Binary image stitching based on grayscale approximation. In a first example embodiment, a method for automatic binary image stitching of first and second binary images includes several acts. First, the first and second binary images are converted into first and second thumbnail grayscale images. Each thumbnail grayscale image is a lower resolution than its corresponding binary image by a scaling factor. Next, a thumbnail transform is calculated between the first thumbnail grayscale image and the second thumbnail grayscale image. Then, a full-size transform is calculated as a function of the thumbnail transform and the scaling factor. Next, the stitched binary image is created by projecting the first binary image onto the second binary image using the full-size transform and combining overlapping portions of the first and second binary images.