Introduction to Basic Instrumental Land Survey


                   Regardless of the specific nature of the research, one thing common to almost every
                   field based report in physical geography is a map of some sort, be it a site map to
                   indicate the location of the study or a detailed geomorphological map from which data is
                   derived and conclusions drawn. Since much of the research in geomorphology is
                   conducted in small areas - square metres rather than square kilometres - it is almost
                   certain that no adequate commercial map exists which will suit your purpose, and so it is
                   vital that you are familiar with the techniques necessary to produce your own map from a
                   field survey.

                   There are a wide variety of survey techniques available to the geomorphologist, and it is
                   important that you have an understanding of their applications, advantages and
                   limitations in order to select the method which best suits both your objectives and your
                   available resources.

                   1. Compass Traverse

                   This is an excellent system for initial site survey work in remote locations to produce a
                   quick and reasonably accurate map. It can be used as a linear technique for plotting
                   long, narrow features, or as a 'closed-loop' to delimit areas of interest. Both methods are
                   commonly used in conjunction with offsets (see section 2)

                   The technique is based on the measurement of magnetic bearings i.e. compass
                   bearings, which are measured clockwise from magnetic north. It is important that you
                   appreciate that magnetic north differs from grid north, so if you want to transfer your data
                   to published maps you will need to correct it by a given amount or magnetic declination.
                   This amount varies, but is published in the key given on larger scale maps covering your


                   1. Identify the first 'leg' of the survey, and place a ranging pole at the start point, A. The
                   second person now moves to the end of the first leg, point B, trailing a tape measure
                   from A to B.
                   2. A pole is placed at point B and a compass bearing is taken from A to B (the forward
                   bearing) and from B to A (the back bearing).
                   3. The FB and the BB +/-180 should differ by no more than 2o. When they are in
                   agreement by 2 or closer the average of the two is the value recorded.
                   4. The tape between the two stations is held taut, and the distance recorded.
                   5. The person at station A now moves beyond B to the next point of interest, C, and the
                   procedure is repeated for this and subsequent legs. In a closed-loop survey the final leg
                   must finish at the original start point, A.

                   1. On paper, mark a point to represent station A. All points are to be plotted relative to
                   this station.
                   2. Through this point, construct a North/South line. You may find it easier to construct this
                   initial plot on graph paper, and use the printed grid lines as your N-S grid.
                   3. Using a 360o protractor, measure the first agreed bearing clockwise from North using
                   A as the origin. A line is then drawn from A in this direction to represent the measured
                   ground length at the scale selected, ending at point B.
                   4. At point B, and all subsequent points, the original N-S line must be reconstructed and
                   the procedure repeated.

                   Compensating for Errors

                   In a closed loop survey, it is likely that when the data is plotted, the start and finish points
                   will not match. This is due to the cumulative effect of small errors on the individual legs of
                   the survey. In the plotting stage however it is possible to compensate or close these
                   errors as follows :-
                   1. Plot the data as outlined above. If a misclosure error is apparent, measure the
                   direction (from North on your map) from the erroneous finish point to the start point and
                   accurately measure the distance between them.
                   2. On graph paper reconstruct, to scale, the total survey base line . This is the sum of the
                   lengths of the individual legs. Draw this length as one horizontal line at the scale
                   3. Construct a vertical line at the same scale to represent the error distance to be closed
                   (measured in stage 1). Draw this line vertically up from the right hand end of your
                   horizontal base line (B)
                   4. Draw a line joining the two free ends of lines A & B -completing the triangle (line C)
                   5. Now identify the position of station B on your base line. Remember this line
                   represents the total length of your survey, and since you know the length of each leg you
                   can easily work out how far along your baseline each point lies.
                   6. Once you have located station B on the base line, proceed vertically upon your
                   diagram until you meet line C and then horizontally across to scale B and read this value.

                   7. You now have all the information you need to correct your plotted data. Station B on
                   your map needs to be moved by the distance you read in stage 6, in the direction you
                   measured in stage 1. Repeat for all the remaining points.

                   2. Offsetting

                   This technique may be employed as an independent method, but is more commonly
                   used in association with compass traverse work. It is particularly useful for filling in the
                   fine detail missed by the compass traverse and accurately plotting the shape of linear
                   features eg a river bank or morphological boundary.
                   The technique requires that a straight base is established in the field (eg. one 'leg' of a
                   compass traverse). Often a surveyors chain is used rather than a tape as it is less likely
                   to flex, and the technique is often referred to as 'chain and offset'. The distance from this
                   base line to the point of interest is then measured (the offset).
                   Offsets may be taken objectively or subjectively. In the former case offsets are taken at
                   fixed and regular intervals. In the latter, they are taken at points that the surveyor deems
                   significant. In most cases a combination of the two works best.


