[Illustration: FIG. 7. SYMBOLS USED ON CHARTS OF THE BRITISH HYDROGRAPHIC OFFICE.]
[Illustration: FIG. 8. TRIANGULATION OF A BAY, SHOWING LOCATION OF SURVEY SIGNALS AND LANDMARKS.]
#Triangulation.# The main framework of the survey consists of a series of triangles connecting prominently located points which are permanently marked in the ground and the location described so that they can be found at a future time. At long intervals in the survey base lines are laid out and carefully measured with steel tape. Signals are erected over the points, including those at the ends of the base line, and angles are then measured at the various stations. From the measured length of the base and the angles the lengths of the sides of the triangles are computed, and from these lengths and the latitude and longitude of one point the latitudes and longitudes of all the other points are obtained. When several astronomically determined points are connected by such a triangulation a complication arises from what is known as "deflection of the plumb line," which is the angular amount by which the actual sea-level surface of the earth departs from the symmetrical figure of revolution, owing to the variations in the density of the earth's outer layers. The distance between two points as measured by triangulation thus differs from the distance computed from the astronomically determined positions. If this irregularity were not taken care of by adopting mean positions, the discrepancy in joining up different surveys would in extreme cases amount to about half a mile.
[Illustration: FIG. 9. TRIANGULATION STATION AND SIGNAL, ON ALASKA COAST.]
[Illustration: FIG. 10. MEASURING ANGLES WITH THEODOLITE AT TRIANGULATION STATION ON ALASKA COAST.]
#Survey sheets# are next prepared, of suitable size and scale. On each sheet a projection is laid down, that is, the meridians and parallels are drawn, and all the points determined in the triangulation are plotted in their true relation. Usually separate sheets are prepared for the topography or shore survey and for the hydrography or survey of the water area.
#Topography.# The topographic survey of the shore and as much of the adjacent area as is required is usually made with a plane table, on which the map is actually drawn in the field as the work progresses. Points are located on the plane table sheet either by direct reading of the distance on a stadia rod or by intersections from two or more stations. On the plane table sheet it is customary to locate the shore or high-water line, the low-water line, off-lying rocks, streams, rivers, roads, towns, lighthouses, and all prominent features near the coast. Elevations are measured with the plane table or obtained from the triangulation, and are represented on the sheet both by figures and by contours, which are lines joining together points of the same elevation. For instance, a 100-foot contour represents the line where a plane 100 feet above sea level would cut the surface of the ground. It is particularly important in this topographic work to locate accurately objects which are good landmarks and likely to be of use to the mariner. In some regions auxiliary methods are used in filling in the topography, as, for instance, along a difficult coast each feature of importance may be located by sextant angles, or a traverse line may be run along the shore by the transit and stadia method.