## 1 Introduction  
A port crane is a large piece of construction machinery, widely used in major ports, playing an important role in cargo transport. Due to its high frequency and high intensity working characteristics [1], its structural safety has attracted much attention [2]. To evaluate the safety of port machinery, it is necessary to measure the coordinates of its key points or measure the key dimensions. Port machinery works continuously for a long time and its structure is usually tall [3]. Some positions cannot be reached manually, so there are few tools that can complete the measurement, and some key data cannot be measured directly.  
At present, among the effective measurement methods within a certain range, the operation of total station [4, 5] is complicated. If too many points need to be measured, its efficiency will be relatively low. For high-strength, long-working construction machinery such as port cranes, the working clearance is relatively short. If the total station is used for measurement, the crane needs to stop working for a long time, which will cause relatively large economic losses. Laser scanner [6] has a high cost and complex post-processing. The price of a single device usually exceeds USD 120,000. It needs to work for a long time in one location to obtain a sufficient number of point clouds, so it does not practical for large-scale application. Installing sensors [7] is too manual and dangerous. Coordinate measuring machine (CMM) [8; 9] has high measurement accuracy and good versatility, but it belongs to the contact measurement mode with high requirements on the environment, and its measurement range is small. Laser tracker [10] also belongs to contact measurement, but each contact can only obtain the coordinates of one point, and its measurement efficiency is low. Compared with the above measurement methods, photogrammetry is not only simple and flexible in operation and low in cost, but also with the continuous development and progress of computer vision technology [11; 12; 13], image post-processing is very convenient.  
Photogrammetry has the characteristics of non-contact and automatic processing, so the use of photogrammetry has become a new idea of traditional engineering measurement, especially in today's pursuit of automation and the intelligent development trend [14]. The development of instruments, sensors, robots, electronic circuits, chips and other technologies has also injected new vitality into the development of photogrammetry technology. With the continuous emergence of various innovative technologies in recent years, close range photogrammetry has been widely applied in fields, such as deformation photogrammetry, architectural photogrammetry, engineering photogrammetry and so on. Close-range photogrammetry is more advantageous than the traditional geodetic method for measuring deformation of objects, especially for high-rise buildings, bridges, machines and dams. Darmstadt Institute of Engineering published their research results on deformation observation of a 43-story building with a height of 142 m, and Freiberg Institute of Mines in Germany published a report on deformation of a transport bridge in an open-pit coal mine under load conditions, both of which achieved good results [15]. Close-range photogrammetry is also used in reverse engineering, which greatly shortens the development cycle of industrial products. Due to the high detection accuracy of close-range photogrammetry, it has many applications in automobile manufacturing, parts quality control, whole machine assembly, automatic welding, automatic painting and so on [16].