LESSON NOTE ON DEFINITIONS OF TERMS IN SURVEY

DEFINITIONS OF TERMS

 

levelling is the term applied to any method of measuring directly with a graduated staff the difference in elevation between two or more points.

Precise levelling is a particularly accurate method of differential levelling which uses highly accurate levels and with a more rigorous observing procedure than general engineering levelling. It aims to achieve high orders of accuracy such as 1 mm per 1 km traverse.

A level surface is a surface which is everywhere perpendicular to the direction of the force of gravity. An example is the surface of a completely still lake. For ordinary levelling, level surfaces at different elevations can be considered to be parallel.

A level datum is an arbitrary level surface to which elevations are referred. The most common surveying datum is mean sea-level (MSL), but as hydrological work is usually just concerned with levels in a local area, we often use:

An assumed datum, which is established by giving a benchmark an assumed value (e.g. 100.000 m) to which all levels in the local area will be reduced. It is not good practice to assume a level which is close to the actual MSL value, as it creates potential for confusion.

A reduced level is the vertical distance between a survey point and the adopted level datum.

A bench mark (BM) is the term given to a definite, permanent accessible point of known height above a datum to which the height of other points can be referred.

It is usually a stainless steel pin embedded in a substantial concrete block cast into the ground. At hydrological stations rock bolts driven into bedrock or concrete structures can be used, but structures should be used warily as they themselves are subject to settlement. The locations of benchmarks shall be marked with BM marker posts and/or paint, and recorded on the Station History Form.

A set-up refers the position of a level or other instrument at the time in which a number of observations are made without mooring the instrument. The first observation is made to the known point and is termed a backsight; the last observation is to the final point or the next to be measured on the run, and all other points are intermediates.

A run is the levelling between two or more points measured in one direction only. The outward run is from known to unknown points and the return run is the check levelling in the opposite direction.

A close is the difference between the starting level of the initial point for the outward run and that determined at the end of the return run. If the levels have been reduced correctly this value should be the same as the difference between the sum of the rises and falls and also the difference between the sum of the backsights and foresights.

Height of Collimation is the elevation of the optical axis of the telescope at the time of the setup. The line of collimation is the imaginary line at the elevation.

Orders of levelling refer to the quality of the levelling, usually being defined by the expected maximum closing error. These are given in Table 1

 Order                   Purpose                                              Maximum close (m)

Precision order      Deformation surveys                        0.001 x km 

First order              Major levelling control                    0.003 x km

Second order          Minor levelling control                   0.007 x km

Third order            Levelling for construction                0.012 x km 

Table 1 Levelling 

The accuracy requirements for water-level stations relate to the standards; for further information refer to section 1.

Change points are points of measurement which are used to carry the measurements forward in a run. Each one will be read first as a foresight, the instrument position is changed, and then it will be read as a backsight. 

