An Introduction To Horizontal Control Survey Techniques: J. Paul Guyer, P.E., R.A
An Introduction To Horizontal Control Survey Techniques: J. Paul Guyer, P.E., R.A
An Introduction To Horizontal Control Survey Techniques: J. Paul Guyer, P.E., R.A
Horizontal Control
Survey Techniques
1. INTRODUCTION
2. TRADITIONAL HORIZONTAL CONTROL SURVEY TECHNIQUES
3. SECONDARY OR TEMPORARY HORIZONTAL CONTROL
4. BEARING AND AZIMUTH DETERMINATION
5. ELECTRONIC DISTANCE MEASUREMENT
6. COORDINATE COMPUTATIONS
7. TRAVERSE SURVEYS
8. TRAVERSE SURVEY GUIDELINES
9. TRAVERSE COMPUTATIONS AND ADJUSTMENTS
10. TRAVERSE ADJUSTMENT (COMPASS RULE)
11. TRIANGULATION AND TRILATERATION SURVEYS
(This publication is adapted from the Unified Facilities Criteria of the United States government which are
in the public domain, have been authorized for unlimited distribution, and are not copyrighted.)
(Figures, tables and formulas in this publication may at times be a little difficult to read, but they are the
best available. DO NOT PURCHASE THIS PUBLICATION IF THIS LIMITATION IS UNACCEPTABLE
TO YOU.)
1. INTRODUCTION
2.1.3.1 MARKERS. Project horizontal control points should be marked with semi-
permanent type markers (e.g., re-bar, railroad spikes, or large spikes). If concrete
monuments are required, they will be set prior to horizontal survey work.
2.1.3.2 INSTALLATION. Horizontal control points should be placed either flush with the
existing ground level or buried a minimum of one-tenth of a foot below the surface.
2.1.8 TARGETS. All targets established for backsights and foresights should be fixed
and centered directly over the measured point. Target sights may be a reflector or other
type of target set in a tribrach, a line rod plumbed over the point in a tripod, or
guyed/fixed in place from at least three positions. Artificial sights (e.g., a tree on the hill
behind the point) or hand held sights (e.g., line rod or plumb bob string) should not be
used to set primary control targets.
2.1.9 CALIBRATION. All theodolites, total stations, EDM, and prisms used for
horizontal control work should be serviced regularly and checked frequently. Tapes and
EDMs must be periodically calibrated over lines of known length, such as NGS
calibration baselines. Instrument calibrations should be done at least annually.
Theodolite instruments should be adjusted for collimation error at least once a year and
whenever the difference between direct and reverse reading of any theodolite deviates
more than thirty seconds from 180 degrees. Readjustment of the cross hairs and the
level (plate) bubble should be done whenever misadjustments affect the instrument
reading by more than the least count of the reading scales of the theodolite. Forced
centering type tribrachs should be periodically (monthly) checked to ensure the optical
plumb line is correct. Circular or "bulls eye" bubbles on tribrachs, total stations, rods,
etc. should be periodically checked and adjusted. Tribrach or total station optical
plummets (visual or laser) must be periodically checked.
Title Page:
• Instrument make, model, and serial number.
• Instrument operator’s name.
• Recorder’s name.
• Weather description.
o Temperature.
o General atmospheric condition.
o Wind.
Entry Page:
• Designation of the occupied station.
o Full station name.
o Year established.
o Name of the agency on the disk.
The field book or recording form should include the above information for each station
observed. If an instrument, signal, or target is set eccentric to a station (not plumbed
directly over the station mark), that item should be sketched on the recording form. The
sketch should include the distance and the directions that the eccentric item is from the
station. When intersection stations are observed, the exact part of the point observed
must be recorded and shown on the sketch.
Figure 3-3
Sample horizontal field book recording--Directional Theodolite
3.2.3 IF A REPEATING THEODOLITE is used for the horizontal angles, the angle
measurement should be repeated a minimum of two times by alternating the telescope
and pointing in the direct and inverted positions.
3.2.4 IF A DIRECTIONAL THEODOLITE is used for the horizontal angles, the process
(described for primary control) should be repeated two times--for a total of two data set
collections.
Interior angles. If angles in a closed figure are to be measured, the interior angles
are normally read. When all interior angles have been recorded, the accuracy of
the work can be determined by comparing the sum of the abstracted angles with
the computed value for the closed loop (Figure 3-4 below).
