After the drone was photographed, it drew a red circle on the country road with nails in it. Why?

Does the subject want to ask: "After the drone was photographed, a red circle was drawn on the country road, and a nail was placed inside. What are the nails for? " Represents the measuring point. According to the inquiry, Sohu. Com shows that the position of the nail represents the position of the measuring point and has coordinates. The circle of red oil painting is to be eye-catching and convenient to find this point when measuring. In addition, there should be the number of this point, which is also written in red paint. UAV aerial surveying and mapping image control points, choose a wide field of vision, no or less occlusion around, and avoid shadow areas; To spray the number; Try to be as flat as possible, and don't choose a slope with height difference; The mark must be wide enough, at least 5cm wide, so that it can be accurately stabbed on the photo when working in the industry.