1. An image forming apparatus comprising: a control point measuring section for measuring a centrally projected image having a plurality of control points, and obtaining the image coordinates for said control points; a coordinate transformation parameter calculating section for obtaining a transformation parameter for correlating, based on said image coordinates for said control points obtained by said control point measuring section and the three-dimensional coordinates for actually measured control points, said image coordinates with said three-dimensional coordinates; an orthogonally projected image (ortho-image) forming section for forming an orthogonally projected image from said centrally projected image based on said transformation parameter obtained by said coordinate transformation parameter calculating section; and an ortho-image correcting section for correcting the image coordinates obtained by said ortho-image forming section based on the three-dimensional coordinates for actually measured additional points, and then performing correction of said orthogonally projected image.