The first C&GS aerial photographic mission took place along the New Jersey coast, near Atlantic City, in 1919. This photo mosaic, which shows part of the project area, was prepared using images collected during the mission. Aerial photographs became the basis for creating maps of our coastline.
Since the late 1920s, collecting and using high-resolution aerial photography to define the nation’s 95,000-mile shoreline has been a responsibility of NOAA’s National Geodetic Survey (NGS) and its predecessor, the Coast and Geodetic Survey (C&GS).
Aerial photographs are the primary source for creating coastal survey maps and digital map feature files. These maps and files, in turn, provide data for producing NOAA nautical charts. Data showing the accurate location of the shoreline is also used as a source to define the boundaries between private, state, and federal ownership and jurisdictions, including the territorial sea and the U.S. Exclusive Economic Zone.
The art of making coastal maps using aerial photographs has evolved over the years and generally falls into three main groups: graphic, analog stereoscopic, and analytical stereoscopic.
This image shows a chart of the same coast before and after an air photo resurvey. It had become possible, by air photo methods, to obtain the topography shown on the right at no greater cost than the limited ground survey on the left.
Graphic map compilation methods are relatively simple. They rely on having a large number of ground control points (points with known locations on the Earth) visible in aerial photographs.
The control points are surveyed in the field to determine their coordinates (latitude and longitude), and then marked on a map sheet. The transparent map sheet is positioned on top of a photograph (or a mosaic of several photographs), and a group of the marked control points on the sheet are lined up with their corresponding points in the image. The map can only be lined up with one portion of the photo at a time, because of various distortions in the photographic image.
Next, the details in that area of the photo are traced from the photo onto the map sheet. The map is then shifted to align a different group of control points with their corresponding points in the image, and the details in that area are then traced. This procedure is followed until the entire map sheet has been compiled.
The graphic method worked best in flat areas of land and where the camera was not tilted far from vertical when the photos were taken. When the area being mapped was hilly, then more control points were required, and only small areas of the photo could be traced before the map had to be adjusted.
Graphic map compilation was used in the earliest C&GS aerial photo projects in New Jersey, Florida, and Louisiana. This method continued to be used for many years, especially with revisions of previous surveys (where plenty of existing control points were available) and in flat areas where the mapping of topographic contours (elevation lines) was not required.
This stereogram shows two overhead images, taken from two different perspectives, of a lake, stream valleys, and surrounding hills. If you center yourself in front of the images, relax your eyes, and try to stare through the image, as if looking into the distance, you might be able to combine the two images into a single 3-D view of the terrain. It could be easier to first try merging the two white circles above the images into one combined circle in the center, and then slowly letting your eyes slide down for a 3-D image of the terrain.
Many areas of the country are not flat, especially along the West Coast and throughout Alaska. In the most mountainous areas, it was very difficult to survey many control points. A way to create maps was needed that compensated for the variations in height of the terrain, for the tilt of the camera, and for the distortions of the lens and film.
In Europe, various instruments for making accurate measurements and compiling maps from photographs were developed in parallel with advances in photographic technology. Practically all of these instruments were designed to work with strips of overlapping photographs, where part of the image in one photo also appears in the next photo of the strip. When the two photographs are arranged such that the left eye views one photo and the right eye views the other through an instrument known as a stereoscope, an object in the overlap area can appear to be three dimensional, having height or depth. The combined image that you see with both eyes is called a “stereo-model.”
Attached to the stereoscopic instrument used to view the two photos was a drawing or plotting device that was used to mark lines or points on a map sheet as a “floating mark” (a dot superimposed on the images in the instrument) was moved about the stereo-model.
Many varieties of these photogrammetric instruments were designed and built in Europe, Canada, and the United States. Some were more complex and costly than others. Generally, the more complicated and expensive the instrument was, the more accurate the final results. Many models could only be used with photos from a particular type of camera or lens.
The analog stereoplotter instruments were used until the late 1980s, when they were replaced by analytical plotters.
This aerial photo was taken in March 1986, as part of a coastal mapping project in the vicinity of Drum Point in the Chesapeake Bay.
Analog stereoplotters used purely optical and mechanical devices to simulate the stereo-model and compensate for the distortions in the photographic imagery. In contrast, analytical plotters use computers to mathematically solve for the parameters of photogrammetric equations and determine the mechanical adjustments required for an instrument to form the stereo-model, while correcting for all known sources of error. This advancement was made possible by the development of the computer in the late 1950s and early 1960s.
The analytical plotter was a quantum leap in sophistication that allowed photogrammetric mapping to become more flexible and accurate than ever before. Analytical plotters, being universal machines, were much less limited by the mechanical constraints of the instruments. A broader range of photographic formats, scales, focal lengths, camera tilts, and distortions of many types could be handled by a single instrument.
In the late 1980s, the National Ocean Service commissioned to have an analytical plotter built to NGS specifications. The device was aptly called the NOSAP. Soon after, additional analytical plotters were purchased, and eventually all coastal mapping was being conducted on the new instruments.
A softcopy workstation typically consists of a high-end desktop computer running specialized photogrammetric software with plenty of disk space for data storage, a three-dimensional pointing device (usually a trackball or special mouse), and a stereo display system for viewing the digital images in three dimensions.
In 1995, NGS began investigating the use of softcopy photogrammetric systems for coastal mapping. These systems are called "softcopy" because they operate only with digital images (i.e., scans of photographs or images from digital sensors), rather than with "hardcopy" film or glass photographic transparencies. Softcopy photogrammetric workstations use the same computational methods as the analytical plotter systems and have the same highly accurate mapping capability. However, the softcopy systems do not suffer from the physical limitations and high maintenance costs of optical-mechanical instruments. Furthermore, the vendors of softcopy systems continually improve their software to allow for a very high degree of automation in the mapping process, greatly improving production rates. Since 2000, NGS has used softcopy photogrammetry for all of its coastal mapping projects.