### 4. Error Sources and Behavior

Errors affecting differential leveling also affect three-wire leveling. Additional errors by sources include:

#### a. Instrumental

##### (1) Uneven stadia wire spacing
###### (a) Principle and Behavior

While sometimes an error on older instruments, this is generally not an issue on modern ones. If one sub-interval is consistently larger or smaller than the other, then there may be an instrumental error. Sub-intervals are a function of distance, making the error systematic.

###### (b) Compensation

Balancing BS and FS distances differences allow wire spacing errors to compensate.

###### (a) Principle and Behavior

The multiplier is based on the stadia wire spacing with most modern instruments having a multiplier of 100. An incorrect multiplier introduces a systematic error.

###### (b) Compensation

If stadia is used to balance BS and FS distances, then the systematic error is irrelevant.

What if the distances are needed for a proportional adjustment of a level circuit? Does the error in them affect the adjustment? No, because each distance is increased or decreased by the same amount, as is their total.

##### (3) Rod bubble
###### (a) Principle and Behavior

In three-wire leveling, a rod bubble must used to hold the rod vertical. Waving the rod, as for differential leveling, does not work for three-wire leveling. The lines of sight for the top and bottom wires are not horizontal so their correct readings do not occur at their minimum values, Figure G-7.

 Figure G-7Rod Waving Error
###### (b) Compensation

Use an adjusted rod bubble to ensure the rod is held vertically.

#### b. Natural

##### (1) Obstructed wire
###### (a) Principle and Behavior

Because the stadia wire spread increases with distance, the bottom wire may "ground out", Figure G-8(a), or top wire pass over the rod, G-8(b).

 (a) Bottom Wire (b) Top Wire Figure G-8Wire Missing Rod

The behavior is systematic because it is based on distance.

###### (b) Compensation

Sight distances should be kept short to ensure all three wires can be read on the rod. Distances may need to be further shortened on rolling terrain, Figure G-9.

 Figure G-9Rolling Terrain

The surveyor could use the middle wire reading instead of the average of all three and double the single sub-interval to determine the distance. This shouldn't be done because:

• there are no reading checks
• the center reading isn't as accurate as an average is
• doubling the sub-interval magnifies the reading errors

#### c. Personal

Because there are more readings and running computations, there is greater opportunity for reading, recording, and calculation mistakes. Mistake reduction comes from experience and diligence.