| Lettering of the Stake. | Depth from Datum Line. |
| Silt Basin | 18.20 |
| C 1 | 15.44 |
| C 2 | 14.36 |
| C 3 | 12.85 |
| C 4 | 12.18 |
| C 5 | 11.79 |
| C 6 | 11.69 |
| C 7 | 11.55 |
| C 8 | 11.37 |
| C 9 | 11.06 |
| C 10 | 8.94 |
| C 11 | 8.52 |
| C 12 | 7.86 |
| C 13 | 7.70 |
| C 14 | 7.39 |
| C 15 | 7.06 |
| C 16 | 6.73 |
The levelling should be continued in this manner, until the grades of all the points are recorded in the field book.
Fig. 21 - PROFILE OF DRAIN C.
Horizontal Scale, 66 ft. to the inch.
Vertical Scale, 15 ft. to the inch.
1 to 17. Numbers of Stakes.
(82) etc. Distances between Stakes.
18.20 etc. Depths from datum-line to surface.
2.50 etc. Depths of ditch.
20.70 etc. Depths from datum-line to drain.
If, from too great depression of the lower parts of the field, or too great distances for observation, it becomes necessary to take up a new position with the instrument, the new level should be connected, by measurement, with[pg 107] the old one, and the new observations should be computed to the original plane.
It is not necessary that these levels should be noted on the map,—they are needed only for computing the depth of cutting, and if entered on the map, might be mistaken for the figures indicating the depth, which it is more important to have recorded in their proper positions, for convenience of reference during the work.
The Depth and Grade of the Drains.—Having now staked out the lines upon the land, and ascertained and recorded the elevations at the different stakes, it becomes necessary to determine at what depth the tile shall be placed at each point, so as to give the proper fall to each line, and to bring all of the lines of the system into accord. As the simplest means of illustrating the principle on which this work should be done, it will be convenient to go through with the process with reference to the main drain C, of the plan under consideration. A profile of this line is shown in Fig. 21, where the line is broken at stake No. 7, and continued in the lower section of the diagram. The topmost line, from "Silt Basin" to "17," is the horizontal datum-line. The numbers above the vertical lines indicate the stakes; the figures in brackets between these, the number of feet between the stakes; and the heavy figures at the left of the vertical lines, the recorded measurements of depth from the datum-line to the surface of the ground, which is indicated by the irregular line next below the datum-line. The vertical measurements are, of course, very much exaggerated, to make the profile more marked, but they are in the proper relation to each other.
The depth at the silt-basin is fixed at 2-1/2 feet (2.50.) The rise is rapid to stake 3, very slight from there to stake 7, very rapid from there to stake 10, a little less rapid from there to stake 11, and still less rapid from there to stake 17.
To establish the grade by the profile alone, the proper[pg 108] course would be to fix the depth at the stakes at which the inclination is to be changed, to draw straight lines between the points thus found, and then to measure the vertical distance from these lines to the line indicating the surface of the ground at the different stakes; thus, fixing the depth at stake 3, at 4 feet and 13 hundredths,[15] the line drawn from that point to the depth of 2.50, at the silt-basin, will be 3 feet and 62 hundredths (3.62) below stake 1, and 3 feet and 92 hundredths (3.92) below stake 2. At stake 7 it is necessary to go sufficiently deep to pass from 7 to 10, without coming too near the surface at 9, which is at the foot of a steep ascent. A line drawn straight from 4.59 feet below stake 10 to 4.17 feet at stake 17, would be unnecessarily deep at 11, 12, 13, and 14; and, consequently it is better to rise to 4.19 feet at 11. So far as this part of the drain is concerned, it would be well to continue the same rise to 12, but, in doing so, we would come too near the surface at 13, 14, and 15; or must considerably depress the line at 16, which would either make a bad break in the fall at that point, or carry the drain too deep at 17.