                   1. Lay out a straight base line (as for a single leg of a compass traverse) as close to the
                   features to be recorded as possible - the shorter the offset, the less room for error.
                   Record the length and bearing of this base line.
                   2. At each determined point measure the distance to the point of interest at 90o to the
                   base line.
                   3. Record the ground length along the base line to the offset.


                   1. Reconstruct the base line of the survey, at the selected scale and in relation to North,
                   as for a compass traverse.
                   2. Plot offset data as recorded, identifying each offset point as a dot.
                   3. If you were mapping a linear feature - join the dots !

                   The above methods allow us to plot the spatial location of points of interest relative to
                   one another. What they do not allow us to do is record any information about the shape
                   of the land we are working on and the nature of any slopes at the site, normally essential
                   information. The following method allows us to construct a cross-section of the slope, or
                   slope profile.

                   3. Slope profile Survey

                   In many ways this method is very similar to a compass traverse, except that the angles
                   measured are slope angles rather than bearings, and each 'leg' is a 'measured length'
                   along one straight baseline. Two simple hand-held instruments are commonly used, the
                   Suunto Clinometer, which can be read to the nearest 0.5o, or the Abney Level, which
                   can be read to the nearest 10'.
                   Measurements are commonly taken over fixed, regular ground lengths.


                   1. Determine the line of the survey. This is usually taken as the line of steepest slope i.e.
                   at 90 degrees to the line of natural contours. Lay a tape along this line and fix it at both
                   ends. Record the total length and the bearing of this line.
                   2. Two operators are required, and are initially stationed at A (the top of the survey) and
                   B (the end of the first 'measured length' segment), each with a ranging pole and a
                   clinometer or Abney.
                   3. Each person reads the slope angle i.e. both upslope and downslope readings are
                   taken and must agree. Be sure to sight the instrument on equivalent points on the two
                   ranging poles - i.e. if you are holding the instrument at the 1.5m mark on one pole, read
                   to the 1.5m mark on the other.
                   4. The operator at A now moves beyond B to C (one further measured ground length)
                   and the procedure is repeated to the foot of the slope.


                   The data is easiest plotted on graph paper, each angle plotted from the horizontal as a
                   line representing the measured ground length at the appropriate scale.

                   A combination of the methods outlined above will allow you to produce reasonably
                   accurate maps with the minimum of equipment. However, certain projects may require
                   greater accuracy than is possible with these methods, or the nature or size of the area to
                   be mapped may preclude their use. In these instances more sophisticated instruments
                   are available to the geomorphologist.

                   4. Levelling

                   This technique allows the operator to rapidly gain information about the relative height
                   and location of points in view. The principle is based on the intersection of an accurately
                   levelled line of sight with a graduated staff. The Geography Department currently uses
                   Sokkia C32 Levels. These are modern instruments with an automatic levelling
                   mechanism, and are very easy and quick to use. Levelling is particularly suited to
                   applications where relatively small changes in elevation need to be accurately mapped
                   e.g. surveying river cross profiles. It may be less suited to surveying areas with a great
                   elevation range - the standard staff height is 4m, so from a single station the maximum
                   measurable height range is +/- 4m. If this range is exceeded, it becomes necessary to
                   establish several base stations following the procedure below.


                   1. Select a suitable site to establish the base station. Use common sense here, and
                   select a site with a clear view of the area to be surveyed, bearing in mind the range of
                   elevations to be measured.
                   2. Set up the tripod, extending the legs to offer a convenient working height, and bed the
                   tripod feet firmly into the ground. Make sure that all the locking nuts on the tripod legs are
                   securely fastened.
                   3. Take the level from its box. At no stage is the level to be placed on the ground or held
                   upside down. Carefully place the level on the tripod head and tighten the captive bolt to
                   hold it in place.
                   4. Slacken the captive bolt by half a turn. It is now possible to move the level around on
                   the domed head of the tripod. Manipulate the level until the circular spirit level on the
                   body of the instrument indicates that it is approximately level and re-tighten the captive
                   5. Use the three Levelling screws on the base of the instrument to bring the spirit bubble
                   into the centre. The level is now ready to use.
                   6. Extend all 4 sections of the staff and check that they are locked in position.
                   7. One person now takes the staff to the first point of detail to be recorded. It is important
                   that the staff is held vertical and still. Watch out for any overhead cables or branches
                   before raising the staff to its full height.
                   8. The instrument operator points the level at the staff. The level can be turned by hand,
                   but fine adjustments can be made with the transit screws located on either side of the
                   instrument body.
                   9. Looking through the telescope focus the image of the staff using the focus control on
                   the right hand side of the instrument.
                   10. In the telescope field of view you will see three horizontal cross hairs. If your survey is
                   purely for elevation data at known points, such as when surveying a river cross section,
                   you need only be concerned with the middle one of these. Take the reading on the staff
                   to the nearest mm. This is your elevation value
                   11. If you require distance and direction data, you will need to note the intersection of all
                   three cross-hairs or stadia with the staff, and the bearing to the staff in the window below
                   the telescope eyepiece. Note that this is not a magnetic bearing. In order to reference
                   these directions to North you will need a compass bearing to at least one of your
                   stations as well as the bearing on the level.
                   12. Repeat for all points of interest visible from your base station.
                   13. If not all of your points are visible from the base station, or the change in elevation is
                   beyond the range of the staff, you will need to establish secondary stations. In order to
                   do this it is vital that the staff man remains at the last point visible from your primary
                   station while the level is moved to a new location from which this last point AND the new
                   points to be measured are simultaneously visible. A reading is first taken to the last
                   point measured (a BACKSIGHT), and then the survey continues. In this manner relative
                   heights for the whole area can be derived.

