Introduction to Surveying Methods

1. 1. Introduction • Surveying Method chosen depends on: • by the purpose of the survey e.g. map making, location of specific points, definition of land ownership etc., • by the nature of the survey itself e.g. hydrographic, terrestrial, astronomic, • according to the scale or accuracy of the survey, • the type of instrument or instruments used • e.g. • prismatic compass, • level • theodolite, • photograph (terrestrial or aerial).

2. 1. Introduction • Other factors: • Curvature of earth over 5 km, vertical angle difference will approach 2.5 minutes which can readily be detected even with most basic theodolite. • Errors collimation errors in instruments need to be calibrated otherwise “rivers could flow uphill” • Surveying involves • transfer of levels between two points • measurement of angles and lengths. • requires solution of triangular shapes using basic trigonometry • (or by graphical means). • If distances are large: planar geometry no longer applies.

3. 1. Introduction • Geodetic surveys allow for curvature of Earth • 1:2500 scale maps actually vary in scale • On extreme east and west coasts, scale is approximately 1:2501 • Reference is taken along 2 oW where scale is 1:2499 • Along Greenwich Meridian and 4oW scale is 1:2500 • Surveying Instruments can be very accurate • Instruments in ENV are capable of accuracies of 1 part in 20000 with ease if used correctly. • Maps in UK are based on cartesian co-ordinates • North is represented by a bearing of 000. • East a bearing of 090, • South-west a bearing of 225 etc. • Referencing: True North: Grid North: Magnetic North • .

4. 2. Basic Surveying Methods N  O where is the length of the line, and  is the bearing where Eo is Easting and No is Northing of the reference station. Difference in the Easting (E) is given by:- • Point location - radial line and distance method.. Northing difference (N)is given by:- True co-ordinates of the second point:- Easting = Northing = This method can ONLY be used if there is an INBUILT reference direction in the instrument - e.g. magnetic north

5. 2. Basic Surveying Methods N R O  • Point location - radial line and distance method.. • No inbuilt reference Two horizontal angles areALWAYS needed i.e. a reading to R (a reference object) as well as object of interest • Applies to most instruments: • Total Stations • Theodolites • Levels etc

6. 2. Basic Surveying Methods A B N C B C A B C A B C D A Internal Triangle of Errors • Point location - Resection External Triangle of Errors Coordinates of A and B are known Point C found from bearings at A and B or bearings from C to A and B Methods to distribute errors are needed P

7. 2. Basic Surveying Methods a a a b b b A1 A1 A1 c c d c A2 e • Open Traverse • Errors accumulate • Point location - Traverse Methods • Closed Traverse • Errors can be distributed • Closed Loop Traverse • Errors can be distributed

8. 2. Basic Surveying Methods 2 1 d2 d1 A B D1 D2 • Point location - Offset Methods Useful for mapping features Not suitable where accuracy is required

9. 2. Basic Surveying Methods • Height Measurement • Level Ground • Base Accessible • Level Ground • Base Not Accessible

10. 2. Basic Surveying Methods ds   H H • Sloping Ground • Base Accessible • Observations to same height above ground • Height Measurement • Sloping Ground • Base Accessible • Observations to different height above ground

11. 2. Basic Surveying Methods H2 H1 • Sloping Ground • Base Accessible • Base and Top above and below observer • Height Measurement

12. 3. Planning a Survey • Careful Planning is needed • A single missed reading will make whole survey of no value • Need to provide checks • Abstract raw data in field • Repeat readings if necessary before leaving site. • Remember an extra set of readings may take 15 minutes - but to remobilise and set up again may take many hours • schedule breaks effectively.

13. 3.2 Basic Requirements • A clear statement of purpose of survey is needed • e.g. • mapping vegetation boundaries; estimating river bank plan shape or erosion rates; • determining flow characteristics in rivers; • establishing fixed reference stations for future use; • locating the point at which a particular set of measurements have been taken; • measurement of the profile of a slope; • assessment of regions liable to flooding. • scale of the map required (if relevant). . The purpose of the survey will dictate the scale and accuracy required and ultimately the methods to be used.

14. 3.2 Basic Requirements • Secondary Planning Requirements include:- • what equipment is actually available • what time is available • what man-power is available • what access and transport are available. • over what distance will the surveying party be spread during the surveying? • Will it be necessary to return to the same site at a later date to take repeat measurements, and if so when (within a few days, or several weeks or months later?). • How will contact between members of the surveying team be maintained at distance?

