Digital photogrammetry is an efficient and cost-effective alternative to analog and analytical technologies.
For many years, institutional planners have used aerial photography and photogrammetry as valuable resources in their campus-planning efforts. The ability to photograph large areas in great detail is an asset to planners, engineers, and both building architects and landscape architects.
Today, photogrammetrists use soft-copy technology to offer an array of products and services to school administrators and design professionals. Soft-copy uses digital imagery rather than traditional film photographs. One process used is orthophotography.
Orthophotography is the means by which aerial photographs are scanned as high-resolution electronic images. The software removes distortion and results in a true-to-scale photograph map. Orthophotographs enable planners to accurately measure distances between objects, such as curbs or buildings. They also allow a technician to superimpose additional information, such as proposed buildings, contours and text.
EFFICIENT AND COST-EFFECTIVE
Digital photogrammetry is an efficient and cost-effective alternative to analog and analytical technologies. It can be set up faster, and the compilation, viewed as a line-drawn map over the photographic image, enables quick updates or revisions. This process increases production by allowing more than one individual to work on the project.
In addition, a digital orthophoto can be produced at a much lower cost than previous aerial photos. It can be created as a stand-alone item or used as part of a mapping process. In a mapping project, production time can be reduced by as much as 75 percent.
Precision is also greater. Unlike aerial photographs, these photographic maps show true relationships between known points on the ground. They allow a user to plot accurate, high-quality photographs at any scale desired.
The level of accuracy of orthophotographs depends on two factors: the altitude from which the photograph is taken and the precision with which the relationship of the points visible in the photograph is obtained. The distances between points can be measured by scaling their position on a quadrangle map or by using surveying methods such as Global Positioning Systems (GPS), which produce extremely accurate orthophotographs.
GPS uses orbiting satellites to accurately determine a position on the surface of the Earth. In photogrammetry it is used primarily to establish survey control on unknown points. Control is defined by establishing X, Y and Z coordinates on points on the ground. Traditionally, survey control is performed by running a traverse — a closed loop of intervisible stations using two or more known points — around a project area. In this manner, X, Y and Z coordinate values on the unknown point are established to within five millimeters horizontally and 12 millimeters vertically.
GPS also makes it possible to mount an antenna onto a range pole and collect data in a “stop” or “go” mode. Once control is established on a project site, topographic features can be collected by occupying known points as base units and the pole mount unit as a rover. Locations of such features as curb lines and utility castings can be collected in this manner. This is a valuable tool for field-editing aerial maps; areas that were obscured in shadows cast by buildings or trees can be checked and completed on the ground.
DEVELOPMENT OF PHOTOGRAMMETRY
Digital photogrammetry is the latest method of preparing mapping from aerial photos. In the middle of the 19th century, aerial photos were taken from hot-air balloons. In later years, analog photogrammetry evolved through the development of stereo photogrammetry, which made it possible to see in three dimensions when viewing the photo. After World War I, aerial photography was used widely in military field reconnaissance and became recognized as an effective surveying and mapping method.
Analog photogrammetry is the use of a mapping system with such features as optical equipment, gear-driven plotting tables and other mechanical devices. Hard copies of the aerial photography, called dispositives, were placed in the machine with lighting devices and adjustment capabilities to set the stereo model so that the photo could be viewed in three dimensions.
With the advent of computers came the first analytical stereoplotters. The stereoplotters used the mechanical devices from an analog system in conjunction with a computer to enhance the capabilities of the newer generation of plotters. As computers advanced from transistors to minicomputers to microprocessors, so did the plotters.
Digital photogrammetry employs digital imagery rather than analog photographs. As computers became powerful enough to store, retrieve and manipulate large image files, the digital photogrammetric workstation evolved. Aerial photographs are scanned on high-resolution equipment to produce electronic images of the photos. The images are then used to collect information into electronic drawing formats.
Photogrammetrists compile the outlines of curbs, buildings, utilities, tree lines and boundaries with the drawings. Using 3-D glasses (stereo viewers) electronically linked to a computer, photogrammetrists can view the photo in three dimensions, allowing them to see the changes in height within the image. The result is a precise electronic topographic map that shows all the streets and surface hardware, such as manholes, valves and street furniture.
Education institutions employ these drawings as base maps of the campus. Information such as underground utility lines between manholes and buildings often are inserted to develop an electronic composite of the campus' infrastructure. The drawings typically are constructed so technicians can view everything in a given area or isolate a specific utility line. Designers commonly prepare construction documents in an electronic format, so these base plans have become essential.
An increasing number of institutions and agencies are relying on photogrammetric mapping to house and depict data on their facilities. There is virtually no limit to the amount or type of data that a university can incorporate in a geographic information system. By clicking on a building, it is possible to ascertain how many classrooms it houses, which courses are offered, how many students are in the course and who teaches there.
The principal advantage of aerial photography is that it covers large areas in great detail. The refinement of digital imagery has given school and university officials the capability of mapping their campuses with the precision required for growth and development.
Stigner, L.S., P.P., is a partner with Vollmer Associates, an engineering firm based in New York City. The firm worked on the SUNY Farmingdale project (see sidebar).
Photogrammetry helps map campus
State-of-the-art photogrammetric techniques were used to prepare a topographic and utility survey of the 375-acre SUNY Farmingdale campus on Long Island for the New York State University Construction Fund.
The most recent mapping of the Farmingdale campus had been performed in the late 1970s before many improvements, including the construction of new buildings. An up-to-date map also was needed because the main campus roadways were to be rebuilt.
Using new color photography of the campus, photogrammetrists selected visible points on the ground to establish photo control. These points were marked on the ground during the initial field reconnaissance of the site.
The survey team established ground control for the aerial photography using dual-frequency global-positioning equipment. This technology also was used to establish the supplemental baseline work for the project.
The aerial mapping was compiled into AutoCAD format, and check plots of the campus were prepared. A walkthrough (field edit) of the site was conducted to check for missing features and to clarify areas that may have been obscured by tree canopy or building shadows. Any changes were incorporated into the mapping.
The drawings depicted topographic features on the campus, including all buildings, curbs, retaining walls, roads, fences and trees. Also shown were all surface utility hardware such as manholes, fire hydrants and valve boxes. The mapping showed the elevation of the campus, including finished floor elevations of the buildings, spot elevations on the ground and contours at two-foot intervals.
Drawings of the campus infrastructure were reviewed, and all underground utility systems, such as storm and sanitary sewers, water, gas, electric, street lighting and telephone-distribution systems were incorporated into the mapping to provide an as-built record of the campus.
The scope of work for the project also called for the preparation of a new boundary survey. Deeds of the campus and the adjoining properties were researched, including any easements. Field evidence was collected to estimate the true location of the campus boundaries.
The basic map was then used to rebuild the main campus roadways. Extensive coordination among the design team, the contractor and campus officials was required to maintain access to all buildings during construction.