NON-CONTROLLED CLOSE-RANGE PHOTOGRAMMETRY METHOD BASED ON PHOTOMODELER SCANNER

Aiming at the high demand of close-range photogrammetry for the object control points and the inconvenient carrying of digital camera to take photogrammetry in the field, a method of detail survey based on PhotoModeler Scanner software is proposed. USB camera combines with the centering rod, USB camera captures image data, Total station obtains the coordinates of the centering rod, the coordinate of projective center is calculated by using the coordinate of the centering rod, PhotoModeler Scanner software processes image data and coordinate data. Ultimately, image stereo measurement of the no object control point was realized. The experimental results showed that the error of the plane position of the solved target point was less than 10cm at a shooting distance of about 10m, and it met the accuracy requirements of detail survey. Therefore, the method can reduce the workload of field detail survey, reduce the cost and volume of the photographic equipment, and has certain application value.

for the measurement which requires simple and quick operation and lower precision. In this paper, USB camera is adopted to shoot images. Although the imaging effect is not as good as that of digital camera, the weight and volume of the device are reduced, making field measurement more convenient. By using the characteristics of PhotoModeler Scanner, a measurement method is designed, which gets rid of the dependence on object control points in photogrammetry. The method can meet the precision requirements of 1:500 digital mapping within a certain distance.

COMPOSITION AND DESIGN IDEAS OF EQUIPMENT
The purpose of the design is to use the three-dimensional coordinates of three known shooting sites as the basic data in the case of no object control point coordinates, and to build a digital model in PhotoModeler Scanner software to realize the three-dimensional coordinate measurement of the target points to be measured. For this purpose, a set of close-range photogrammetry equipment is designed, including a centering rod, a prism, a USB camera, an angle sensor, a smartphone, an OTG data cable. A total station observes the prism mounted on the centering rod, and the prism and centering rod are to obtain the coordinates of the shooting sites, and then the coordinates of the shooting sites will be converted into the coordinates of the projective center in computer. The USB camera is fixed on the centering rod, and the projective center light of the camera is perpendicular to the centering rod. The angle sensor is mounted on the camera to measure the azimuth angle of the main optical axis of the camera. The smartphone is mainly used for manipulate USB cameras and angle sensors and stores data. The OTG data cable is used for data transmission between smartphone and USB camera and angle sensor. The close-range photogrammetry equipment is shown in figure 1.

General design idea:
First, the centering rod is placed on an appropriate point, which is called the shooting site. The USB camera aims at some target points to shoot an image, and the three-dimensional coordinates (x, y, z) of the shooting sites are measured by the total station. In this way, image data and location data are collected at three locations that are not collinear.
Secondly, the vertical height from the bottom of the centering rod to the projective center is h, the plane distance from the centering rod to the projective center is d, and the azimuth of the centering rod to the projective center is . The three-dimensional coordinates of the projective center point are as follows: ( sin cos )   x+d , y +d ,z +h .
Finally, we can complete the absolute orientation of the image and the image stereo measurement by using the calculated coordinates of the three projective center points.

Calibration of Cameras
Before using the ordinary digital camera to carry on the close-range photogrammetry, it must carry on the strict calibrating, in order to restore the relative geometry relation between the projective center and the image [11]. USB camera must be calibrated before taking images. The main content of the camera calibration is to obtain the basic information of the camera, lens distortion parameters and the inside azimuth element of the camera. In this paper, a camera calibration method based on 2D targets proposed by Mr. Zhang zhengyou is adopted. This method only needs the relationship between corresponding points in multiple images to achieve camera calibration, which is simple and flexible, and its accuracy can meet the requirements of this experiment [12][13].
The calibration steps of USB camera are as follows. Frist, The main optical axis of photography keep 45°from the indoor Finally, eight images are imported into PhotoModeler Scanner software for camera calibration [14]. The images and part of the processing process are shown in figure 2. At the same time, the data of lens distortion and accuracy evaluation were showed, which showed that the accuracy of the calibration results met the requirements. different origin" in mathematics. The transformation formula is as follows:

Images
In formula (1) Photogrammetric coordinates of all model points can be transformed into ground measurement coordinates.    As could be seen from table 1, the standard deviations of , and were 0.0270m, 0.0137m and 0.0030m respectively. Horizontal position error was 0.0301m after calculation. In 1:500 digital mapping, the horizontal position error was required to be less than or equal to 10cm. Therefore, the results of this photogrammetry can meet the requirements of 1:500 digital mapping.

EXPERIMENTAL ANALYSIS
In second experiment, 10 reflectors were attached to the surfaces of stone benches or pillars on the roof of a teaching building. The distances between the shooting sites and the reflectors were 8 to 12 meters. The method of photogrammetry is the same as that adopted in first experiment. The process of outdoor photogrammetry experiment is shown in figure 5.  coordinates of the target points and the measurement accuracy met the requirements of digital mapping.

CONCLUSION
In this paper, the USB camera is combined with the centering rod. Three pieces of image and three-dimensional coordinates of three shooting sites are taken as the basic calculation data, which are processed in PhotoModeler Scanner software. Finally, the three-dimensional coordinates of the target point are calculated and will be applied to detail survey.
The experiment showed that when the shooting distance was The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-3/W10, 2020 International Conference on Geomatics in the Big Data Era (ICGBD), 15-17 November 2019, Guilin, Guangxi, China about 10 meters, the error in the plane position of the target point was less than 10cm, and the accuracy met the requirements of detail survey. This method effectively solves the limitation of the object control points which close-range photogrammetry needed for, and makes full use of the advantage that close-range photogrammetry can carry out stereo measurement without contacting the target points. In addition, the equipment can reduce the field workload of detail survey, and reduce the volume, weight and cost of photographic equipment, thus it has certain application value.
In order to improve the accuracy of stereo measurement, the system error is minimized when the equipment is installed.