1. A measurement apparatus for measuring a relative position and orientation of an image-capturing apparatus with respect to a measurement object, wherein the measurement apparatus comprises: an image acquiring unit configured to acquire a captured image of the measurement object from the image-capturing apparatus; a projection unit configured to project geometric features of a 3D model of the measurement object onto the captured image based on a position and orientation of the image-capturing apparatus to obtain projected geometric features; a selecting unit configured to select projected geometric features to be used in calculation of the relative position and orientation from the projected geometric features obtained by the projection unit based on a positional relationship between the projected geometric features in the captured image; a search unit configured to search the captured image for image geometric features corresponding to the projected geometric features selected by the selecting unit; and a calculating unit configured to calculate the relative position and orientation of the image-capturing apparatus with respect to the measurement object using positions of the projected geometric features selected by the selecting unit and the corresponding image geometric features searched out by the search unit, wherein the geometric features are detected based on a pixel density between search start points and search corresponding points, and a point where a density gradient of an adjacent pixel exceeds a predetermined threshold value is detected as a position of a geometric feature.