A target including a reference object of a known size is prepared. The reference object from target image data, which is generated by photographing the target using a survey instrument, is detected through image processing to calculate first size data on dimensions of the reference object. A defect from external appearance image data, which is generated by photographing an external appearance of a structure using the survey instrument, is detected through image processing to calculate defect data on dimensions of the defect. The first size data is compared with second size data on actual dimensions of the reference object to calculate correction data, which is used to correct the defect data.