### 2. Natural

#### a. Climate

##### (1) Description

Weather can have various detrimental effects on leveling accuracy:

- Heat waves are atmospheric anomalies that can randomly bend the LoS or make the rod difficult to read.
- Wind gusts will cause the compensator to bounce as well as make it a challenge to hold an extended level rod vertical.
- A large temperature difference between equipment storage and use requires acclimation time or tripod leg locks may loosen and slip.
- Cold uncomfortable temperatures can result in haphazard work.

##### (2) Behavior

Cumulative effect is **random**.

##### (3) Compensation

In as much as possible select times and locations minimizing climate impacts. Allow equipment temperature acclimation. Dress for conditions.

#### b. Curvature

##### (1) Description

Recall that a level line is curved and the LoS is horizontal. Both coincide at the instrument but separate as the distance from the instrument increases.

##### (2) Behavior

Figure D-11 shows that curvature causes the rod reading to be too high.

Figure D-11 Curvature Error |

The effect of curvature is **systematic**; it is a function of distance, Equation D-1.

c = -0.667M^{2 }= -0.0239F^{2} |
Equation D-1 | |

c: reading correction, ft M: distance to rod, miles F: distance to rod, 1000s of ft |

Note: F in 1000s of feet means, for example, that at 100 ft, F = 100/1000 = 0.1

##### (3) Compensation

(a) Mathematical - Curvature can be accounted for by computing and applying the correction, Equation D-1, to the rod reading.

(b) Procedural - Because curvature is a function of distance, balancing BS and FS distances, Figure D-12, allow it to cancel: C_{BS} (added) is the same as C_{FS} (subtracted).

Figure D-12 Curvature Compensation |

#### c. Refraction

##### (1) Description

Even if there are no atmospheric anomalies, the fact that the LoS has to pass through atmosphere causes it to bend, introducing a reading error.

##### (2) Behavior

Refraction causes the LoS to be bent downward, Figure D-13, resulting in a rod reading that is too low.

Figure D-13 Refraction Error |

The effect of refraction is **systematic** and is a function of distance, Equation D-2.

R = +0.093M^{2 }= +0.0033F^{2} |
Equation D-2 |

R: reading correction, ft M: distance to rod, miles F: distance to rod, 1000s of ft |

##### (3) Compensation

(a) Mathematical - Refraction can be accounted for by computing and applying the correction, Equation D-2, to the rod reading.

(b) Procedural - Being a function of distance, balancing BS and FS distances allows refraction to cancel, Figure D-14: R_{BS} (added) is the same as R_{FS} (subtracted).

Figure D-14 Refraction Compensation |