Presentation on theme: "Surveying Method chosen depends on:"— Presentation transcript:
1 Surveying Method chosen depends on: 1. IntroductionSurveying 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 usede.g.prismatic compass,leveltheodolite,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.Errorscollimation errors in instruments need to be calibrated otherwise “rivers could flow uphill”Surveying involvestransfer of levels between two pointsmeasurement 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 scaleOn extreme east and west coasts, scale is approximately 1:2501Reference is taken along 2 oW where scale is 1:2499Along Greenwich Meridian and 4oW scale is 1:2500Surveying Instruments can be very accurateInstruments in ENV are capable of accuracies of 1 part in with ease if used correctly.Maps in UK are based on cartesian co-ordinatesNorth 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 Point location - radial line and distance method. .NODifference in the Easting (E) is given by:-Northing difference (N)is given by:-True co-ordinates of the second point:-Easting =Northing =where is the length of the line,and is the bearingwhere Eo is Eastingand No is Northingof the reference station.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 Point location - radial line and distance method. .No inbuilt referenceNOTwo horizontal angles are ALWAYS neededi.e. a reading to R (a reference object) as well as object of interestRApplies to most instruments:Total StationsTheodolitesLevels etc
6 Internal Triangle of Errors External Triangle of Errors 2. Basic Surveying MethodsABCPoint location - ResectionInternal Triangle of ErrorsNCBACExternal Triangle of ErrorsBACoordinates of A and B are knownPoint C found from bearings at A and Bor bearings from C to A and BBCDAMethods to distribute errors are neededP
7 2. Basic Surveying Methods Point location - Traverse MethodsOpen TraverseErrors accumulateabA1cbacA1Closed TraverseErrors can be distributedA2bacA1Closed Loop TraverseErrors can be distributeded
8 2. Basic Surveying Methods Point location - Offset Methods2d21d1ABD1D2Useful for mapping featuresNot suitable where accuracy is required
10 2. Basic Surveying Methods Height MeasurementSloping GroundBase AccessibleObservations to same height above groundHdsHSloping GroundBase AccessibleObservations to different height above ground
11 2. Basic Surveying Methods Sloping GroundBase AccessibleBase and Top above and below observerHeight MeasurementH1H2
12 Careful Planning is needed 3. Planning a SurveyCareful Planning is neededA single missed reading will make whole survey of no valueNeed to provide checksAbstract raw data in fieldRepeat 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 hoursschedule breaks effectively.
13 3.2 Basic RequirementsA clear statement of purpose of survey is needede.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 Secondary Planning Requirements include:- 3.2 Basic RequirementsSecondary Planning Requirements include:-what equipment is actually availablewhat time is availablewhat man-power is availablewhat 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 Difference in water surface elevation in a river is small, 3.2 Basic RequirementsWater Slope MeasurementDifference 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 the collimation error is known is 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 Surveying River Banks Methods Decisions needed 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.~ mDecisions neededwhat constitutes the edge of the channel?
19 Markers: Permanent: Temporary???? 3.2 Basic RequirementsSize of Survey Party:-what equipment is to be used for accuracyaccess for vehiclese.g.a theodolite requires a tripod and targetsmay also require targets mounted on tripodscould require a minimum of three tripods and ancillary equipment.Often makes sense to establish control stations separate from detailed purpose of surveyCommunicationRadiosFlags?Markers: Permanent: Temporary????
20 3.3 Booking of Data in waterproof notebooks should be logical should always be done in the fieldif necessary it can be transcribed BUT the ORIGINAL BOOKING MUST ALWAYS be accessible.Cross checking should be booked in fieldCritical Information which should always be presentPurpose of Survey: (River Bank Mapping at Maes Mawr)Date: Time: Weather:Specific LocationSketch of AreaBooker’s Name:Observer’s Name
22 3.5 Treatment of Errors Systematic Errors Random Errors collimation errors in instrucmentsmagnetic errors affecting all readings at a particular locationSystematic errors (unless large) can be compensated - will always be present and calibration is important.Random Errorsobserver variations in reading a scaleMean is standard deviation is 0.09
23 3.5 Treatment of ErrorsSnedecor’s RuleWhere R is range of readings
24 Gross Errors should always be discarded 3.5 Treatment of ErrorsRandom ErrorsMean is should this value be used?Exclude and mean isstandard deviation is 0.10value of is 12 standard deviations from mean and should be excluded.Gross Errors should always be discarded
25 Gross Errors should always be discarded 3.6 Provision of ChecksGoing to repeat a survey is time consumingAlways provide a checkpace out a distancesight on three positions rather than basic twoif measuring three stations - take readings at all three stations - sum of angles should be 180oFor Level transferse.g. elevation profile of longstream of rivertrace river from start to finishtransfer level back along back - differences should be sameif errors are unacceptable (e.g. 20 mm per 1 km) then repeat until consistency is achievedMean is should this value be used?Exclude and mean isstandard deviation is 0.10value of is 12 standard deviations from mean and should be excluded.Gross Errors should always be discarded
26 3.7 An example of Bad Planning ENV students doing their third year project in 1992 in Derbyshireneeded to map meandersdecided to use radial line method and tachymetry using levelmeasured A to ROROmeasured radial lines at AAmoved to BBmeasured B to Ameasured radial lines at BCmoved to CetcDEDespite careful planning points B, C, D and E could not be located!Lady Bower Reservoir
27 3.7 An example of Bad Planning ROWhat should have been done?Before leaving Derbyshire, map should have been plotted.ABefore moving from A to Bmeasure A to BBWhen at B measure from B to AC