1. A computer implemented method comprising: providing, on a transformation device, an interface that enables identification of geo-coordinates on a source map that correspond with known geo-coordinates on a pre-existing online reference map, the interface including two viewing panes, a source pane to display the source map, and a reference pane to display the online reference map; using the identified geo-coordinates to determine an approximate coordinate transform relating to geo-coordinate relationships between geo-coordinates on the source map and geo-coordinates on the online reference map, the online reference map having images formed by a plurality of tiles; and using the approximate coordinate transform to transform an image of the source map to form an image of a modified source map having geo-coordinates that substantially align with corresponding geo-coordinates on the online reference map, wherein: the image of the source map is transformed using a reprojection function that is automatically selected based on a number of correspondence points established between the source map and the online reference map, and an affine reprojection function is automatically selected when at least two correspondence points are established.