Chapter 6: Field surveying
Introduction
Chapters 1 to 5 are concerned with the interpretation of data on existing topographic maps. This
chapter is designed to introduce you to the various surveying techniques used to obtain the original field
data on which such maps are based. An understanding of these surveying techniques not only provides
us with some very useful field mapping tools but it also can give us further insight into the nature of
topographic maps in general.
Geographers work with all scales of maps, from global to very local. Global maps - at least base
maps - are readily available in published form.
Maps of intermediate scale down to 1:20 000 also are
available for many, although certainly not all, areas of Canada. But detailed topography at larger scales
is rare and is not produced by government agencies as a matter of routine although they may be drawn
for special projects such as highway and dam construction or for floodplain hazard mapping, for example.
At these larger scales it usually is necessary to prepare an original map based on a photographic or field
survey. If aerial photographs are unavailable or the topographic detail required is beyond their resolution,
such maps must be based on a field survey.
This chapter is concerned with field survey techniques which will allow the production of detailed
large-scale maps of moderate to high-level accuracy. Although some of these techniques are very simple
and use readily available and inexpensive equipment, the resulting maps are quite adequate for many
purposes in geography (or in geology, archaeology, or any other field science).
Principles of spatial location
The purpose of a field survey is to accurately locate points in the field so that their positions
relative to each other can be plotted on a map. Regardless of the actual survey technique used, plotting
positions of points in the field are determined by one or more of four basic positioning principles:
(a) location by three measured sides
(b) location by offset
(c) location by intersection
(d) location by resection
The application of these principles is illustrated in Figure 6.1 where A, B, and C are known
positions, the bearing of AB is known, and X is some location to be determined and plotted on a map.
Location by three measured sides requires the field measurement of the distance from points A to
X and from points B to X. These two distances are then plotted at the appropriate scale on the map as
Chapter 6: Field surveying
arcs centred on A and B respectively. The intersection of the arcs locates the position X; this positioning
principle is the basis of 'chain triangulation'.
Location by offset is based
on
the
measured distance along a line offset from the
base line A-B through the point X. If a compass is
not available, the offset can be fixed as a normal
to AB by repetitive measurements XY' until the
minimum distance XY is obtained.
Once this
minimum distance is determined, the angle AYX
must be a right-angle and further measurement of
the distance AY and AX by tape or pacing allows
the offset to be plotted on the map.
If a compass is available the field surveyor
simply proceeds along the base line AB a known
distance until they are about opposite the point X
at some point Y'.
6.1: Positioning principles in surveying
Noting the distance AY, the
compass bearing of X from Y', and the offset distance XY', provides sufficient data to plot the position
X on the map. This compass-based location by offset is commonly used in pace and compass traverses
and in plane table surveys.
Location by intersection involves positioning by the intersection of bearing lines to X from the
known positions A and B. Clearly an instrument such as a prismatic compass must be available to use
this principle of positioning.
Location by intersection is frequently used in both pace and compass and plane table surveys.
In a small area in which all the points to be mapped are visible from two points,
the measurement of a
single base line and the use of intersection will serve to locate all features. This technique is particularly
useful for the location of inaccessible but visible features. Accuracy is maintained at a satisfactory level if
the subtended angle AXB approaches 90o but it declines rapidly as it becomes more acute or obtuse.
Location by resection
can be employed if the positions of at least three landscape features are
known. Compass bearings are taken from the plotting position (X) to each of these known positions (A,B
and C) and the back bearings plotted on the map.
The correct position of X is indicated by the
intersection of the back bearing lines. If they do not meet exactly at one point (as they almost never do!)
a small triangle of error will result. Conventionally, the point X is located in the centre of this triangle. If
the triangle is large relative to the area being mapped then measurement error is unacceptable and the
survey must be repeated.
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Chapter 6: Field surveying
Field Survey methods
The principles of spatial location described above are the basis of three commonly used field
survey methods: chain triangulation, pace and compass traverse, and the plane-table survey. After we
review these methods we will consider the instruments and techniques necessary to add the third
dimension to a field map.
