baseline vectors) are computed forward around the loop and back to the starting point. The forward-
computed misclosure provides an estimate of the relative or internal accuracy of the observations in the
traverse loop, or more directly, the internal precision of the survey. This is perhaps the simplest method
of evaluating the adequacy of a survey. These point misclosures (usually expressed as ratios) are not the
same as relative distance accuracy measures.
b. The coordinates (and reference orientation) of the single, fixed starting point will also have some
degree of accuracy relative to the network in which it is located. This external accuracy (or inaccuracy)
is carried forward in the traverse loop or network; however, any such external variance (if small) is
generally not critical to engineering and construction projects. When a survey is conducted relative to
two or more points on an existing reference network, misclosures with these fixed control points provide
an estimate of the absolute accuracy of the survey. This analysis is usually obtained from a final
adjustment (usually a fully constrained least squares minimization technique) or by another recognized
traverse adjustment method.
5-49. Internal Versus External Accuracy. Geodetic surveying is largely concerned with absolute
accuracy or the best fit of intermediate surveys between points on a national network, such as the
NGRS. In engineering and construction surveying and, to a major extent, in relative or local boundary
surveying, accuracies are more critical to the project at hand. Thus, the absolute NAD-27 or NAD-83
coordinates (in latitude and longitude) relative to the NGRS datum reference are of less importance;
however, accurate relative coordinates over a given project reach are critical to design and construction.
a. For example, when establishing basic mapping and construction layout control for a military
installation, developing a dense and accurate internal relative-control network is far more important than
the values of these coordinates relative to the NGRS. Surveys performed with GPS-S and the final
adjustment thereof, should be configured or designed to establish accurate relative (local) project
control. This is of secondary importance in connection with NGRS networks.
b. Although reference connections with the NGRS are desirable and recommended and should be
made where feasible and practicable, it is critical that such connections (and subsequent adjustments) do
not distort the internal accuracy of intermediate points from which design, construction, or project
boundaries are referenced. Connections and adjustments to distant networks (such as NGRS) can result
in mixed datums within a project area, especially if not all existing project control has been tied in. This
can lead to errors and contract disputes during both design and construction. On existing projects with
long-established reference control, connections and adjustments to outside reference datums or networks
should be performed using caution. The impacts on legal property and project-alignment definitions
must also be considered prior to such connections.
c. On newly authorized projects, or on projects where existing project control has been largely
destroyed, reconnection with the NGRS is highly recommended.
5-43
EN0593
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