1. A survey data processing device comprising: a data receiving unit 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, which is later than the first timing, respectively, while the camera travels; a selection receiving unit that receives a selection of reference points from among multiple reference points, which are included in both the first still image and the second still image and have known location information; an exterior orientation parameter calculating unit 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 reference points; a coordinate integrating unit that integrates a coordinate system of the unselected reference point and a coordinate system of a location of the camera, which is calculated by the exterior orientation parameter calculating unit, so as to obtain an integrated coordinate system for describing the location of the unselected reference point and the location of the camera in the same coordinate system; a back-projected image generating unit that back-projects the unselected reference point in the integrated coordinate system, to the location of the camera at a specific position, thereby obtaining a back-projected image; and an estimating unit that estimates a position of the unselected reference point in a still image that is photographed by the camera at the specific position, based on the position of the unselected reference point in the back-projected image on a screen.