Chain triangulation can be conducted by two field surveyors using:
(a) a measuring tape or surveyor's chain
(b) two plumb bobs
(c) several markers or survey rods
(d) a field note book for recording field measurements
Chain triangulation, like most field survey techniques, is best suited to an open landscape where
the features to be mapped are separated by flat or gently undulating country. It becomes difficult to
impossible to undertake chain triangulation in heavily timbered and mountainous terrain.
Before commencing the survey it is important to decide what features are to be mapped and how
the sides of the chained triangles are to be laid out. A little careful planning of the survey in the first place
may save considerable time and effort later.
Imagine that the area to be mapped is that depicted in Figure 6.2. It is a rural district with a local
hill to the north from which a small stream flows southward before it forks on more gentle slopes. A few
houses dot the landscape and a district road crosses the river over two bridges. Ultimately we want to
include all of these features in our survey.
The survey commences at house A and the first side of the triangle is run to the water tower at B.
The first member of the survey team, we'll call her Elaine, sets out from A towards B with the forward end
of the tape or chain and a number of survey markers. The survey markers simply consist of stakes with
coloured survey tape attached to the top. The second member of the survey team, we'll call him Ted,
keeps his position at A while Elaine walks out the full length of the tape. When the tape is fully extended
Ted, who can see both Elaine and the water tower at B, directs Elaine to move to the right or left as
required so that the water tower, Elaine, and the house at A, all lie on the one straight survey line. With
this alignment checked the tape is swung until it lies in a straight line between them and then it is
tightened. Elaine places a survey marker at her end of the tape and then moves forward towards point B
with Ted following. When Ted is abreast of the survey marker he calls a halt and lines up Elaine with the
water tower as before. Once again the tape is tightened and the end marked with a survey stake. As
measurement of the line proceeds, Ted collects the survey markers in order to check the number of
complete lengths of tape laid out.
If the river is small it may be possible to wade across it with the chain in order to complete this leg
of the survey. If it is too large to wade across, however, it will be necessary to throw the chain across
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Chapter 6: Field surveying
6.2: Chain triangulation survey (see text for explanation)
the stream and to use the bridges for access. If the river is too wide to throw the chain across obviously it
will be necessary to walk the chain over the bridges.
Clearly, there will be some circumstances in which
this procedure also is not practical and chain triangulation should be abandoned for a more appropriate
survey method.
From the water tower at B the distance is measured to the house at C and then from C back to
the house at A. Since the length of each side of the triangle ABC is known the relation of one to the other
can be plotted on the map at the appropriate scale. The position of the tall tree at D can be located by
triangulation in the same way, in this case with measured sides of the triangle ADB (side AB is known
already from the first leg of the survey).
The leg AD of this survey may present a problem because of the steepness of the slopes on the
hillside. In other words the ground distance is not a good measure of the horizontal distance and the
two diverge as slope increases. For this reason it may be necessary to step chain this leg as depicted in
Figure 6.3. This is achieved by placing the uphill end of the tape at ground level, while the downhill end
6.3: Step chaining on steep slopes.
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Chapter 6: Field surveying
is held over a marker by means of a plumb bob. Where slopes are steep the distance between survey
markers may be reduced to as little as 6 to 8 metres. In this case the horizontal distance between the
house at A and the tall tree at D is the sum of the distances bc,de, fg, hi, jl, kn, mp, and or.
Chain triangulation clearly has the advantage of being a simple technique requiring a minimum of
equipment while yielding quite accurate planimetric information. Its principal disadvantage is that it is
laborious and slow, particularly in hilly terrain. Also, it does require that the features to be mapped be
accessible to the surveyor. For example features on the far side of a wide river or ravine, or an island in a
lake, present obvious problems to the surveyor using this technique! In such cases the pace and
compass traverse may be a more useful basis for preparing a map of the area.
