2 mm at the measuring distance of 8 m. This accuracy meets the requirement of general tunnel deformation monitoring. Reference [13] studied the application of sub-pixel displacement measurement method in soil strain monitoring. Based on the spatial correlation function iteration, the sub-pixel displacement of soils was measured. [PERSON] et al. [14] applied the close-range photogrammetry technology in measuring bridge deflection and proved that a desirable accuracy can be achieved, i.e., \(\pm\)1 mm. However, the accuracy is greatly affected by the positioning of the artificial marking points required by the method [15]. Although the above studies validated the feasibility of the application of close-range photogrammetry technology in structural deformation monitoring, the above methods are still unable to measure the structural overall deformation. In order to explore the feasibility of photogrammetry in structural overall deformation monitoring, [PERSON] et al. [16] explored the use of consumer-grade cameras and projectors for the deformation monitoring of structural elements. A low-cost digital camera deformation monitoring system is proposed. Static load tests of concrete beams are carried out in the laboratory. The experiments proved that it was possible to detect sub-millimeter-level overall deformations given the used equipment and the geometry of the setup. However, this technology requires high texture characteristics of the structure surface and needs to project random pattern on the structure surface, which is difficult to achieve in the actual bridge structure deformation monitoring. On another hand, the close-range photogrammetry requires the measuring equipment to be located in the orthogonal projection position of the measured surface, which is usually difficult in engineering practice. Taking the bridge structure as an example, the surroundings near the bridge are usually complex, such as mountains, rivers, and trees, which makes it difficult for the camera to maintain the orthogonal projection position with respect to the measuring bridge. In the deformation monitoring of structures, environmental factors must be considered. The complex geographical conditions of the structures means the photogrammetric camera is unable to work in the ideal measuring position. Therefore, it is necessary to study a new photogrammetric method for the overall deformation of structures under the condition that the camera is in the inclined position. In view of the actual needs of the structure deformation monitoring, this paper studies the overall deformation monitoring method under the condition of tilt photography. The steel truss concrete composite beam specimens were made in the laboratory. The static deformation images of the specimens were obtained by oblique photography. The overall deformation of the specimens was obtained by perspective transformation and edge detection technology. The error sources of this method were analyzed.