ENV-3C16 FLUVIAL GEOMORPHOLOGY
FIELD COURSE (2003)
A modern Surveying Level
SURVEYING METHODS
(based on Tovey (1996) – Surveying
Sections 1 – 3
Section 1 Introduction
Section 2 basic Surveying Methods
Section 3 Organisation and planning of a Survey
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N.K. Tovey ENV-3C16 Fluvial Geomorphology Field Course Sections 1 - 3
1. SURVEYING
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N.K. Tovey ENV-3C16 Fluvial Geomorphology Field Course Sections 1 - 3
1.1 Introduction
An integral part of many fieldwork observations is Surveying and a good working knowledge of the methods involved is essential for most Environmental Scientists. Surveying methods are varied and may be divided in several ways:-
a) by the purpose of the survey e.g. map making, location of specific points, definition of land ownership etc.,
b) by the nature of the survey itself e.g. hydrographic, terrestrial, astronomic,
c) according to the scale or accuracy of the survey,
and d) by the type of instrument or instruments used e.g. prismatic compass, level theodolite, photograph (terrestrial or aerial).
Many of these aspects will be of interest to the Environmental Scientist, and must be considered fully in the planning stage. There are also other factors to consider, for instance with surveys which stretch more than about 1 km, the curvature of the earth can start to become significant. Thus even the most basic theodolites likely to be used are calibrated to 20 seconds of arc and can be estimated to 5 seconds. Over a sighting distance of 5 km, the difference in vertical angle arising from the curvature of the earth will be about 2.5 minutes which is about 30 times greater than the accuracy achievable by those instruments.
Most surveying involves either the transfer of levels between two points, or the measurement of angles and lengths. Much of the analysis needed requires solution of triangular shapes using basic trigonometry (or by graphical means). However, when the area covered by the survey becomes large then the effects of the curvature of the earth becomes important, the normal rules of trigonometry and planar triangles no longer apply and consideration must be made of curvilinear triangles (on the surface of the earth) where angles do not add up to 180 degrees.
Such complications require geodetic surveys and are beyond the scope of this chapter. However, the student of surveying should be aware of the consequence of this on the production of maps such as the Ordnance Survey maps in Great Britain. One of the more common series of maps are the 1:2500 scale maps in which 1 mm on the map represents 2.5 m on the ground. Because of the curvature of the earth, it is impossible to cover an area the size of the United Kingdom with planar maps of exactly the same scale, and a compromise must be reached. Thus on the extreme east and west coasts of mainland Britain the true scale is about 1:2501, while along the longitude 2o W it is 1:2499. Only along the Greenwich Meridian and 6oW the scale is close to the nominal one of 1:2500.
Many textbooks on surveying are written for professional engineers and surveyors and cover many topics which are beyond the scope of interest of the Environmental Scientist. Nevertheless he or she will wish to be sufficiently conversant with the methods to map features in an area at an accuracy consistent with the study in hand. This chapter attempts to cover key aspects of surveying, to describe simple techniques, often little covered in more advanced texts, and to introduce at a basic level some of the more accurate techniques. Where relevant, important issues such as instrument calibration and checks for errors are included. In section 7 there are several text books to which the reader should refer for further information.
For the Environmental Scientist surveying is important in many field ranging including the profile of slopes or glaciers, the distribution of vegetation or soil types, the erosion of river banks, the assessment of areas prone to hazards such as landslide or flooding, assessments of land-classification as well as in planning and transportation. Equally, the Environmental Scientist may wish to determine the position where say a series of meteorological or geophysical observations were taken or the position of soil samples. In yet other cases it may be necessary to relocate at exactly the same point for future series of observations.
Surveying involves measurements in the field, but despite this, measurements taken in surveying can be among the most accurate in any branch of science. Thus with modern theodolites reading to 1 second of arc, accuracies of 1 part in 105 and better are possible, and even with the more basic theodolites for routine work in Environmental Sciences accuracies of 1 part in 20 000 are readily achievable.
Reference is made in several parts of this section to bearings. These may relate to measurements taken relative to magnetic north (as with a compasses), or to grid north according to the convention used on maps, or to true north. All systems measure bearings relative to North, and angles are measured in degrees clockwise so that due East is 090, while south west is 225 (some instruments are calibrated in grads instead of degrees where 100 grads = 1 quadrant). The differences between the systems of bearings relate solely to the direction of the zero reference. The difference between the origins will vary from place to place. Thus in 1995, the magnetic north pole was 5 degrees west of grid north in Norwich, England and any magnetic bearings taken in that region must be corrected by subtracting 5 degree to convert the bearings to grid bearings (or by 3 degrees to convert to bearings based on true north.
The chapter is divided into several sections. Following this introduction is a section covering some of the general methods of surveying which are relevant whatever equipment is used. A key aspect to successful surveying is careful planning and organisation, and this is covered in the next section . This is followed by a discussion of the various instruments and their use. Errors will be present in all surveying and it is important to minimise them. While a general discussion of errors is included in the sections 2 and 3, specific methods to compensate for systematic errors are included as section 5. The construction of maps ( or plans) are often important in Environmental Science applications and these, included as section 6, precede a concluding summary.