The pace and compass traverse is a convenient and rapid surveying technique for mapping the
line of a road or river course along with features in the adjacent areas. Little equipment is required for this
type of survey and the accuracy which can be obtained is entirely adequate for maps used for local field
work in geography. The detail of the map is located by traversing a continuous route such as those along
a road, stream, or fence line, and locating the features on either side by intersection and offset.
The compass used in this type of traverse can be of any style that allows the accurate
determination of a bearing while sighting to the object of interest. The instrument specifically designed for
this purpose is the prismatic compass. It usually consists of a metal housing containing a card attached
to a magnetized needle which is immersed in an oil bath to dampen oscillations. The circular card is
graduated in degrees from 0o to 360o and rotates freely on a central pivot so that the north point is
oriented to the magnetic north pole. The compass also has a viewing window with a vertical sighting hair
and a prism which allows the graduations on the card to be observed while sighting.
Use of this
instrument will be demonstrated in class.
A number of basic precautions must be observed when taking a compass bearing. First, you
must ensure that the compass card is unlocked and rotating freely (the compass housing should be held
with the face horizontal to prevent the card sticking against the glass). Second, it is prudent to take at
least two readings to each feature as a check against magnetic influence and human error.
Third,
compass bearings should not be taken near any structure which distorts the natural magnetic field.
Candidate sources of error in this respect include steel poles, fencing wires, motor vehicles, and power
lines.
Where repeated check observations do not yield a stable bearing you should suspect local
magnetic distortion and avoid taking readings in the area in question.
Remember that a compass yields a magnetic bearing: an angular measurement from local
magnetic north. If these readings are to be compared with others on a base map drawn with respect to
true north, then the appropriate conversion of bearings must be made.
The measurement of distance in a survey of this nature is required to fix positions on the traverse
as well as those of features on either side of it. Because these distances are measured by pacing, it is
necessary to first establish as accurately as possible the length of your pace. This is accomplished by
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Chapter 6: Field surveying
pacing a known distance over similar terrain to that being mapped . You should also carry with you the
same equipment that you will be using during the survey. The measured distance should be at least 50
50 metres
metres in length, in which case, pace length =
. During the course of a pace and
number of paces
compass survey all distances in the field should be measured and recorded as paces and not converted
to metres until later when the map is being plotted.
The traverse is located in the field by a combination of bearings and paced distances.
For
example, say that a map is required of the area shown in Figure 6.4A. Here the data for the map clearly
can be obtained by a traverse of the road.
Station 1 is established at the road junction and from here a bearing is taken along the road or
along the bordering fence line to the point at which the road changes direction (Station 2). It would be
helpful but not necessary to have a field assistant walk ahead of you to stand at the change in road
direction. The distance from Station 1 to Station 2 is then paced and the distance recorded. Together
these data (bearing and distance) are sufficient to plot this leg of the traverse on the map. Station 3, also
at a bend in the road, is located in the same manner, as is Station 4.
Location of detail is fixed by the principles noted earlier. Features such as the house at H1 which
are located well clear of the traverse route
can be located by intersection.
Since the bearing and
distance of each leg of the traverse is known, bearings to the house from Stations 1 and 2 will allow the
location of the house to be fixed and plotted. Houses H2 and H4 can be located in the same manner.
Checks on the location of these houses may be provided by additional bearings from a third station. For
example a third bearing to H1 could be obtained from Station 3 and to H4 from Station 4.
The angle of the intersecting bearings should always be between about 60o and 120o, and
preferably close to 90o, in order to minimize plotting errors. Outside of this angular bracket small errors in
plotting bearings will result in large errors in locational fix.
In the case of features which are close to the traverse, their locations can be fixed by offset. For
example, the house H3 in Figure 6.4A has been located in this manner by noting the traverse distance
from Station A (100 paces) and the normal offset distance of 15 paces. The bridge site also is located in
a similar manner by the traverse distance from Station 2 (a zero offset applies here and no bearing is
necessary). The tower at location T has been located by an offset from a point 60 paces from Station 3;
o
here the 36 pace offset is not at right-angles to the traverse but rather on a bearing of 165 .
