WHAT IS PHOTOGRAMMETRY ?

            Simply stated, photogrammetry is the art and science of making measurements from imagery. Historically, this meant using photographs, but today, digital images are becoming the medium of choice for many photogrammetric applications. In fact, the distinction between remote sensing and photogrammetry has gradually become blurred as conventional photogrammetrist employ "non-conventional" imagery for mapping purposes. One of the main advantages of photogrammetry is that the photograph/image forms a permanent record of the situation at that instant in time. This is ideal when change detection is important in a project.

         Imagery is generally collected from an airplane. The advantage is that the camera can capture a very wide image from which the mapping can be performed. For example, most aerial photography uses a film size of 9” x 9” (23 cm x 23 cm). If the scale of the photograph was 1” = 500’ then each photograph would image approximately 4,500’ x 4,500’ on the ground. Thus, with one click of the shutter, the image captures approximately 465 acres of land1. Another advantage of photogrammetry there is no line of sight problems that surveyors may encounter on the ground since we are looking at the land from above. There is, though, a need to have an nobstructed view from the camera.

               Increasing the flying height, or distance from the camera to the object being mapped, has two effects. First, it can increase the area imaged on the photograph. In our example, if the scale was decreased to 1” = 1,000’ then about 1,860 acres would be imaged on the photograph (a four times increase in coverage). This will save time in subsequent measurements on the photograph. But, the second effect of increasing the flying height results in a decrease in the resolution of the image. In other words, features that are too small may not be imaged on the photograph. Thus, the photogrammetrist is left with the task of determining the optimal flying height to match the requirements of the project. Close-range photogrammetry (sometimes called non-topographic photogrammetry) is another area that has found widespread applications. Here, the photography is obtained from the ground, or near the ground level. It is used for a myriad of purposes such as medical photogrammetry, accident reconstruction, aircraft and ship construction, architectural studies and construction sites, just to mention a few. This form of mapping is ideal when the object to be

measured is too hot, cold, unsafe, radioactive, inaccessible and delicate or when objects are moving so as to make direct measurement hazardous.

            Satellite photogrammetry is a relatively new area of practice, especially among conventional photogrammetric companies. Historically, photogrammetry had been used to locate satellites in space in order to establish a more global network of control. With the advent of Global Positioning System (GPS) receivers, the use of photogrammetry with satellites has taken on a new role. Added to this is the emergence of very high-resolution imagery, on the  order of one-meter or less. Thus, the future will see more utilization of this high-resolution imagery to both supplement and supplant conventional aerial imagery. 1 acre = 43,560 square feet