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N.K. Tovey ENV-3C16 Fluvial Geomorphology Field Course Sections 1 - 3
2 Basic Surveying Methods.
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N.K. Tovey ENV-3C16 Fluvial Geomorphology Field Course Sections 1 - 3
2.1 Introduction
Most surveying can be reduced to a two basic requirements a) the location of a point in the field relative to others, and b) the determination of a height difference between two or more points. There are several methods by which each can be achieved, and for each method, there are also several different instruments which may be used. The choice of which one to use often depends on availability, the precision required from the survey, and logistical constraints. In this section we shall consider the basic methods which can be employed. They will be generally applicable to many instruments irrespective of their precision. In general we shall not consider in this section the precautions needed when using particular equipment as specific considerations of the different instruments are left to section 4. Nor will we consider the adjustment of readings to compensate for errors as this is covered in section 5. General map making methods which draw on many of the basic techniques described here have a separate section 6 devoted to them.
The position of a point of interest may be determined by one of four methods: a) radial line and distance; b) resection; c) traverse methods; d) offset methods. These topics are covered separately in the next four section before a discussion of height determination using vertical angle measurement methods. Height measurement using a Surveyor's level is specific to that instrument and is covered in section 4.5.
2.2 Point location - radial line and distance method.
Starting from a known fixed point, it is possible to identify the position of a second point by measuring the bearing to that point and also the horizontal distance. The bearing may be measured with a hand held compass or with a tripod mounted compass for greater accuracy. The distance may be measured directly with a tape (4.6), optically by tachymetry (section 4.5), by electro-magnetic distance measurement (section 4.6) and using a Total station (section 4.7). For methods other than the optical methods, the raw distance measured will normally be the slope distance, and a correction must be made for any slope in the ground by measuring the vertical angle (see section 2.6). Once both bearing and horizontal distance have been measured, it is a simple matter to plot the information on a plan at the appropriate scale, and thus locate the position of the second point (see Fig. 2.1 ).
Fig. 2.1 Point location - radial line method. Point P can be located by measuring distance and a compass bearing from O.
One method for constructing a map involves the location of several points of a feature (see section 6.4), such as a river bank using this radial line and bearing method from a single fixed point (or if the survey covers a large area, radial lines from several fixed points). Plotting the data will only locate the point with an accuracy which is related to the scale of the map. Thus on a plan of 1:1000 it will not be possible to position points closer than 0.1 - 0.2 m. For defining features on a map this will often be of sufficient accuracy, but for defining co-ordinate information of fixed or control station, a higher accuracy will normally be required, particularly if measurements are done with a theodolite. Where possible, the location of the second point should be determined numerically by trigonometry. To do this the co-ordinates of the initial reference station must be known, or alternatively given an arbitrary value say 1000.000 (Easting), and 1000.000 (Northing). If arbitrary values such as these are given, they should be chosen so that all co-ordinate values are positive.
The difference in the Easting (DE) from the fixed point to the second is given by:-
while the corresponding difference Northing (DN)is given by:-
where is the length of the line,
and q is the bearing
Finally the true co-ordinates of the second point are then:-
Easting =
Northing =
where Eo and No are the Easting and Northing of the reference station respectively.
Fig. 2.2 Point location - radial line method using a theodolite or level. Readings must be taken to both a reference object (R) and also point P. Unlike the case in Fig. 2.1, it is not possible to locate P by a single reading from O to P.
When this radial method to locate the position of a second point is used with instruments such as theodolites and levels, there is no in built reference direction (corresponding to the North for compasses), and there must be a third station whose direction from the first is preferably known (or arbitrarily assumed as say North in cases of difficulty). This is illustrated in Fig. 2.2. Horizontal angles are determined relative to this reference direction by measuring the horizontal angle to this reference direction and that to the unknown point and subtracting the two readings. With instruments such as these it is not possible to determine the bearing by a single angular measurement. Once the horizontal angle with respect to the reference station has been determined it is then a simple matter to locate the unknown point either by direct plotting or by trigonometry in a manner similar to that outlined above.
2.3 Point location - Resection.
Often it is either not possible or it is inconvenient to use the radial line method described above to locate a point. Instead, taking bearings from the unknown point to two well defined points (either on a map or previously established) will allow the position of the unknown point to be determined. Graphically the situation is represented in Fig. 2.3. The bearings from the known points may be plotted to locate the unknown point at the intersection of the two construction lines. For greater accuracy, trigonometry may be used to locate the co-ordinates of the unknown point, and this method should be used whenever moderate or high accuracy is required. To do this the co-ordinates of the two fixed points must be known or measured from the map. From this information, the distance and the bearing of the line between the two fixed stations can be determined, and then the two internal angles of the triangle A and B. Once this has been done it is a simple matter to apply the sine rule to determine either the distance AC or BC and hence the co-ordinates of C.
Fig. 2.3 Point location - resection. Point C may be located by taking bearings from points A and B without the need for distance measurement. If theodolites or other instruments which have no in-built reference direction are used, then the included angles CBA and BAC should be measured instead of taking bearings.
When the co-ordinates are read from a map it is essential that the same reference direction is used. Thus if a magnetic compass is used for measurements then the readings must be corrected for the magnetic variation. Remember during this correction that some maps may have been plotted according to true north while others will have grid north as the reference direction. When instruments which do not have an in built reference direction are used, it is not possible to locate the position of an unknown point with just two sightings from the unknown point (C). Instead separate angles at both A and B must be measured to the point C.
Once point C has been determined, any pair of the stations A and B, or B and C, or C and A may be used to locate a fourth point. This process may be repeated several times to cover the area of interest and is known as triangulation. .