Preliminary Survey.
For the preliminary survey, to design the highway on the
selected route and to prepare detailed construction plans, all sur-
veying must be adequate in detail, sufficient in scope, and of the
accuracy which will permit the actual staking of the designed highway
on the ground exactly where intended. This staking will include the
center line, the structures, the slope stakes, and all essential points
on boundaries of property acquired for the highway right-of-way.
Generally, the basic control for the preliminary survey in
rural areas should be not less accurate than third order, with prefer-
ence for second order, and in metropolitan areas, where land values are
high and land use is intense, second order accuracy is essential, with
preference for accuracies approaching first order. Third order accu-
racy usually suffices (sometimes less accuracy is acceptable) for
supplemental control in using the aerial photographs photogrammetric-
ally to make precision measurements and compile topographic and other
essential maps, and for preparing photographic mosaics as required.
The orders of accuracy on the ground which are generally
required for highway surveys are included in the subsequent tabula-
tion, Table 1.
TABLE 1%
MAXIMUM ERROR OF CLOSURE
FOR BASIC CONTROL ON HIGHWAY SURVEY PROJECTS
Unadjusted | Unadjusted Horizontal Angles Unad justed
Order of | Horizontal Triangle Closure Vertical
Accuracy | Distances Traverses | Average | Maximum* Distances
First 1:25,000 | 2 Sec. |/N| 1 Sec.| 3 Sec. 0.017 Ft. yM
4.0 m.m. VK
Second 1:10,000 | 10 Sec. /N | 3 Sec.| 5 Sec. 0.035 Ft. yM
8.4 m.m. K
Third 1:5,000 | 30 Sec. /N| 5 Sec. | 10 Sec. 0.05 Pt. JN
12.0 m.m. |X
Fourth 1:2,500 | 60 Sec. y/N | 15 Sec. | 30 Sec. 0.50 Ft. /M
120.0 m.m. VK
The maximum should seldom be exceeded.
is the number of angle points in the traverse.
is the number of miles in the level circuit.
is the number of kilometers in the level circuit.
* Reference Guide Outline, Specifications for Aerial Surveys and
Mapping by Photogrammetric Methods for Highways, prepared by
Photogrammetry for Highways Committee of the American Society
of Photogrammetry.
