This article first introduces oblique aerial imagery, then describes how vertical distances can be measured once the pixel distances of the original pictures are known. The calculations require that, not only all camera settings be known, but also that one relies on the availability of detailed digital terrain and digital surface models (DSM and DTM), in order to provide the necessary ground level for calculating vertical distances. The algorithm is finally implemented in an online viewer.
Molinari, M., Medda, S., & Villani, S. (2014). Vertical Measurements in Oblique Aerial Imagery. ISPRS International Journal of Geo-Information, 3(3), 914–928. https://doi.org/10.3390/ijgi3030914