Field notes constitute the record of a field survey and they should be simple and unambiguous.
For this reason it is important to record your observations in a standard format such as that depicted in
Figure 6.4B. A column centred on the page of a 'reporter' type notebook records the data necessary to
plot the traverse and the landscape features are recorded to the right or left of this column.
The number of each traverse station is shown within the column of the field notes. The bearing
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Chapter 6: Field surveying
to the next station is recorded immediately above each station and the distance in paces from the
previous station is recorded immediately below.
Landscape detail on either side of the traverse is shown on the respective side of the field notes.
In order to keep the notes uncluttered and legible, no more than two traverse legs should be recorded on
one page of the field notebook. The detail on either side of the traverse is shown by an arrow from each
station (or intermediate substation) from which the feature is observed accompanied by a precise
description of the feature, together with the magnetic bearing. If a number of similar features are being
mapped between two stations some sort of identifying code should be used to avoid confusion (for
example, several similar houses might be labeled H1, H2, H3, H4, etc., as in Figure 6.4).
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Plotting the traverse is a straightforward process involving the drafting of traverse legs of given
length and bearing. It always is a useful exercise to first sketch the traverse in order to see its shape and
overall size. At this point all the distances in paces should be converted to metres. When this is done an
appropriate scale can be selected for the final map. At each station a reference north line should be
constructed so that the bearings can be plotted. When all the stations are located and the features
plotted by intersection or offset, construction lines should be removed and the remaining detail inked;
addition of a title, scale, north point and legend complete the map.
The traverse described above is an open traverse in the sense that it starts at a particular station
and ends at a different station. This is commonly the type of traverse that is used to map linear strips of
landscape. A disadvantage of the open traverse, however, is that there is no way to assess the accuracy
of the survey unless the start and end points can be located independently on a pre-existing base map.
In the case of a closed traverse, however, the closure itself is a measure of the accuracy of the survey.
Consider the traverse shown in Figure 6.5A. Here a long closed traverse was surveyed from
Station 1 to Station 1 through five intermediate stations and when the traverse was plotted up from the
6.5: Compass traverses with (A) a perfect closure and (B) misclosure and correction.
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Chapter 6: Field surveying
field notes the closure was found to be perfect. That is, the plot from Station 6 ends exactly on the start
point at Station 1. This circumstance of perfect traverse closure is only possible if there are no errors in
the initial field survey.
Because of instrument limitations and human error, however,
this particular
outcome is very rare indeed! If there are errors the traverse will not close and a correction must be made
to the plotted results.
If the misclosure is small it sometimes may be possible to attribute the whole error to one leg of
the traverse. For example, one of the legs may have been surveyed through difficult terrain or under
power lines and represents a likely candidate for most of the error. In most cases, however, it is not
possible to identify the precise source of error and it is usual to assume that it has accumulated over the
whole length of the traverse in proportion to the length of each separate leg.
Figure 6.5B presents a solution to such a misclosure. Here a closed traverse that should have
returned to A is in error by the distance AF (that is, A and F should coincide). The steps for closure are:
(a) Join AF and draw lines parallel to AF through stations B through E.
(b) Draw a line ABCDEF to represent the total length of the traverse with AB representing the first leg,
BC the second, and so on. This line ABCDEF can be any length but the relative leg lengths must be
correctly proportioned.
(c) At F construct a perpendicular FA equal in length to the misclosure and join AA. The distances BB',
CC', DD', EE' then represent the error accumulated at Stations A,B,C,D, and E, respectively.
(d) These distances are transferred to the traverse plot. F already is in the correct position A while the
remaining stations must be adjusted in the direction FA by the distances indicated.
(e) The corrected traverse is represented by the new closed traverse AB'C'D'E'. Once the traverse is
corrected the detail from these stations and from any substations then can be plotted on the map. Some
error in a closed traverse almost always will be present and will need correction. It also is rather
obvious, of course, that substantial misclosure implies errors that can only be truly corrected by repeating
the pace and compass survey.
