Singapore Polytechnic Surveying Levelling PDF

Official (Closed), Non-Sensitive SINGAPORE POLYTECHNIC SCHOOL OF ARCHITECTURE & THE BUILT ENVIRONMENT COURSE: DCE 1 FT MODULE CODE: BE8133 MODULE NAME: GEOMATICS TOPIC 2: LEVELLING LE...

Official (Closed), Non-Sensitive SINGAPORE POLYTECHNIC SCHOOL OF ARCHITECTURE & THE BUILT ENVIRONMENT COURSE: DCE 1 FT MODULE CODE: BE8133 MODULE NAME: GEOMATICS TOPIC 2: LEVELLING LEVELLING Levelling Terminology Process for : Level surface Find the height difference of points on the E.g. the surface of a pond or still lake where earth’s surface. all parts of the surface is perpendicular to Finding the reduced level of a ground point. the direction of gravity. In practice, it is Many construction processes where heights regarded to be a spherical surface with its and height differences are required e.g. centre at the centre of the earth. control of slopes, checking design levels of buildings/roads etc. Level line - is a line on a level surface and its height Ordinary Levelling above datum line is constant. Height differences can be determined by Horizontal line - levelling through the application of: is a straight line that is tangential to a level line. A device (level instrument) that gives a truly horizontal line of sight Datum line (surface) - is any lines (surface) to which elevations A graduated staff (levelling staff) for reading (heights) on the ground are referred. vertical heights (In precise leveling, while the principles are Mean Sea Level (MSL) - the same, special levels, staffs and procedures is the average level of the sea, which is are used to obtain highly accurate and refined obtained by measuring the mean heights of results.) the tide at hourly intervals over a period of time (e.g. 1 year, 19 years). In Singapore, Equipment MSL is the reference surface used for The level is an instrument that is designed to measuring heights or elevations. give a horizontal line of sight (line of collimation). Reduced level (RL) - is the height or elevation of a point above or 4 main types below a datum.  Dumpy level  Tilting level  Automatic level – most “popular”  Digital level BE8133 – Topic 2 Page 1 of 4  Official (Closed), Non-Sensitive Levelling Staff For short distances, the level line and horizontal line are assumed to coincide Backsight (BS) - is the 1st reading after the instrument has been For longer distances, the two lines diverge set up. and therefore there is a need to correct for curvature. Foresight (FS) - is a reading taken before the instrument is For distances of 100m the curvature moved, i.e. the last staff reading before correction is less than 0.001m. moving the instrument. Intermediate sight (IS) - is a reading taken after a BS and before a FS. Change point (CP) - is a point on which a staff is held and a FS and BS reading taken to it. Benchmark (BM) - is a point or mark of known height above datum, a point / mark of known reduced level. Temporary Benchmark (TBM) - same as BM, but likely to be of a temporary nature. Collimation Height - is the height of the line of sight of the level above datum – the RL of the line of sight, Further Notes A level line is a curve line which is located at a constant height above MSL A horizontal line is a straight line which is tangential to the level line at any point because it is perpendicular to the direction of gravity. The level is an instrument which is designed to give a horizontal line of sight (line of collimation) BE8133 – Topic 2 Page 2 of 4  Official (Closed), Non-Sensitive Levelling Procedures Which Method to Use? We use levelling to find out height If the levelling exercise has many CPs (e.g. differences between a series of points. If in a vertical control survey, where BMs are the RL of the starting point is known (BM), established) the Rise & Fall method is we apply the height differences to preferred. successive points to get the RLs of those points. If the levelling exercise has many IS, the Height of Collimation Method should be As we go from one point to another, there used. can be: Arithmetic Check Rise – the 1 point is lower than the st The “summation” check is used for checking 2nd point (diff in staff reading +ve) arithmetic errors (addition & subtraction) and cannot detect other errors (e.g. taking a Fall – the 1st point is higher than the wrong staff reading). A closed level circuit 2nd point (diff in staff reading –ve) is required for detecting mistakes. Readings observed in the field are recorded Inverted Staff Readings as BS, IS or FS. If you want to find the RL of a point above The other data, rise, fall, ht. of collimation the line of sight (e.g. on the ceiling or the and reduced levels are derived from BS, IS underside of a bridge), then an “inverted” and FS. staff must be used. We start levelling with a BS (staff held on The reading is recorded as (-ve) and if the a BM) and end levelling with a FS (usually normal rules of algebra are followed, also on a BM) addition and subtraction will give the correct results. Each row of the booking form represents a staff position. For a change point (CP), the BS and FS would be recorded along the same row. Reduction of Readings There are 2 methods of reducing the readings:  Rise & Fall Method  Height of Collimation Method BE8133 – Topic 2 Page 3 of 4  Official (Closed), Non-Sensitive Level Circuits (Closed / Open) Level Datum Verification Closed Before we adopt a BM for use as a datum,  Start from a BM and end levelling on we must make sure that it is reliable, i.e. it is the same BM i.e. return to start. still in its original position. Physical Self –closing level circuit. appearance can be deceiving and a BM may  Start from a BM and end levelling on look “good” but have actually “settled”. another BM.  We should always use closed circuits In Singapore, the practice is to use 3 existing for levelling as we can check for gross BMs, level between them and use the results “misclose” for verification. Open  Start from a BM and end levelling on a “new” mark. Should be avoided as there is no check on the work. Sources of Errors Causes Precautions Instrument errors  Line of sight not horizontal  Adjust level (i.e. conduct 2-Peg test).  Staff not properly extended  Maintain equal distance for BS/IS/FS.  Make sure that all sections of the staff are properly extended. Errors in handling equipment  Instrument not stable  Tighten the joints of tripod  Bubble not centered  Tilting level – ensure bubble is centered  Staff not vertical before and after taking the reading.  Use staff bubble or swing staff back and forth slowly and accept the lowest reading. Errors due to displacement of equipment  Settlement of level  Choose stable ground for instrument and  Unstable ground for change points press tripod feet firmly into the ground.  Choose stable CP, use a peg on soft ground Errors in reading and recording  Inverted staff image  Some older levels give inverted image –  Read & record wrongly be careful  Be more careful BE8133 – Topic 2 Page 4 of 4