When a level survey system covers a large area, you, in turn, adjust the interconnecting network in the whole system. Adjustment of an interconnecting network of level circuits consists of adjusting, in turn, each separate figure in the net, with the adjusted values for each circuit used in the adjustment of adjacent circuits. This process is repeated for as many cycles as necessary to balance the values for the whole net. Within each circuit the error of closure is normally distributed to the various sides in proportion to their lengths. Figure 7-5 represents a level net made up of circuits BCDEB, AEDA, and EABE. Along each side of the circuit is shown the length of the side in miles and the observed difference in elevation in feet between terminal BMs. The difference in elevation (plus or minus) is in the direction indicated by the arrows. Within each circuit is shown its total length (L) and the error of closure (Ec) that is determined by summing up the differences in elevation in a clockwise direction. Figure 7-6 shows the computations required to balance the net. The circuits, sides, distances (expressed in miles and in percentages of the total), and differences in elevation (DE) are listed.

For circuit BCDEB, the error of closure is 0.40 ft. This is distributed among the lines in proportion to their lengths. Thus, for the line BC, the correction is 

(Notice that the sign is opposite to that of the error of closure.) The correction of +0.07 ft is entered on the first line of the column headed CORR and is added to the difference in elevation (10.94 + 0.07 = +11.01). That sum is entered on the first line under the heading CORR DE (corrected difference in elevation). The same procedure is followed for the remaining lines CD, DE, and EB of circuit BCDEB.

The sum of the corrections must have the opposite sign and be equal to the error of closure. The algebraic sum of the corrected differences in elevation must equal zero. The lines in circuit AEDA are corrected in the same manner as BCDEB, except that the corrected value of ED (+27.08 instead of +27.15) is used. The lines of EABE are corrected using the corrected value of EA (+17.97 instead of +17.91) and BE (+5.13 instead of +5.23). In the column Cycle II, the procedure of Cycle I is repeated. You should always list the latest corrected value from previously adjusted circuits before computing the new error of closure. The cycles are continued until the corrections become zero. The sequence in which the circuits are taken is immaterial as long as they are repeated in the same order for each cycle. Computations may be based on corrections rather than differences in elevation.