The plane-table survey is a very efficient and accurate method for constructing maps in the field.
Unlike chain triangulation and the pace and compass traverse in which measurements are recorded in the
field and plotted up in the office, plane tabling involves simultaneous measurement and direct plotting of
field data in one procedure.
The plane table is a drawing board centrally mounted on a tripod so that it can be rotated in the
horizontal plane. Plotting paper is attached to the top of the table and all bearings are recorded directly
on the paper through the use of an alidade or sighting rule. The alidade serves the same function as the
compass in a pace and compass survey. It is about 30 cm in length and has a sighting arm at each end
with sighting hairs in a line parallel to the ruling edge. Some plane tables also come equipped with bubble
levels for leveling and with a built-in compass for orientation.
Although mapping with a plane table will be demonstrated in class it may provide a useful
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Chapter 6: Field surveying
6.6: The plane-table survey
preamble to consider briefly how a plane-table survey is conducted. The steps in mapping the locations
of points A, B, C, D, and E in the area depicted in Figure 6.6 are as follow:
(a) The plane table is set up in the field at Station 1 and oriented so that its long axis lies along the field
base line between Stations 1 and 2.
(b) Station 1 is plotted (arbitrarily) on the plane table and the edge of the alidade is set against this
plotted position of Station 1 while at the same time taking a sight on Station 2 in the field. The base line
through Station 1 towards Station 2 is drawn on the plane table.
(c)
In turn each of the points A through E are sighted with the alidade edge on the plotted position of
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Chapter 6: Field surveying
Station A and rays drawn on the plane table to indicate their direction (bearing). Each ray is labeled
clearly to indicate its target.
(d) When all the target location rays are duly recorded and labeled, Station 1 is clearly marked with a
survey rod and then the plane table is moved to station 2, as shown in Figure 6.6. The distance between
Stations 1 and 2 is measured by pacing or by tape and recorded.
(e) At Station 2 the plane table is reset in a horizontal position and oriented so that the base line on the
plane table and that between Stations 1 and 2 on the ground are in line. This is achieved by placing the
edge of the alidade along the base line and rotating the plane table until the two hairs of the alidade
sighting window are in line with Station 1; the plane table is then locked in position.
(f) Station 2 is then plotted on the plane table. Its plotting position on the base line determines the scale
of the map and should be chosen carefully.
(g) With the alidade edge on the plotted position of Station 2, rays are again drawn through the positions
A to E, as before. These points are now automatically located by intersection on the plane-table map.
(h) This procedure can now be repeated for a third and subsequent stations, as required, in much the
same way as we might undertake a pace and compass survey.
The plane table, tripod, and alidade clearly are less portable than a compass but nevertheless,
the equipment is light enough to carry considerable distances into areas which are inaccessible to
vehicles. It provides an inexpensive means of producing quite accurate field maps with only minimal
surveying effort.
The third dimension can be added to a field map in several ways. Some alidades have an
adjustable forward cross hair which, if set on a distant object, can read an angle of declination or
inclination. More sophisticated versions have a telescope (a telescopic alidade) with stadia that allow
distances to be read directly from a surveyor's staff.
We will consider just two instruments designed for this purpose: the abney level and the dumpy
level. The first is a small highly portable hand-held instrument which nicely illustrates the principle of
angle-based height measurement, and the other is a somewhat more cumbersome but relatively
sophisticated and distinctly more accurate instrument which makes use of stadia for distance
measurement.
The abney level is a small hand-held instrument designed to measure angles in the vertical plane.
By sighting through the abney-level eyepiece an object can be located and aligned with a cross-hair.
Subsequent adjustment of the level also aligns a leveling bubble and ensures that an adjustable datum on
the level is in a horizontal position. The angle between the observer and the object can then be read
directly from the level. If the observer is above the object the angle is one of declination; if the observer is
below the object the angle is one of inclination.
