surveys. All observations are made on the actual curved surface of the earth. This curvature is later
PART D - SURVEY METHODS
2-4. General. Topographic surveyors use theodolites, levels, and distance measuring equipment
(DME). The automated integrated survey instrument (AISI) provides topographic surveyors with the
capability to extend control through the use of a total station. On the other hand, the NAVSTAR GPS is
capable of determining accurate positional, velocity, and timing information. The GPS provides
positional and navigational data to civilian and military communities in the form of two positional
services. The Standard Positioning Service (SPS) encompasses the civilian user and the United States
Coast Guard (USCG). When using a single GPS receiver (absolute positioning), the SPS users are
denied the high accuracy and instantaneous positioning capability of the GPS. The Precise-Positioning
Service (PPS) consists of military users and authorized representatives. The PPS user can obtain high
accuracy and instantaneous positioning if the receiver is capable of accepting the necessary cryptologic
2-5. Global Positioning System. Absolute and differential (relative) positioning methods, use the GPS,
provide accurate and timely positional data. The method of choice depends on the accuracy required,
the equipment available, and the logistical requirements. At present, the PPS GPS receiver capable of
performing relative positioning is the global positioning system-survey (GPS-S) differential global
positioning system (DGPS).
a. Absolute positioning uses a single GPS receiver and does not require known survey control.
Absolute positions can provide instantaneous (real-time) or postprocessed positions. Known survey
control is unreliable or nonexistent in immature theaters. Topographic surveyors can establish SCPs by
using absolute positioning.
b. Differential positioning uses two or more GPS receivers. One GPS receiver (reference receiver)
is resident over a known SCP. The remaining receivers (remote) are used to position points of interest.
Differential positioning can be performed in real time or through postprocessing. If real-time-
positioning results are required, a communications link that is capable of transmitting digital data must
be established at the reference and remote-receiver locations. This method supports distances up to 100
kilometers between the reference and remote stations. Topographic engineer battalions within the Army
have PPS GPS receivers that are capable of real-time and postprocessed differential positioning and
provide accuracy of about 1 centimeter.
2-6. Accuracy. The accuracy of a GPS is dependent on the user's classification (precise lightweight
GPS receiver [PLGR] or GPS-S) and the surveying method employed (absolute or differential).