15. 3.2 Basic Requirements • Mapping vegetation boundaries: • accuracy ~ 1:1000 (1 m is represented by 1 mm), • Suitable surveying methods: • compass and tape traverses, • chain and offset mapping, • point resection using a prismatic compass. • height variations, Abney levels will often be adequate. • Alternative methods, if the equipment is available, • use of a surveyor's level and tachymetery, • use of electro-magnetic distance measurement.

16. 3.2 Basic Requirements • Water Slope Measurement • Difference in water surface elevation in a river is small, • Measurement requires accurate measurement of height differences over distances which are usually between 10m and 500m apart. • A good surveying level for which thecollimation error is knownis required. • Otherwise river may appear to flow uphill!!

17. 3.2 Basic Requirements • Fixed Control Points • Measurements will be needed to the nearest millimetre (centimetre) even if associated mapping detail is not required at this level of accuracy. • Sometimes, such as in the vegetation survey, simple methods can be used including prismatic compasses to establish stations, • Control stations will be located more accurately using a theodolite and associated equipment.

18. 3.2 Basic Requirements ~10 - 20 m • Surveying River Banks • Methods • Radial Line Techniques using tachymetry for general plan shape of meanders (general profile ~5m). • More accurate methods involving the establishment of short permanent base lines on the bank parallel to the long stream direction of the river are needed for erosion studies. • accurate profiles of bank are determined using metre rule offsets from this reference line to the edge of the river bank. • Decisions needed • what constitutes the edge of the channel?

19. 3.2 Basic Requirements • Size of Survey Party:- • what equipment is to be used for accuracy • access for vehicles • e.g. • a theodolite requires a tripod and targets • may also require targets mounted on tripods • could require a minimum of three tripods and ancillary equipment. • Often makes sense to establish control stations separate from detailed purpose of survey • Communication • Radios • Flags? • Markers: Permanent: Temporary????

20. 3.3 Booking of Data • in waterproof notebooks • should be logical • should always be done in the field • if necessary it can be transcribed BUT the ORIGINAL BOOKING MUST ALWAYS be accessible. • Cross checking should be booked in field • Critical Information which should always be present • Purpose of Survey: (River Bank Mapping at Maes Mawr) • Date: Time: Weather: • Specific Location • Sketch of Area • Booker’s Name: • Observer’s Name

21. 3.4 Permissible Errors

22. 3.5 Treatment of Errors • Systematic Errors • collimation errors in instrucments • magnetic errors affecting all readings at a particular location • Systematic errors (unless large) can be compensated - will always be present and calibration is important. • Random Errors • observer variations in reading a scale 148.1 148.2 148.0 148.1 148.1 148.1 148.2 147.9 148.1 148.2 • Mean is 148.1 - standard deviation is 0.09

23. 3.5 Treatment of Errors • Snedecor’s Rule Where R is range of readings

24. 3.5 Treatment of Errors • Random Errors 148.1 148.2 148.0 148.1 148.1 148.1 148.2 147.9 149.3 148.2 • Mean is 148.22 - should this value be used? • Exclude 149.3 and mean is 148.10 • standard deviation is 0.10 • value of 149.3 is 12 standard deviations from mean and should be excluded. • Gross Errors should always be discarded

25. 3.6 Provision of Checks • Going to repeat a survey is time consuming • Always provide a check • pace out a distance • sight on three positions rather than basic two • if measuring three stations - take readings at all three stations - sum of angles should be 180o • For Level transfers • e.g. elevation profile of longstream of river • trace river from start to finish • transfer level back along back - differences should be same • if errors are unacceptable (e.g. 20 mm per 1 km) then repeat until consistency is achieved 148.1 148.2 148.0 148.1 148.1 148.1 148.2 147.9 149.3 148.2 • Mean is 148.22 - should this value be used? • Exclude 149.3 and mean is 148.10 • standard deviation is 0.10 • value of 149.3 is 12 standard deviations from mean and should be excluded. • Gross Errors should always be discarded

26. 3.7 An example of Bad Planning RO A B C D E Lady Bower Reservoir • ENV students doing their third year project in 1992 in Derbyshire • needed to map meanders • decided to use radial line method and tachymetry using level • measured A to RO • measured radial lines at A • moved to B • measured B to A • measured radial lines at B • moved to C • etc Despite careful planning points B, C, D and E could not be located!

27. 3.7 An example of Bad Planning RO A B C What should have been done? Before leaving Derbyshire, map should have been plotted. Before moving from A to B measure A to B When at B measure from B to A