Lesson Note On Carrying out a level traverse

Carrying out a level traverse To determine the difference in level between points on the surface of the ground a 'series' of levels will need to be carried out; this is called a level traverse or level run. There are two method of levelling: Rise & Fall method and Height of collimation (height of instrument) methods Click the link to see the animated rise&fall methods then click next for the height of collimation method. Please note when the shifting of the staff or level can be done using the rise&fall method Leveling or Field Procedures The leveling or field procedure that should be followed is shown in Figure 1 below.. Figure 1 Procedure: Set up the leveling instrument at Level position 1. Hold the staff on the Datum (RL+50 m) and take a reading. This will be a backsight, because it is the first staff reading after the leveling instrument has been set up. Move the staff to A and take a reading. This will be an intermediate sight. Move the staff to B and take a reading. This also will be an intermediate sight. Move the staff to C and take a reading. This will be another intermediate sight. Move the staff to D and take a reading. This will be a foresight; because after this reading the level will be moved. (A changeplate should be placed on the ground to maintain the same level.) The distance between the stations should be measured and recorded in the field book (see Table 1) Set up the level at Level position 2 and leave the staff at D on the changeplate. Turn the staff so that it faces the level and take a reading. This will be a backsight. Move the staff to E and take a reading. This will be an intermediate sight. Move the staff to F and take a reading. This will be a foresight; because after taking this reading the level will be moved. Now move the level to Leveling position 3 and leave the staff at F on the changeplate. Now repeat the steps describe 8 to 10 until you finished at point J. Field procedures for leveling All staff readings should be recorded in the field book. To eliminate errors resulting from any line of sight (or collimation) backsights and foresights should be equal in distance. Length of sight should be kept less than 100 metres. Always commence and finish a level run on a known datum or benchmark and close the level traverse; this enables the level run to be checked. Booking levels There are two main methods of booking levels: rise and fall method height of collimation method Table 1   Rise & Fall Method Back- sight Inter- mediate Fore- sight Rise Fall Reduced level Distance Remarks 2.554 50.00 0 Datum RL+50 m 1.783 0.771 50.771 14.990 A 0.926 0.857 51.628 29.105 B 1.963 1.037 50591 48.490 C 1.305 3.587 1.624 48.967 63.540 D / change point 1 1.432 0.127 48.840 87.665 E 3.250 0.573 0.859 49.699 102.050 F / change point 2 1.925 1.325 51.024 113.285 G 3.015 0.496 1.429 52.453 128.345 H / change point 3 0.780 2.235 54.688 150.460 J 10.124 5.436 7.476 2.788 54.688 Sum of B-sight & F-sight, Sum of Rise & Fall -5.436 -2.788 -50.000 Take smaller from greater 4.688 4.688   4.688 Difference should be equal The millimeter reading may be taken by estimation to an accuracy of 0.005 metres or even less. Backsight, intermediate sight and forsight readings are entered in the appropriate columns on different lines. However, as shown in the table above backsights and foresights are place on the same line if you change the level instrument. The first reduced level is the height of the datum, benchmark or R.L. If an intermediate sight or foresight is smaller than the immediately preceding staff reading then the difference between the two readings is place in the rise column. If an intermediate sight or foresight is larger than the immediately preceding staff reading then the difference between the two readings is place in the fall column. A rise is added to the preceding reduced level (RL) and a fall is subtracted from the preceding RL Arithmetic checks While all arithmetic calculations can be checked there is no assurance that errors in the field procedure will be picked up. The arithmetic check proves only that the rise and fall is correctly recorded in the appropriate rise & fall columns. To check the field procedure for errors the level traverse must be closed. It is prudent to let another student check your reading to avoid a repetition of the level run. If the arithmetic calculation are correct, the the difference between the sum of the backsights and the sum of the foresights will equal: the difference between the sum of the rises and the sum of the falls, and the difference between the first and the final R.L. or vice versa. (there are no arithmetic checks made on the intermediate sight calculations. Make sure you read them carefully) Table 2  Height of collimation method (height of instrument)  Back- sight Inter- mediate Fore- sight Height of collimation Reduced level Distance Remarks 2.554 52.554 50.00 0 Datum RL+50 m 1.783 50.771 14.990 A 0.926 51.628 29.105 B 1.963 50591 48.490 C 1.305 3.587 50.272 48.967 63.540 D / change point 1 1.432 48.840 87.665 E 3.250 0.573 52.949 49.699 102.050 F / change point 2 1.925 51.024 113.285 G 3.015 0.496 55.468 52.453 128.345 H / change point 3 0.780 54.688 150.460 J 10.124 5.436 54.688 Sum of B-sight & F-sight, Difference between RL's -5.436 -50.000 Take smaller from greater 4.688   4.688 Difference should be equal Booking is the same as the rise and fall method for back-, intermediate- and foresights. There are no rise or fall columns, but instead a height of collimation column. The first backsight reading (staff on datum, benchmark or RL) is added to the first RL giving the height of collimation. The next staff reading is entered in the appropriate column but on a new line. The RL for the station is found by subtracting the staff reading from the height of collimation The height of collimation changes only when the level is moved to a new position. The new height of collimation is found by adding the backsight to the RL at the change point. Please note there is no check on the accuracy of intermediate RL's and errors could go undetected. The rise and fall method may take a bit longer to complete, but a check on entries in all columns is carried out. The RL's are easier to calculate with the height of collimation method, but errors of intermediate RL's can go undetected. For this reason students should use the rise and fall method for all leveling exercises. Closed and open traverse Always commence and finish a level run on a datum, benchmark or known RL. This is what is known as a closed level traverse, and will enable you to check the level run. Closed level traverse Series of level runs from a known Datum or RL to a known Datum or RL. Misclosure in millimeter   24 x √km Closed loop level traverse Series of level runs from a known Datum or RL back to the known Datum or RL. Misclosure in millimeter   24 x √km Open level traverse Series of level runs from a known Datum or RL. This must be avoided because there are no checks on misreading