If other information is available, then object heights can be determined graphically or by using
simple trigonometric relations. For example, consider the cases in Figure 6.7; if a survey of a rock
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Chapter 6: Field surveying
platform backed by a sea cliff is being undertaken, it may be necessary to determine the height of the sea
cliff above the platform, as shown in Figure 6.7A. If the observer paces out a known distance from the
base of the cliff and determines with an abney level the angle BOC, height BC can readily be obtained. If
the angle of inclination is 20o, for example, then
o
tan 20 =
BC
OC
and BC = OC tan 20o
= (200)(0.364)
= 72.8 metres
Figure 6.7B depicts a case where a survey
party needs to establish the height of a point B
below an observer at O.
The distance AB is
paced and found to be 300 metres and an abney
level reads a 15o declination angle from O to B.
The drop CB is calculated readily from:
sine 15o =
CB
,
300
from which it follows that
CB = 300 sine 15o
6.7: Determining heights and drops by
measurement of inclination and declination of
angles. See text for explanation.
= (300)(0.259) = 77.6 metres.
The dumpy level (sometimes called an engineer's level), takes a variety of forms but all these
instruments are based on the same principles of measurement. Some have a self-leveling device while
others have to be leveled manually.
Some have a telescope with a single cross-hair for height
measurement on a surveyor's graduated staff while others have a telescope with a full stadia between
which measurements indicate horizontal distance as well as elevation. All are mounted on a tripod which
will allow rotation of the instrument in the horizontal plane. Setting up and using the instrument will be
explained in class. Here we will be concerned only with the principle of the dumpy-level survey.
The measurement of a simple height difference with a dumpy level is illustrated in Figure 6.8.
The level is set in a perfectly horizontal position at A and the graduated staff is held by the 'staffperson' at
B and C, in turn. At each location height readings on the staff are sighted and recorded. The vertical
height between stations B and C is given by the difference between the two dumpy-level readings. In the
case shown in Figure 6.8B the dumpy level must be rotated in the horizontal plane in order to read the
height on the staff in the two diametrically opposed positions B and C. As in the former case, the height
of B above C is given by the difference in the two readings on the staff.
By combining movement of both the level and the staff, a traverse can be surveyed along a
continuous line, as depicted in Figure 6.9. The dumpy level is set horizontally at B and a reading is made
to the staff at A. The horizontal distance between B and A is measured by tape to a plumb bob on the
level or by the difference in readings at the upper and lower stadia in the telescope sight. The staffperson
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Chapter 6: Field surveying
6.8: Taking elevation sightings with a dumpy level and staff.
6.9: The dumpy-level traverse (see the text for explanation).
moves the staff to a new position at C and the surveyor rotates the dumpy level to take the next
reading. Because the reading from B to C is in the direction in which the traverse is progressing, it is
called a forward sight . Once the height reading from the forward sight is recorded and the distance
between B and C noted, the staffperson stays at C while the surveyor moves the dumpy level to the new
sighting location at D. Here the instrument must be set up and leveled once again before any further
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Chapter 6: Field surveying
6.10: Sample page from a dumpy-level survey. Note that an arbitrary datum is set (in this case, 100 m)
and that positive height differences imply a positive (uphill) slope while negative height differences
indicate a negative (downhill) slope.
measurements can be made. The surveyor then obtains another reading to the staff at C . Thus, there
are now two readings to the staff at C, one from a forward sight and this second reading from a backsight.
Once the distance CD is measured and recorded, the staffperson then moves the staff to a new position
at E while the surveyor stays at D and rotates the dumpy level to make a new forward sight to the next
station (E). This procedure is repeated for station G and for as many others as required.
The readings from forward sights and back sights need to be carefully recorded in a field
notebook so that reduced levels later can be calculated for the traverse. A sample page from a field
notebook listing a set of observations from a dumpy-level traverse is shown in Figure 6.10.
