Measurement of Vertical Distances and Leveling Methods

Instrumental Errors

These errors are attributed to imperfections in the instruments, faults in their manufacture, or improper adjustment.

Instrument Out of Adjustment

The most common instrumental error caused by the level being out of adjustment, particularly when the line of sight of the telescope is not parallel to the axis of the level vial.

Rod Not Standard Length

This is usually due to imperfections in manufacture, where inaccurate rod graduations cause errors in measured vertical distances.

Defective Tripod

The movement of the level due to settling of the tripod legs, often in soft ground or due to vibration from traffic, can cause errors.

Personal Errors

Although personal errors occur largely due to the limitations of the senses, the skills, training, and teamwork of the leveling party are major factors.

Bubble Not Centered

Rod readings will be in error when the bubble is not centered in the level vial.

Parallax

The effect of parallax is to cause relative displacement between the image of the cross hairs and the image formed by the focusing lens.

Faulty Rod Readings

An incorrect rod reading is usually the result of the length of sight, poor weather conditions, and the skill of the instrument man and the rodman.

Rod Not Held Plumb

If the rod is held off the vertical, the line of sight will intersect it farther from the base, resulting in a reading greater than it should be. The reading is lowest when the rod is held plumb.

Incorrect Setting of Target

The rodman at times fails to properly set the target, or the target may slip downward because it is not clamped firmly.

Unequal Backsight and Foresight Distances

It is good practice to make backsight and foresight distances nearly equal to reduce or eliminate errors due to imperfect instrument adjustment and those due to curvature and refraction.

Natural Errors

These errors are due to natural sources and cannot be totally removed, but their effects can be reduced by applying corrections and using good judgment.

Curvature of the Earth

The effect of curvature of the earth is to increase the rod reading, an error introduced even if the instrument is in perfect adjustment.

Atmospheric Refraction

The bending of the line of sight (refraction) caused by atmospheric conditions, such as heat waves, makes the rod appear unsteady, leading to reading errors.

Temperature Variations

Changes in temperature cause leveling rods to either expand or contract, which can introduce errors when taking rod readings.

Wind

A strong wind can shake a leveling instrument, making it difficult to center the bubble, or cause an extended rod to vibrate, making it hard to read or plumb.

Settlement of the Instrument

In soft or thawing ground, the instrument may settle between rod readings. This is a cumulative error, increasing the computed elevations of all other observed points.

Faulty Turning Points

A poorly chosen turning point in differential leveling can be a source of error, similar to the settlement of the instrument.

Misreading the Rod

The instrument man may occasionally read the rod incorrectly, most frequently when the line of sight is partially obstructed.

Incorrect Recording

The recorder should always call out the reading as he records them to prevent the recording of incorrect values.

Erroneous Computations

Mistakes in simple additions and subtractions are still committed in level notes.

Rod Not Fully Extended

When using a Philadelphia rod, it is important that it is fully extended and the two sliding sections lock properly when reading the high (or long) rod.

Moving Turning Points

A turning point carelessly or accidentally moved out of its position by a rodman will cause a serious mistake in leveling work.

Adjustment of the Cross Hairs

The reticle is adjusted to ensure the horizontal cross hair lines in a plane perpendicular to the vertical axis of the instrument.

Adjustment of the Level Vial

The purpose of this adjustment is to make the axis of the level vial perpendicular to the vertical axis of the instrument.

Adjustment of the Line of Sight

The line of sight is adjusted to make it parallel to the axis of the level vial. When this is done, the line of sight is truly horizontal when the bubble is centered.

Curvature and Refraction

The effects of earth curvature and atmospheric refraction are taken into account since the measurements are made in vertical planes and these effects all occur in the same plane.