The natural water courses mark not only the lowest lines, but the lines of the greatest longitudinal slope in the valleys through which they flow.
The direction and position of the principal streams give also the direction and approximate position of the high ground or ridges which lie between them.
The positions of the tributaries to the larger stream generally indicate the points of greatest depression in the summits of the ridges, and therefore the points at which lateral communication across the high ground separating contiguous valleys can be most readily made.
The instruments employed in reconnoitering, are: The compass, for ascertaining the direction; the aneroid barometer, to fix the ap proximate elevation of summits, etc.; and the hand level, to ascertain the elevation of neighboring points. If a vehicle can be used, an odometer may be added, but distances can usually be guessed or ascertained by time estimates or otherwise, closely enough for pre liminary purposes. The best maps obtainable and traveling com panions who possess a local knowledge of the country, together with the above outfit is all that will be necessary for the first inspection.
The reconnoissance being completed, instrumental surveys of the routes deemed most advantageous should be made. When the several lines are plotted to the same scale, a good map can be pre pared from which the exact location of the road can be determined.
In making the preliminary surveys the topographical features should be noted for a convenient distance to the right and the left of the line, and all prominent points located by compass bearings. The following data should also be obtained: the importance, magnitude, and direction of all streams and roads crossed; the character of the material to be excavated or available for embankments, the position of quarries and gravel pits, and the modes of access thereto; and all other information that may effect a selection.
Topography. There are various methods of delineating upon paper the irregularities of the surface of the ground. The method of most utility to the engineer is that by means of "contour lines." These are fine lines traced through the points of equal level over the surface surveyed, and denote that the level of the ground throughout the whole of their course is identical; that is to say, that every part of the ground over which the line passes is at a certain height above a known fixed point termed the datum, this height being indicated by the figures written against the line.
The intervals between the lines vertically are equal and may be 1, 3, 5, 10 or more feet apart; where the surface is very steep they lie close together. These lines by their greater or less distance apart have the effect of shading, and make apparent to the eye, the undulations and irregularities in the surface of the country.
Fig. 4 shows an imaginary tract of country, the physical features of which are shown by contour lines.
Map. The map should show the lengths and direction of the different por tions of the line, the topography, rivers, water courses, roads, railroads, and other matters of interest, such as town and county lines, dividing lines between property, timbered and cultivated lands, etc.
Any convenient scale may be adopted; 400 feet to an inch will be found the most useful.
Memoir. The descriptive memoir should give with minuteness all information, such as the nature of the soil, character of the several excavations whether earth or rock, and such particular features as can ' not be clearly shown upon the map or profile.
Special information should be given re garding the rivers crossed, as to their width, depth at highest known flood, velocity of current, character of banks and bottom, and the angle of skew which the course makes with the line of the road.
Levels. Levels should be taken along the course of each line, usually at every 100 feet, or at closer intervals, depending upon the nature of the country.
In taking the levels, the heights of all existing roads, railroads, rivers, or canals should be noted. "Bench marks" should be established at least every half mile, that is, marks made on any fixed object, such as a gate post, side of a house, or, in the absence of these, a cut made on a large tree. The height and exact description of each bench mark should be recorded in the level book.