chapitre 4 : limnimétrie · projet niger - hycos – formation en hydrométrie eléments de...

Projet Niger - HYCOS – Formation en hydrométrie

Eléments de Topographie – Nivellement , Niamey, Mai 2006

TOPOGRAPHIC SURVEY - LEVELINGJean – Pierre BRICQUET (IRD - US OBHI)

WMO / OMM

Volta-HYCOS Project

Presentation plan

Introduction

Leveling

Leveling techniques

Closing

Topographic profiles

Objectives of topographic survey

The main objectives of topographic survey are to allow for the establishment of maps and graphic plans on which all information in connection with land topography and its natural and artificial details are represented under symbolic form.

The cartographic data will for instance guide one in the field or allow to study a building project.

Cartography : it is the whole set of scientific, artistic and technical operations based on direct observations or on the use of a document in order to draw maps, plans and other means of expression.

Planimetry aims at representing horizontally the projection of all the points situated in the field. To that effect, distances between various points as well as the angles linking them should be measured.

Leveling or altimetry is the set of topographic operations allowing for the determination of the altitude of a reference point from a known altitude, after calculating the height between these two points.

Some definitions

The reference networks

• NGF, General levelling of France• IGN 69 (French National Geographic Institute)• Normal Null

The information sources

The best source at national level is the National Geographic Institute :

• Coordinates• Projection parameters• Map reference Numbers• Description of points…

The level

The parts of the level

1 fixing screw 2 spring plate 3 focussing knob 4 fixing plate 5 eyepiece of the telescope6 sighting tube 7 micrometer knob

Types of levels

The simple level

The automatic level

The digital level

The setting up

BubbleSpirit level

The setting up

The leveler

Spirit level on an ‘N2 WILD’ level

Direction of rotation of the screws

Reticles

Reading of the staffThe reticle of a level is generally made up of four threads:- The upper thread (s´), which gives the upper reading m1 on the staff; - the lower thread, which gives the reading lower m2 on the staff;- The leveler thread (n), which gives the middle reading m on the staff; - the vertical thread (v) allows for pointing to the staff.The reading on every thread is visually estimated to the nearest millimetre. The threads allow for obtaining an approximate value of the horizontal distance. For every reading, it is advisable to read the three horizontal threads so as to avoid reading mistakes: a checking is indeed carried out in the field that : m =( m1 +m2 )/2

Positioning of the staffThe staff is a linear rule that should be held vertically (it is fitted with a spherical bubble) and placed on the point whose elevation is to be measured. The precision of its graduation and the manner the staff is held in position strongly impacts on the accuracy of the measured height. The classical staff is generally graduated in centimeters. Numbering is often in decimeters.

Reading of the angles

In degrees In radians

In degrees

Checking of the level

Moving

The staff and the level should NEVER be moved simultaneously

The leveling principle

Spirit level

Horizontal line

Leveling technique

StaffHorizontal axisAltitude of Horizontal axis

Calculation of differences in level

∑ −+= )(21 AVARZZ RR

thobs HHf Δ−Δ=

Optical level

Climbing

Descending

Altitudes

• Extension leveling• Plane leveling• Section leveling• Surface leveling

Extension levelling

Reference point

Extension levelling

Plane levelling

Leveling pegs

Axis of staff

Altitude

Surface levelling

Measurement of the distance

• Historically, by the base amplification method defined by a two-meter invar staff

• Presently, by electronic telemetry or tachometry• Also with a level using stadimetric threads.

The scope is the distance from the level to the staff:

It varies following the equipment and the level of precision needed, and should be 60 m maximum in ordinary levelling and 35 m in precision levelling. To the extent possible, the operator should position the level at equal distance from A and B, so as to achieve the equality of the scopes.

Measuring of distance

Measuring uncertainties

All measurements are always affected by some error. These are:

• Systematic errors• Accidental errors• Mistakes

Error in the calibration of the staff;

Failure in the verticality of the staff: faulty bubble;

Tilting mistake of the optical axis: optical axis not at right angle with the main axis;

Defect in the operation of the compensator.

Measuring uncertaintiesSystematic errors

The parallax error which is a wrong focussing of the telescope;

a wrong fixing of the bubble;

reading mistake on the staff due to the estimation of the millimetre;

a wrong selection of intermediary point: unstable point;

the blazing of air: sighting below the staff nearer to the ground when the weather is hot should be avoided

Measuring uncertaintiesAccidental errors

Measuring uncertainties

The following mistakes are noted:

fixing: forgetting to fix the bubble, jammed compensator;

reading: confusing of leveller line with stadimetricline; confusing of graduation or of units;

transcription on the notebook: wrong transcription of the value read.

Mistakes

Leveling and closing

Starting point

Plane closing

−−=n

iiA Agonnf

1200).2(Angular closing :

cfX XXf −=Planimetric closing (as X) :

222ZYXL ffff ++=Linear closing :

Weighted compensation

−= n

Compensation by the least squares

−±=∑=1

).(VPVA t ..=

Longitudinal section

NoWrong

Axis of a longitudinal profile

Or follow flood marks line

Cross section

Real altitude

Benchmark

Altitude of flood (20.11.1927 : 144.06 m)

Left Bank Right Bank

Scales :

Cross section

Wrong drawing

Actual altitudeMain benchmark (from National Geographic Institute)

Secondary benchmark

Imaginary Altitude

Secondary benchmarkSecondary benchmark

Main benchmark

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