                   Analysis and plotting

                   After a few basic calculations. data can be plotted in much the same way as for the
                   simpler techniques

                   1. It is common, unless you know the absolute height of one of your stations (rare !) to
                   identify one of your stations as a reference datum and calculate all of your detail heights
                   relative to it. If you want heights relative to the instrument station do not forget to add or
                   subtract the tripod height from the staff reading.

                   2. The distance between the level and the staff is calculated as follows :

                   (Upper stadia reading - Lower stadia reading) in cm x 100 = Distance (in m)

                   With practice, levelling is a very accurate and rapid technique.

                   5. Total Station Surveying

                   The most advanced optical instrument for land survey is the total station . This is an
                   electronic instrument based on a traditional theodolite, which enables us to measure
                   elevation, slope angle, horizontal and slope distance and direction from one station in a
                   matter of seconds. The limitations of the level are overcome because the instrument
                   does not rely on a horizontal line of sight. A signal is sent from the instrument to a simple
                   reflecting prism which replaces the 4m staff used in levelling. The transit time of the
                   signal is used to calculate distance to the nearest mm over several kilometres. The only
                   limitations of this system are the size and weight of the equipment required. Without
                   vehicle access it is far from ideal for remote locations, and for small area surveys an
                   efficient team could complete a perfectly adequate compass traverse in the time taken
                   to set up the Zeiss Elta 4 total station - the most accurate does not automatically mean
                   the best for the job!

                   There are many other techniques and instruments available for land surveying, but they
                   are all based in large part on the principles behind the techniques outlined above,
                   techniques which have proved the most useful and the most frequently employed by the
                   field scientist. The most appropriate technique for a given job will depend on many
                   factors - the desired level of accuracy, site accessibility, time available, area to be
                   covered, etc.

                   6. Global Positioning System (GPS) Mapping

                   The methods of field surveying described above all rely on the basic principle of
                   establishing a line of sight i.e. being able to see the point to be surveyed from a fixed
                   point or instrument position, and all require careful thought if they are to be 'tied in' to
                   existing mapping such as the Ordnance Survey National Grid. To overcome these
                   limitations, geomorphologists can now make use of the Global Positioning System
                   (GPS) to obtain accurate position information for any point on the Earth's surface simply
                   by visiting that point with a GPS receiver.

                   The system is based on a constellation of 24 satellites, operated by the US Department
                   of Defense, that orbit the Earth at very high altitude and constantly transmit information
                   regarding their exact location and a time code. A portable GPS receiver on Earth that
                   receives signals from any four of the satellites can calculate its location, in theory, to a
                   high degree of accuracy. Until Summer 2000, the signals available to civilian users were
                   subject to a deliberately introduced error to reduce accuracy, but even though this error
                   has now been removed, other natural sources of error mean that a single GPS receiver
                   used alone (such as those sold for recreational use) will yield an accuracy of around 30
                   metres from the true location most of the time. While this level of accuracy may be
                   acceptable for most navigation purposes, it is of little use for surveying purposes.

                   In order to overcome these errors, it is common to employ a technique called Differential
                   Correction to GPS measurements (DGPS), based on the simultaneous use of two
                   receivers. Using the College Trimble ProXR DGPS system we can fix locations with an
                   accuracy of better than 50cm. Furthermore, this information is stored on a dedicated
                   Data Logger that allows us to enter any other data about the features that we map as we
                   go along - effectively creating a GIS that we can later download to a PC for plotting and

                   Clearly, there are many techniques and instruments available to the field scientist for
                   mapping and surveying, and choice of the appropriate technique will depend on a
                   number of factors - the desired level of accuracy, site accessibility, time available and
                   area to covered to name but a few.

                   Suggested Reading

                   Bannister, A., Raymond, S. & Baker, R. (1998) Surveying (7th ed). Longman.
                   Goudie, A.S. (1990) Geomorphological Techniques. Unwin Hyman.
                   Hurn, J. (1989) GPS: A Guide to the Next Utility. Trimble. *
                   Hurn, J. (1993) Differential GPS Explained. Trimble. *
                   Petts, G.E. (1983) Rivers. Butterworths
                   Pugh, J.C. (1975) Surveying for Field Scientists. Methuen.

                   * Available from Patrick Hopcroft