Other nations. Existing data is sometimes received from the national agency charged with
the mapping of that nation. Local municipalities and city governments also have survey
information within engineering or land-planning offices.
Do not evaluate existing material until all material has been assembled and the information has been
annotated on the available maps or aerial photographs. Plot the required SCPs from the project
directive, and evaluate the usability of existing controls. Compare the required control method with the
existing control method to determine if additional basic control is needed. It is possible that many
required stations may be eliminated because adequate control already exists. For required stations that
must be established, a tentative route of survey is annotated on the maps.
b. The field-reconnaissance phase is different for each survey project. A party chief must consider
the lessons learned from previous projects and apply the methods and techniques to suit the conditions
of the current project. A successful party chief employs the knowledge and ingenuity of the survey-
party personnel. Time permitting, the party chief and one other person should conduct a preliminary
field inspection of the area. When gathering information concerning the area to be surveyed, include the
terrain; tree heights; the road width; the road surfaces; the spacing between roads; the microclimate,
such as fog, haze, and heat waves; and any other factors that may have an effect on the distance
measuring and intervisibility between the proposed stations. The inspection may be conducted using
vehicles, helicopters, or airplanes. The results of the inspection determine the scheme and the route for
the survey.
2-48. Recovery and Verification of Existing Control Stations. In areas where control is to be
extended or established, there may be control stations from earlier surveys that must be recovered and
verified. These stations should have been identified and annotated on overlays during the office
reconnaissance phase and will serve as starting points for proposed GPS networks, traverse lines, or
level lines. The existing stations should be located, described, and verified for accuracy before using
them for extending control.
a. Recognize that existing control stations (and their establishing surveys) follow similar patterns.
Recognizing and associating the patterns with the terrain types will assist the surveyor in locating
existing stations. Triangulation stations are usually found on the highest point of a hill or a mountain.
In areas of little relief, the stations may be located at prominent points or sites where a tower can be
easily erected. BMs and traverse stations are typically located along roads, railroads, pipelines, or other
transportation routes which permit intervisibility and accessibility. BMs and traverse stations may also
be found along waterways, rivers, canals, and coastlines.
b. Evaluate all existing information. In some areas, urbanization has changed existing road or
drainage information. In rural areas, land may have been cleared
2-25
EN0593
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