Trig elevations are used only to control vertical picture points. The precision of the derived elevations is insufficient to permit its use as basic control. Depending upon the situation, direct leveling with fewer precision restraints is often used to control vertical picture points.
To further advance knowledge of the earth, as well as for orientation of geodetic datum's, the force of gravity at a large number of photo-identifiable points is usually determined with a device called a Gravity Meter The meter is a small portable unit, the readings from which can provide the force of gravity at each observed station.
Field classification requires people on the ground to provide the names of roads, towns, and other features shown on a map. Interpreters are also needed to interview local residents and officials. Often the same feature will be called by different names and help of the interpreters to needed to determine the best or most common name.
Mapping a country, prior to satellites, required a tremendous amount of time, equipment, and skilled personnel. Logistics, maintenance, transportation, communications, and emergency situations could be the most difficult parts of a survey project, as many will attest. Memoirs of the men involved in the projects of the 64th Topo Bn are a testament to the fortitude required of those on the ground to complete these missions. The experiences were unique and yet common among its members but are probably not to be seen again.
David P. Moore
Traverse consists of a sequence of intervisible stations at which angles to the last station and the next station are observed, the distances between the sequential stations are directly measured. Traverse, as a method of extending geodetic control, was previously seldom used due to limited sight distances and no internal accuracy checks. With the advent of portable electronic lightwave and microwave distance measuring equipment (EDME) the use of traverse to extend control began to gain favor. The lightwave devices, which provided for the most precise distance measurement, lead to traverse supplanting triangulation as the basic geodetic control method. A traverse begins at an existing station of sufficient accuracy, at which the azimuth to another existing station is known, the traverse line progresses through the area where horizontal control is required. The line terminates at another pair of stations with existing horizontal control. Traverse control points are always called traverse stations.
Establishment of a vertical geodetic control net is, with rare exception, done by means of leveling. More correctly it is spirit leveling (but more commonly being called direct or differential leveling), using a precise instrument in which the level line was established with the use of a sensitive level vial, reading of the instrument being made on precise vertical measuring rods (more commonly called level rods). Vertical control stations are always called benchmarks or BM's, intermittent points upon which the level rod is set are called turning points or TP's. Level lines are divided into sections of varying lengths with a BM at the end of each section.
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The methods of providing horizontal control of sufficient accuracy to position the HIRAN ground stations consisted of triangulation or traverse. Control of high quality, which may be used to extend additional control, is usually called geodetic control. Triangulation consists of observing the angles of a sequence of intervisible overlapping triangles, which form a net, beginning at a base line between two previously existing stations of sufficient accuracy If only one station was available a base was measured by precisely taping the distance to a newly established station. The net extended then through the area in which horizontal control was required. As the mathematical conditions require, additional bases within the net are measured. The net finally terminates at another location with existing horizontal control. Measurement of all the angles between visible stations provides for calculation of all line azimuths and lengths and therefore the horizontal location of all stations. Triangulated control points are referred to as triangulation stations.
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Vertical control may also be established by measurement of the vertical angle of each horizontal control line, which in combination with its measured or computed length provides the difference in elevation between the stations. In practice, the vertical angle at each end of a line is measured, this provides a check on the results. This method of vertical control is called trigonometric (or trig) elevations.
"Making Topographic Maps (or what we were really doing)" Copyright © 2008 is written by Mr. David P. Moore who has graciously provided Ethi-USMappingMission.com with a copy for publication on this Web site. All Rights Reserved.
Mapping surely predates written language with a stick drawing lines in the dirt to represent geographic features. The first known map is a Babylonian clay tablet made before 2000 BC. The Greeks and Romans were highly skilled mapmakers. The science and art reached a high level of sophistication by the 19th century. The data was gathered by surveyors, on the ground, drawn into maps by cartographers, with printers producing the paper maps on various types of presses. The pairing of the camera with the airplane in the early 20th century provided images of the surface of the earth that allowed precise measurements through photogrammetry. With the surveyor providing location of points on the earth the photogrammetrist was able to quickly draw images on paper that the cartographer combined to provide uniform and detailed map reproducibles. These were then used to make the printing plates. The maps were printed, generally with the offset press, this being the fastest production method and still in use today
Introduction of electronic distance measurement (EDM) in the 1950's provided the surveyor a means of quickly making accurate long distance measurements. It also provided a means to find the location of an aircraft taking the photos, which reduced the amount of surveying required to produce certain types of maps. Introduction of satellite photography and the Global Positioning Satellites in the 1980's has eliminated the need for both the surveyor and the aircraft in many types of mapping in the 21st century
The ultimate product of the surveyor is control of aerial photography to determine the location on the surface of the earth, horizontal and/or vertical, of a point on the ground that may also be seen as an image on the aerial photograph. These points are called a picture points.
Horizontal control always involves two components, i.e., how far north or south and how far east or west. These components may be expressed as latitude or longitude or as coordinates on a rectangular grid. As a part of the horizontal control also azimuth (direction of north from point to point) and the distance between points may also be determined.
Vertical control is almost always elevation above sea level.
Aerial photography used by the 64th was produced by the US Air Force and controlled horizontally by HIRAN (High Range Navigation Radar) If the location of two widely and usually non-intervisible points are known and each measures a distance to an airplane at the instant the aerial photo is made the horizontal component of location of the airplane and therefore the photograph is determined. In order to get the maximum coverage, the airplane was usually flown at an altitude of above 30,000 feet. The equipment, both the air and ground stations, was large and heavy so that large aircraft were required, e.g., an RB-50.
Accurate and consistent altitude of the aircraft was required so that ground elevation could be determined. The altimeter of the airplane did not have sufficient precision for that use. For determination of ground elevations a device called a Terrain Profile Recorder was used It used the boiling point of a liquid chemical to maintain the aircraft at the same pressure altitude during a flight. The aircraft would fly to the boiling point of the liquid, use a gyroscopically stabilized radar altimeter to determine height above a large body of water (i.e. known elevation), and then fly a line (at the pressure altitude), while making both a series of aerial photos and a continuous strip chart trace of both the radar altimeter reading and the pressure altitude, to a junction point within the country being mapped. A number of the flights were flown to provide a series of flight line intersections and junctions. Such junctions were called TPR areas. Photo identifiable points in each TPR area were controlled vertically by survey and the flight altitudes assigned to each photo adjusted mathematically to provide elevations for the entire area
Each project had differing requirements with respect to quality of the control, and spacing (density) of the control. The projects of the 64th Engr Bn in Africa and the Near East generally had a requirement to provide horizontal control of high quality for Air Force HIRAN station location and of adequate vertical quality for the TPR areas. A somewhat higher quality of horizontal and vertical control was required for areas of special interest.
Most of the projects involved production of maps generally at a horizontal scale of 1:250,000 (1 inch = approx 4 miles) and with a contour interval of, usually, 100 feet, both of which were within the accuracy range of HIRAN and the TPR. Areas of special interest were generally at a scale of 1:50,000 (1 inch =approx 0.8 mi) and with a contour interval of 20 feet, which required additional survey control.