1. A survey data processing device configured to detect multiple targets, of which locations are preliminarily identified, from an image, the device comprising: a processor programmed, or dedicated hardware configured, to perform as a data receiver that receives data of a first still image and data of a second still image, the first still image and the second still image being obtained by photographing the same object by a camera at a first timing and at a second timing respectively, while the camera travels, the second timing being later than the first timing; a selection receiver that receives a selection of respectively, while the camera travels, the second timing being later than the first timing; a selection receiver that receives a selection of some of the multiple targets, which are included in both the first still image and the second still image; an exterior orientation parameter calculator that calculates exterior orientation parameters of the camera at the first timing and exterior orientation parameters of the camera at the second timing, based on the selected-targets; a coordinate integrator that integrates a coordinate system of-an unselected target and a coordinate system of a location of the camera, which is calculated by the exterior orientation parameter calculator, so as to obtain an integrated coordinate system for describing the location of the unselected target and the location of the camera in the same coordinate system; a back-projected image generator that back-projects the unselected target in the integrated coordinate system, to the location of the camera at a specific position, thereby obtaining a back-projected image; and an estimator that estimates a position of the unselected-target in a still image that is photographed by the camera at the specific position, based on the position of the unselected target in the back-projected image on a screen; wherein the device performs processing for setting a search area by enlarging an area containing the location of the unselected target, which is estimated by the estimator, wherein the processor is further programmed, or the dedicated hardware is further configured, to perform as a determiner that judges a target as an erroneous target when a difference between known location data of the target detected in the search area and location data of the target, which is calculated from multiple still images by an intersection method, satisfies a predetermined condition; and the survey data processing device performs processing of evaluating variation of the difference and changing an alarm level depending on a degree of the variation.