Leveling, like any other surveying operation, is carried out by following a prescribed ORDER OF PRECISIONmeaning that the instruments you use and the methods you follow have to be those that can give you the specified standard of accuracy.

FIRST-ORDER leveling is used to establish the main level network for an area and to provide basic vertical control for the extension of level networks of the same, or lower, accuracy in support of mapping projects, cadastral (recording property boundaries, subdivision lines, buildings, etc.), and local surveys. Level lines must start and end on proven, existing BMs of the same order. New levels must be run between the starting BM being used and at least one other existing BM and must show there is no change in their relative elevations.

SECOND-ORDER leveling is used to sub-divide nets of first-order leveling and to provide basic control for the extension of levels of the same, or lower, accuracy in support of mapping projects and local surveys. Second-order levels are divided into two classes: Class I and Class II. CLASS I is used in remote areas where the line must be longer than 25 mi because routes are unavailable for the development of additional or higher order networks and for spur lines. CLASS II levels are used for the development of nets in the more accessible areas. In Class I leveling, it is required that all lines start and close on previously established BMs of first or second order. New levels have to be run between the existing BM being used and at least one other existing BM to prove that they have not changed their relative elevations. The criteria for Class II are the same as for Class 1, except that Class II lines are run in one direction only.

THIRD-ORDER leveling is used to subdivide an area surrounded by first- and second-order leveling and to provide elevations for the immediate control of cadastral, topographic, and construction surveys for permanent structures. The following criteria should be observed in third-order leveling:

1. All lines have to start from, and close on, two previously established BMs of third, or higher, order of accuracy if the new leveling indicates they have not changed in their relative elevations.

2. In the United States, third-order lines should not be extended more than 30 mi from BMs of first or second order. In foreign or remote areas, the distance may be extended according to the evaluation of the existing control and the situation. They may be single-run (one direction) lines but should always be loops or circuits that close upon BMs of an equal or a higher order.

3. When a line from previously established third-order marks is extended, the maximum length of the new line is greatly reduced. The distance and allowable error are to be carried back through the existing line to the nearest tie BM of the second or higher order.

4. Balanced sights should not be greater than 300 ft. BS and FS distances maybe measured by pacing and approximately balanced between BMs, Rod readings are read to thousandths and the rod waved for extended rod readings. The bubble is checked to make sure it is exactly centered before each sighting and reading. Turning point pins or plates or well-defined points on solid objects are used for TPs.

FOURTH-ORDER leveling is used to sub-divide an area within a third-order network. This is the method of leveling used in connection "with the location and construction of highways, railroads, and most other engineering works that concern the SEABEEs in advanced base projects.

But, in practice, trying to shoot for a higher degree of accuracy is advantageous if it does not affect the proper progress of the work. The following criteria should be observed in fourth-order leveling:

1. All lines are to start from, and close on, previously established BMs of the third or fourth order of accuracy.

2. Maximum sight distance is about 500 ft. Rod readings are read to hundredths of a foot. BS and FS distances are roughly balanced only when lines of great lengths are run, either uphill or downhill. TPs are taken on solid or any well-defined, firm objects.

The instrument commonly used in third- and fourth-order leveling is the engineers level and the Philadelphia rod. Always check the proper adjustments of the instrument before using it.