This procedure is used for either differential or trigonometric leveling when along sight across a wide river, ravine, or similar obstacle must be made. This long sight will be affected by curvature and refraction and by any small error in aligning the line of sight with the bubble axis. The alignment error can be minimized by balancing the long sight and computing the curvature. The atmospheric conditions will vary so much over an open expanse that the refraction correction will be quite erratic. Reciprocal leveling is desired to minimize the effect of the atmosphere as well as the line of sight and curvature corrections. To do this, take the following actions:

1. In reciprocal leveling, balance the BSs and FSs as carefully as possible before you reach the obstacle. In figure 14-15, a TP, N, is selected close to the edge of the obstruction so that it is visible from a proposed instrument location, B, on the other side. A second rod is held on the other side of the obstruction at F. Point F should be selected so that the equivalent distances, AN and FB, and AF and NB, are almost equal. The instrument is setup at point A and leveled carefully. A BS reading is taken on the N rod and an FS on the F rod. These readings are repeated several times. The instrument is moved to point B, set up, and carefully leveled. The rods remain at their stations. Again, a BS is taken on the N rod and an FS on the F rod, and repeated several times. Since instrument leveling is especially critical on reciprocal leveling, you need to check the bubble before each reading and center it carefully. If it is off-center a slight amount, the procedure must be repeated. The difference in elevation between N and F is computed from the readings at A setup and from the readings at B setup separately. Because of the errors in the long sight, the two results will have slightly different values. Note, however, that the long sight is an FS from A and a BS from B. The true difference in elevation is the average of both values, since the errors have opposite signs and will cancel each other.

2. For more accuracy, make several long sight readings for each short sight and average them. You should use a target on the rod and reset it for each reading. Average each series of long sights and combine this average with corresponding short sights for the computations.

3. Changes in atmospheric density and temperature affect the refraction of a line of sight. The longer the time interval is between reciprocal long sights, the greater the chance of an atmospheric change and a variation in the refraction value. For this reason, you should keep the time lapse between the long sights as short as possible.

4. An excellent method of avoiding the time lapse problem is simultaneous-reciprocal observation. The object is to read both long sight values at the same time. This requires two instruments and two observers and two rods and two rodmen. Some method of communication or sequence of operations must be agreed upon.

5. The note keeping for reciprocal leveling is identical to differential leveling. Take a series of either BS or FS readings on the far rod from one setup and take only one sighting on the rear rod. Average the series of readings, and use a single value to make the elevation computations.