Aiming at the issues of camera calibration and orientation in large spatial volumes, a novel joint calibration and orientation method for dual cameras is proposed. In this method, an unmanned aerial vehicle carries a scale ruler to create a spatial virtual calibration field, while two cameras simultaneously take photos of marker points at both ends of the scale ruler. Then, the spatial length of the scale ruler is used as constraint; the self-calibrated bundle adjustment model is developed to calculate the internal azimuth parameters and distortion coefficients of the two cameras and the external azimuth parameters between both cameras. The length of the rebuilt scale ruler is then used to assess the error and accuracy. To validate the proposed method, calibration tests were performed in an outdoor environment with the spatial dimensions of 40 m×10 m×14 m. The results demonstrate that after calibrating the dual cameras using the proposed method, the average length error of the proposed method for reconstructing the scale ruler is 0.193 mm, the root mean square error is 2.316 mm, and the relative accuracy is better than 1/20000. The proposed method can easily and correctly calibrate all the dual-camera system’s parameters in a large measurement volume. Furthermore, the proposed method is simple, costeffective, highly accurate, and has a broad applicability. Moreover, it serves as a reference method and database for the visual assessment of extremely large scale objects.