Measuring vertical distances

Most surveyors now use levels or transits. Since these instruments are costly and rarely available in rural communities of developing countries, other alternatives may be useful.

It is not given that near surveyed site any benchmark point is to be found. In that case network of benchmarks will be established by surveyor. Because BP can suffer from damage, at least 3 BPs have to be established. In case of damage, based on two others, the damaged point can be re-established. Benchmarks should be established before leveling is required. Almost any fixed or permanent object can serve as BP. Sometimes nail in tree, wooden stake or clearly identified position on the pavement.

The figure above describes the idea of working with equipment called level. General idea should be obvious from the picture. Level instrument is set to horizontal position and has telescopic sight to make a reading of both rods.

Instrument is leveled into a horizontal position by means of spirit bubble. Spirit bubble is in a transparent container filled with a low viscosity liquid, e.g. alcohol or ether. The precision of bubble depends on the radius of curvature.

In general larger radius will enable better precision. If the precision is to high, bubble becomes too sensitive to any move, and it can be very time consuming to establish horizontal position and high precision might be sometimes unpractical.

For self-leveling and digital instruments, circular bubble is built-in. Circular container is of lower precision than tubular vial, but lower precision is compensated by other internal optical components of the instrument.

When moving over long distances, we need to move instrument and rods continuously according to points measured. We talk about turning points (TP) then. We turn the instrument over TP to the next setup.

Let's have an example: we need to take level of station B and we know coordinates of benchmark point BM A. The characteristic of terrain doesn't allow us to make a mere one setup to read BS to BM A and FS to B to find out the level of B. We have to use path around, through Setup 1 + Setup 2.

BM A is given. Station B is given and its altitude is to be found. We need a temporary station at TP: either a new station can be established at TP or it can be just a temporary position without any station.

It was said that setting the instrument should be done nearby center of points being measured. In that case, errors from BS and FS almost cancels each other.

Then check of measurement can be made. Otherwise, without closing the level circuit, there is no way to detect or prevent errors and blunders. It is much less expensive to find and correct blunder in the field by closing the loop.

Procedure called three-wire leveling has long been applied for precise work. The improvement stands in crosshair, which is built into telescopic sight. Instead of the cross, there are 2 additional stadia hairs. Surveying engineer reads central value together with lower and upper. The average computed provides more accurate reading. It provides great protection against blunders as well.

The more up-to-date method is level instrument equipped with optical micrometer, which is able to move its vertical sight. The crosshair can be accurately aligned to mark on the rod then and correction from micrometer is added to the reading.

Ordinary level rods are not used for precision leveling. Instead, a precise level rod is used. It is constructed to enforce holding in a vertical position.