While the practice of photogrammetry has changed considerably over the years, especially since the advent of high-speed desk-top computers, the basic principles have remained the same. The ultimate goal is obtaining reliable information about objects and the environment through the use of photographs or other non-contact data sources. Photogrammetry's best known application is in the generation of topographic maps; this is the area that Mapmaker focuses on.

     Aerial photography is our most common data source. Typically the cameras used have a 9"X 9" negative size and a 6" focal length lens. The cameras and film used (which can be black and white or color) are designed to precisely replicate the geometric properties of the captured scene. The altitude from which the images are taken determine the amount and precision of the data that can be extracted. Lower altitudes provide more data and greater precision, but have less field of view and so require more images and ground control to cover the target site. We normally have the photography captured at altitudes between 1200' and 5000' above mean terrain depending on the accuracy and detail necessary for the project.

     The photography we use for mapping is taken at regular intervals along the flight line so that each image overlaps the previous one by about 60%. In this common overlap area, the same object on the ground will appear in both images, but is seen at different angles. When two adjacent images are oriented in a stereoplotter, a virtual 3D model of the overlap area is formed.

     The resultant virtual 3D model in the stereoplotter is geo-referenced  by measuring the position in the model of an object with known real world coordinates (ground control points). These coordinates are usually obtained through conventional or GPS field survey techniques. A measuring mark (often called the "floating mark) in the stereoplotter is superimposed into the 3D model and can be moved in x, y, and z by the operator to obtain precise photo coordinates.

     In the pre-computer analog stereoplotters, the motion of the floating mark was transferred mechanically to a pencil. As the operator traced a feature in the model, it would be drawn on the map sheet. Today, a graphics program allows the operator to digitize the features and save the vectors and points to a digital file.