SYN scan is another form of TCP scanning. Document imaging requirements differ from those of image scanning. There are other types of scanning probe microscopy. It can be used as a stain in scanning electron microscopy. An example would be scanning a photograph and having a digital copy on a computer. This method had several disadvantages which could be overcome using an electronic scanning system. The safety of frequent ultrasound scanning has not be confirmed. Development of electronic scanning and display made an all-electronic system possible. It involves aerial surveys conducted with terrestrial laser scanning (LiDAR), drones, and photogrammetry exercises. SHARAD has the capability scanning up to about below the surface at intervals. Pixels in an image obtained by CT scanning are displayed in terms of relative radiodensity. Signature updates for a number of network scanning applications are now available. They were found on the Atlantic seabed using advanced sonar scanning. Early 21st-century laser scanning of the carvings supports this interpretation. A specialized form of document scanning is book scanning.