Levelling

Aim: to learn the basic leveling principles, theory and applications and to be able to book and reduce leveling data. Leveling refers to height measurements for representing the relative difference in height (altitude) between various points on the earth’s surface. Basic equipment (a) A device which gives a truly horizontal level (the Level). (b) A suitably graduated staff for reading vertical heights (the Leveling Staff. Types of levels (a) Tilting – adjustment of level bubble needed before every measurement. (b) Dumpy – adjustment of level bubble needed only once after level set up. ) Automatic levels – self leveled instruments. Bench Mark and Reference Datum In order to calculate the heights of points a datum is required, I. E. A reference level. This is usually the mean sea level. For this purpose, the use of Bench Marks is necessary, and these are classified as follows: Bench Mark (MM) – a point with known height above mean sea level (or other reference datum). These are permanent points (e. G. Unchanged by weather conditions) and are provided by the Department of Lands and Surveys. Temporary Bench Mark (TAB) – a point of known height above a pre-defined level.

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This level is not absolute and is defined locally by the surveyor for the purpose of the survey. Based on the TAB the survey may then later be reduced to absolute levels if the level of TAB is known. The height of any target point is referred to as Reduced Level (RL), because it is reduced to a known datum. Fig. L Leveling line. For large areas a correction is required for curvature of the earth. Common sources of errors in leveling 2. 3. 4. 5. Instrument not correctly leveled. Telescope not correctly focused. The wrong cross-hair reading recorded (e. G. Top instead of middle).

Staff incorrectly read or not held vertical. Staff incorrectly booked. All the above are mistakes (blunders) and cannot be corrected unless the work is discussed later in detail. Leveling operations Level readings can be taken either from one location (one set-up leveling) or from various stations, if some points of interest are not visible. A level survey can be used for setting-out purposes, the presentation of soil profiles etc. , as will be discussed later in the subject. Basic definitions Backstitch (BBS): first staff reading taken immediately after setting up the instrument.

Foresight (FSP): last staff reading taken before moving the instrument to another location. Intermediate sight (IS): all readings taken between a BBS and a FSP. So, if the instrument is set up at one location only, there will be Just one BBS (first reading to a TAB), one FSP (last reading) and several IS. Fig. 2 Leveling set-up and height calculation RI-A HOP RL datum Now consider Figure 2 above. The level is set up as shown, and using the staff at points A and B, height readings are recorded. This is Just the height read through the telescope horizontal line of sight (known as line of collimation).

If no reduced level is known only the difference in height can be found between A and B, not their absolute levels. Staff Reading at A is 1. Mum Staff Reading at B is 1. Mum difference in height is = 1. 875 – 1 . 135 = 0. Mum If we know that REAL = +120. Mum (above datum), then RL = 120. 00 – 0. 740 = +119. Mum I. E. A fall from A. If RL was known we would calculate a rise in level. Hence, the following can be defined: Rise – staff reading is less than previous reading. Fall – staff reading is greater than previous reading. The above definitions are used in the Rise & Fall method a level booking and deduction.

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