The upper set of measurements recorded in Figure 6.10 include distances obtained from a field
measuring tape. In the lower set of measurements distances have been obtained directly from the three
stadia visible in the engineer’s level (Figure 6.11). The upper and lower stadia typically are designed to
provide a 1:100 ratio between the stadia interval on the staff and the distance on the ground. In Figure
6.11 the staff is marked in tenths of a metre and the upper and lower stadia readings are respectively
1.965 m and 1.715 m. The difference (or stadia interval) is 1.965-1.715 = 0.250 m indicating that the
engineer’s level is 25.0 m from the staff (100 x 0.250 m).
This level of precision for distance
measurements (to the nearest centimetre) implies stadia measurements to the nearest millimetre.
Similarly, stadia measurements to the nearest centimetre will yield distances to the nearest metre.
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Chapter 6: Field surveying
If stadia measurements are being used
to determine both elevations and distances, all
three stadia measurements must be recorded in
the field notes as shown in the lower data set in
Figure 6.10. Obviously care must be taken when
entering the data in the field.
It is difficult to
overstate the importance of keeping neat and
tidy and well organized field survey notes. With
few exceptions most errors in field surveying
occur at this most important step of recording the
observations. A hurried and unclear notation in
the field can become a major stumbling block to
successfully completing the survey back in the
office.
Surveying is often a team effort and in the case of a dumpy-level survey the level operator must
ensure that the instrument is horizontal and that readings are obtained and recorded accurately. The
staffperson or rodperson must ensure that the staff is vertical and on the same point on the ground at all
times during forward sight and backsight measurements. Keeping the staff vertical is not always easy but
a slight fore-and-aft rocking motion of the staff can ensure that the level operator reads the lowest staff
measurement for each stadia (the staff is vertical at the point of lowest measurement).
The most sophisticated survey levels allow data to be entered in a data logger electronically in the
field and elevation reductions are calculated automatically by an on-board computer. Still more advanced
instruments obtain data using laser orientation and reflection characteristics. But these instruments are
expensive and may not be available or economical to use for small mapping projects.
A simple
engineer’s level together with a computer spreadsheet for computations provides a fast and relatively
inexpensive means of producing very accurate large-scale topographic maps.
Global positioning systems (GPS) may be combined with any of the survey techniques described
above to provide absolute positioning in the field (geographic or UTM coordinates) and potentially,
distance and elevation measurements as well.
GPS is based on a constellation of navigation satellites orbiting the earth. The precise time and
position information transmitted by these satellites is used by a GPS receiver to triangulate a position fix.
The system essentially was completed in 1994 and provides continuous three-dimensional coverage
anywhere on earth.
It was developed for, and is administered by, the United States Department of
Defense to provide consistent and reliable navigation information that is unaffected by the roughness of
terrain or the state of the weather or other sources of interference. Although GPS was designed as a
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military navigation system, its civilian and commercial uses have been recognized and were
accommodated early in its development. Satellites transmit two codes: a military-only encrypted code
(PPS) and a civilian-access, Standard Positioning Service (SPS) code. All commercial and consumer
GPS receivers are SPS receivers.
Each GPS satellite transmits its precise location (position and elevation) and the start time of the
transmission. A GPS receiver acquires the signal, then measures the interval between transmission and
receipt of the signal to determine the distance between the receiver and the satellite (this is known as
ranging). Once the receiver has computed the range for at least three satellites, its location on the
surface of the earth can be determined.
Each satellite transmits two types of data, almanac and ephemeris. Almanac data are general
information on the location and status of each satellite in the system.
Since it contains general
information, an almanac can be collected from any satellite. A receiver with a current almanac in its
memory knows where in the sky to look for satellites, given its last known position and time of the day.
Ephemeris data are precise satellite positioning information used for ranging. Each satellite transmits its
own ephemeris data. Both almanac and ephemeris data are required for a GPS receiver to locate and
acquire satellites quickly and compute a position fix.
