**✍**

**Direct leveling**

**₪**

**1. General**

**₪**

**1.1 Principle**

Direct

**leveling**consists of determining the height difference**ΔHAB**between two points**A**and**B**using a**device :**the level and a vertical scale called a**staff**The

**telescopen level**consists of a sighting optic rotating around a vertical axis : it therefore defines a**horizontal**sighting**plane.**The target is placed successively on the two point**A**and**B**. The**operator**reads the valus**LA**on the staff placed at**A**and the value**LB on the**staff placed at**B.**This drio is an algebraic value whose sign indicates wherher

**B**is higher or lower than**A**(if**ΔHAB**is negativee then**B**is lower than**A**).**The altitude**of a point

**A**noted

**Alta**is the

**distance**counted along the vertical which separates it from the geoind (leveel

**0**surface. If the altitude of point

**A**id knoxn,, we can deduce that of

**point B.**

**₪**

**1.2 Rod Readings**

The staff is linear scale which must bee held vertically (it includes a spherical level) on the point involved in the height difference to be measured. The precision of its graduation and its maintenance in a vertical position strongly influence the precision of the height difference measured.

A level's reticule is usually made up of four wires:

**₪**The upper stadia wire

**₪**The lower stadia line

**₪**The leveling wire

**₪**The vertical thread

**₪**

**1.3 Interpretation of staff readngs**

The reading on each wire is estimated visually to the nearest millimeter, compared to the example on the previous page:

**₪**Upper stadia wire (sup wire) =

**₪**Lower stadia wire (lower wire) =

**₪**Wire leveler wire level =

The stadia wires make it possible to obtain an approwimate value of the range (horizontal distance between the

**device**and**the staff**) using the relatioship:**₪**

**2. The journey**

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**2.1 Simple pathways**

When points

**A**and**B**are located so that a singht (distance, mask,toogreat a difference in height, etx...) the total difference in height must be broken down into elementary differences in level using intermediate points. The set of these decompsitions is called**leveling by progression.****A framed path**starts from an "

**origin point'**known in

**altitude,**passes through a certain number of inteermediate points and closes on an "

**end point**" different from the point of origin and also

**known in altitude.**

When one seeks to determine the altitude of an end point

**B**from that known of a benchmark**A.**when one seeks to determine the altitude of an end point

**B**from that known of a ebnchmark. one generally performs a**rounde**of**B**and to check the validity of the measurementsby finding the**altitude of A.**When a

**traverse**constitutes a loop returning to its starting point**A,**it is called a**closed traverse.**In all cases, thee

**calculation**principale is as flows:**₪**The staff being on the origin pont '

**A',**the opertor parks the leveel in '

**S1'**the distance of which he determines by counting the numbner of steps separating

**A**from

**S1,**so as not to exceed to maximum range of

**60m.**The operator

**reads backward,**that is to say in the chosen direction of travel, on point '

**A**', denoted

**Lar**(

**A**)

**₪**The

**staff**moves to come to the first intermediate point '

**I1'**as stable as possible (stone, metal base called '

**toad',**stake, etc...) and whose distance it determines by counting the number of step separating '

**«**

**S1**» to «

**I1**»

**₪**Still stationed in '

**S1',**the operator reads on the staff the

**front reading**on

**'I1'**denoted

**Lav**

**(I1);**it is then possible to

**calculate**the height difference from

**'A'**to

**'I1'**as flows :

**:**

**₪**The operator moves to choose a station '

**S2'**and so on..

**₪**

**2.2 Applications on simple paths**

Ajouter une légende |

₪ Depuis la station n°1 on fait les lectures suivantes : | ₪ LAR sur R1 = 1,208m LAV sur A = 1,312m |

₪ Depuis la station n°2 on fait les lectures suivantes : | ₪ LAR sur A = 1,735m LAV sur B = 1,643m |

₪ Depuis la station n°3on fait les lectures suivantes : | ₪ LAR sur B = 1,810m LAV sur C = 0,763m |

₪ Depuis la station n°4on fait les lectures suivantes : | ₪ LAR sur C = 1,739m LAV sur R2 = 1,934m |

Point

**R**1 of known altitude 35,000**NGF**Point

**R**2 of known altitude 35,840**NGF****Determine the altitude of points A, B et C ?**

**₪**

**2.3 Mixed pathways**

From any station of the level in a

**traverse,**and after having recorded the**backward reading**on the previous traverse point, the operator aims at several detail points and performs on each of them a single reading which is therefore a**forward reading.**Then it ends the station by

**playing forward**to the next waypoint. For example, in the figure below, the points**1 , 2**and**3**are radiated from the station**S1**whose rear point is the reference (**R**) and the front point**A.**The operation in

**S1 is called radiation.**When a

**traverse**includes radiated points and traverse points, it is said to be a**mixed traverse****The progression**of the figure opposite passes through the points

**R**,

**A**,

**B**,

**C**,

**D**,

**E**and

**R’**. Points

**1**,

**2**,

**3**,

**4**,

**5**,

**6**and

**7**are radiated.

The whole is a

**cheminement mixte encadré**between**R**and**R´.**on the**leveling**book, a radiated point is directly identifiable by the fact thatr it does not include a**back reading**When the measurement is finished,

**the path**is first calculated without taking into account the**radiated**detail points. Then the radiated points are**calculated**and noted, for exeample; in another color.Their calculation is fifferent from that of

**chimney points**:**₪**All points radiated from the same station are calculated from the altitude of the back point of the station. This difference in calculation : first the path alone, then the

**rays**and by using different colors.

**₪**There is no compensation on the height difference of a radiated

**topographic point**since there is no possible control of its value.

**₪**

**2.4 Application on radiation**

**Calculate the altitude**of points

**A**,

**B**,

**C**and

**D**knowing that the leveling mark

**R**is located at a

**glasses altitude**of 38,775

**m**(

**NGF.**) What is the altitude of the sighting plane ?

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**2.5 Rules on compensation**

As a general rule on a

**closed**and**framed**path, the altitude of the true reference point is different from**the altitude of the calculated point,**there is a closing difference that must be compensated for.This spread is calculated by

**summing the back readings-the sum of the forward readings.**The closing gap can come from :

**₪**One more false readings

**₪**Poor horizontality of the device

**₪**A malfunction of the device.

Not knowing the true origin of the closed gap, we established the following rule :

**1.**The closing deviation is small, i.e. thee deviation is less than the standard deviation, in this case the compensation is proportional to the number of elevation changes

**₪**

**2.6 Applications on compensations**

**Exercise n°1 :**We give the diagram of thee following

**framed progression**complete the

**land leveling**book knowing that the altitude of point

**R**1 and

**AltR**1 = 10,354

**m**anand

**Alt**12 = 10,390

**m**.

**Exercise n°2 :**We give the diagram of the following mixed path. Complete the

**Topography leveling**book knowing that the altitude from point

**R**is

**AltR**= 54,870

**m**.

## 1 Comments

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ReplyDeleteSvp le lien