Photogrammetry -- the art of measuring from aerial photographs and drawing those meausrements to scale -- is being used increasingly in the planning of residential communities.
Used by the federal government for its vast map-making operations, photogrammetry has been adapted since World War ii for the making of three-dimensional surface maps. These maps are invaluable for highway planning and building and for developers who are making decisions on how to build a large residential tract.
Bernard Solomon, the Pittsburgh-based director of photogrammetry for Greenhorne and O'Mara Inc., a large, engineering firm with headquarters here in Riverdale, describes his specialty of topographic mapping as an "orthographic representation of the earth's surface."
For Kettler Brothers, which is planning a 213-acre addition to Montgomery Village, photogrammetry gives the lay of the land in advance. Edward De-Simon. Kettler vice president for land development, said that the topographic maps made from aerial photographs and land surveys "enable us to know what grading and earth balancing must be done to obtain a finished contour as the site for new homes."
Thomas Wylie, a civil engineer and land surveyor with Dewberry, Nealon & Davis, another broad-based engineering firm in this area, pointed out that photogrammetry requires "some field work on co-ordinates" but generally provides a result that is "quicker, cheaper and generally accurate."
Ricard Krause, an executive with Photo Science Corp. of Gaithersburg, said that in addition to being adapted into topographic maps, sophisticated aerial photographs have been used to study the effects of Dutch elm disease in a downtown area of Washington and to study peanut crop disease in Suffolk County, Va.
"Right now we are mapping the Delaware coastline to make possible the establishment of new building line limitations near the water," he added.
"Since World War ii, the technology of photogrammetry and topographic map-making has advanced," said James Crabtree of Air Survey Corp. in Reston. He agreed that the aerial photo component has made "topo" map-making less expensive and more accurate for highway designs and residential subdivisions.
Bernard Solomon says a photogrammetric assignment starts with an experienced pilot in a specially adapted airplane and a $100,000 cartographic mapping camera.
"Then the weather and time of the year must be right," he added. "Ninety percent of our work is done in four months of the year when the trees are leafless and the sky is clear and when
Solomon said that a camera built in West Germany is mounted over a 19-inch hole in the center of the plane's floor. "It must be perpendicular to the ground with a tilt less than 5 degrees," he added. Then the flight follows a pre-determined line over the site, for which Solomon has made a prelimary map.
"It's almost like a bombing run, but using a camera with the pilot and photographer in voice communication," He said. Most of the photos, which are overlapped, are taken at altitudes of 1,200 to 6,000 feet. The assignment itself might range from 50 to 600 acres, Solomon added.
The photogrammetric assignment can be done on a medium-size tract for $35 an acre, Solomon said. Conventional, land-based engineering surveys for a map can cost more than twice that.
After the aerial photgraphs are printed on glass plates, they go to Solomon's shop in Pittsburgh, where a $55,000 stereo plotter meshes photos to produce "what the human eye would see from an airplane," he said. There's a plotting table beside the technician, who uses switches to draw lines that make up, the topographic map. "That's where the third dimension is provided by a skilled person," Solomon said.
The last step in the process which also involves a computer-driven machine with a TV screen, is a free-hand drawing by a draftsman. This reproduction is for a permanent record. The map is produced on "mylar," transparent plastic material that is water-proof, does't tear but can be cut or melted. The material, from which blueprints are made, can be corrected.
Solomon said his firm also makes aerial surveys for cities and counties including Rockville, Jamestown, N.Y. and Calvert and Howard counties in Maryland.
Solomon, who lectures on photogrammetry at several eastern universities, said that early earth mapping was done from ground-level sites and that the giant Tennessee Valley Authority power dam was the first big aerial mapping project in the United States. Aerial map-making got its start in central Europe and was introduced in Philadelphia are as early as 1920.
In addition to competing with sevphotogrammetry firms in this area, the Greenhorne-O'Mara division also has rivals in Baltimore, Texas, New Jersey and Cleveland for business it does in areas east of the Mississippi and north of South Carolina. Most of the assignments in this area have been rsidential subdivisions such as the new one for Montgomery Village, the development of the Timberlawn estate, a community near Annapolis and sections of Columbia.
"But we also can use aerial photography, in conjunction with known ground measurements and densities, to inventory piles of gravel and coal," Solomon said.