Positioning with a GPS receiver intended for general use was originally designed to produce
accuracies of ±25 m. Although the US Department of Defense originally degraded the GPS signal for
public use, the system was changed in 2000 so that now the public and military GPS accuracy is
essentially the same (about ±8 m).
GPS receivers vary considerably in price and quality. Small hand-held GPS receivers costing just
a few hundred dollars are now widely available. These instruments usually have one channel and cycle
through the satellites sequentially. They are excellent for obtaining positions at a stationary location in
geographic or UTM coordinates although elevations are usually not sufficiently accurate to be useful for
most mapping applications.
More expensive multichannel receivers lock onto several satellites
simultaneously and give more rapid and consistently reliable position fixes even from a rapidly moving
site (such as a boat or automobile). All GPS receivers are programmed as aids to navigation and allow
inputs of waypoints and will compute distances and directions and travel times between waypoints. In
recent years GPS capacity has been built into most cars, smart phones, and into some wrist watches and
into other domestic and recreational electronic devices.
GPS signal quality is degraded by cover (such as trees) so GPS works best in view of an open
sky. It also is dependent on satellite configuration so that, in Vancouver for example, GPS positioning is
more reliable in the morning (when most satellites are directly overhead) than in the late afternoon (when
most satellites are clustered at a low angle near the horizon).
Like any other surveying technique, GPS is useful for certain applications (for example, the
smaller the scale, the less important the ±8 m positioning error) but not for others and it is up to the field
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Chapter 6: Field surveying
mapper to exercise sound judgment in selecting the technique most appropriate to the mapping problem
at hand.
Contour plotting
Once heights have been established for locations in the area being surveyed they can be added
to a map in the usual way. If only a few prominent features have been surveyed their elevations might be
shown as spot heights.
If a more dense pattern of heights has been determined it may be
appropriate to generalize the information in a contour map.
In the latter case a contour interval
appropriate to the local relief and the spot-height density of the survey must be selected and contours
then are located as interpolated isolines.
The most straightforward way to plot contours through a field of spot heights is based on linear
interpolation; the procedure is illustrated in Figure 6.12.
The task of contouring generally is simpler if the
contour defining the highest region is the first to be drawn; it is often the shortest and it provides a guide
to the shape of the next contour in sequence. In Figure 6.12 the highest point is160 m and the highest
contour is arbitrarily set at 100 m. To establish the locus of the 100 m elevation, interpolation points are
located along all interpolation lines. Interpolation lines join adjacent spot heights which bracket 100 m.
For example, consider the 160 m spot height and the interpolation line to the 43 m spot height. The
elevation difference between these two points is 160 m - 43 m = 117 m. Since the 100 m point is 57 m
57
above the 43 m spot height, the 100 m contour crossing must be located
= 0.49 x length of the
117
interpolation line from the 43 m spot height, a distance of about 1 cm on the map (2 cm x 0.49) in Figure
6.12: Contouring based on linear interpolation among a field of spot heights.
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Chapter 6: Field surveying
6.12. Similarly, the interpolation line between 147 m and 92 m must be crossed by the 100 m contour at
100-92
8
=
= 0.15 x length of the interpolation line from the 92 m spot height, a distance of about 2
147-92
55
mm on the map (1.3 cm x 0.15), and so on.
When the interpolation is repeated for all interpolation lines the locus of the 100 m elevation
should be defined well enough to draw the contour. The same procedure is followed for other contour
lines until the map is completed.
Obviously this contouring procedure can become very tedious on a map of complex topography!
In the case of simple hand-drawn maps the tedium can be reduced by estimating the interpolated contour
intersections rather than measuring them (with a little practice we are all capable of accurately estimating
10ths of the length of an interpolation line). In the case of more complex maps it may be worthwhile
digitizing the spot heights (x, y and z coordinates) for use in a computer contouring program.
It is
important to remember, however, that almost all computer mapping programs are based on the principles
of linear extrapolation described above.
Like most other aspects of practical cartography, field surveying has to be done to become a skill. This
chapter is merely a preamble to the real thing is best combined with 'hands-on' tasks.
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