Deflection angles. In an open traverse (Figure 3-4), the deflection angles are
measured from the prolongation of the backsight line to the foresight line. The
angles are measured either to the left or to the right. The direction must be
shown along with the numerical value.
Vertical angles. Vertical angles can be referenced to a horizontal or vertical line
(Figure 3-5).
Optical-micrometer theodolites measure vertical angles from the zenith (90° or 270°
indicate a horizontal line). Zenith and nadir are terms describing points on a sphere. The
zenith point is directly above the observer, and the nadir point is directly below the
observer. The observer, the zenith, and the nadir are on the same vertical line.
4.1 BEARING TYPES. The bearing of a line is the direction of the line with respect to a
given meridian. A bearing is indicated by the quadrant in which the line falls and the
acute angle that the line makes with the meridian in that quadrant. Observed bearings
are those for which the actual bearing angles are measured, while calculated bearings
are those for which the bearing angles are indirectly obtained by calculations. A true
bearing is made with respect to the astronomic north reference meridian. A magnetic
bearing is one whose reference meridian is the direction to the magnetic poles. The
location of the magnetic poles is constantly changing; therefore the magnetic bearing
between two points is not constant over time. The angle between a true meridian and a
magnetic meridian at the same point is called its magnetic declination. An assumed
bearing is a bearing whose prime meridian is assumed. The relationship between an
assumed bearing and the true meridian should be defined, as is the case with most
SPCS grids.
4.3 AZIMUTH TYPES. The azimuth of a line is its direction as given by the angle
between the meridian and the line, measured in a clockwise direction. Azimuths can be
referenced from either the south point or the north point of a meridian. (Geodetic
azimuths traditionally have been referenced to the south meridian whereas grid
azimuths are referenced to the north meridian). Assumed azimuths are often used for
making maps and performing traverses, and are determined in a clockwise direction
from an assumed meridian. Assumed azimuths are sometimes referred to as "localized
grid azimuths." Azimuths can be either observed or calculated. Calculated azimuths
consist of adding to or subtracting field observed angles from a known bearing or
azimuth to determine a new bearing or azimuth.
Figure 3-6
Geometry of an EDM measurement
Alternatively, the elevations of the occupied hubs (Stations A and B in Figure 3-6 above)
may have been determined by differential levels. Applying the measured HI and HT
yields the absolute elevation of the instrument and target. The measured slope distance
"S" can then be reduced to a horizontal distance "H" given the delta elevation between
the instrument and target. A meteorological correction is applied to the observed slope
distance before reducing it to horizontal. Subsequently, the horizontal distance is
corrected for grid scale and sea level. A traditional field book example of a horizontal
slope distance observation is shown in Figure 3-7 below. In this example, slope
distances are manually recorded along with meteorological data. A series of 10 slope
Figure 3-7
Horizontal distance observations and reductions--manual computations in field book
5.1 ERRORS. Distances measured using an EDM are subject to the same errors as
direction measuring equipment. The errors also include instrumental component errors.
Instrumental errors are usually described as a number of millimeters plus a number of
ppm. The accuracy of the infrared EDM is typically ± (5 millimeters + 5 ppm). The ppm
© J. Paul Guyer 2017 17
accuracy factor can be thought of in terms of millimeters per kilometer, as there are 1
million millimeters in 1 kilometer. This means that 5 ppm equals 5 millimeters per
kilometer. Errors introduced by meteorological factors must be accounted for when
measuring distances of 500 meters or more. Accurate ambient temperature and
barometric pressure must be measured. An error of 1 degree Celsius (C) causes an
error of 0.8 ppm for infrared distances. An error of 3 millimeters of mercury causes an
error of 0.9 ppm in distance.
• The use of a prism typically provides an indicated distance longer than the true
value. Applying a negative correction will compensate for this effect. Each prism
should have its own constant or correction determined individually, and a master
file should be maintained.
• An instrument constant can be either positive or negative and may change due to
the phase shifts in the circuitry. Therefore, a positive or a negative correction
may be required.
• The algebraic sum of the instrument and the prism constants are referred to as
the total constant. The correction for the total constant (equal in magnitude but
opposite in sign) is referred to as the total constants correction, from which the
instrument or prism constant can be computed if one or the other is known.
Figure 3-8
Forward Position Computation
If the traverse leg falls in the first (northeast [NE]) quadrant, the value of the easting
increases as the line goes east and the value of the northing increases as it goes north.
The product of the dE and the dN are positive and are added to the easting and
northing of Station A to obtain the coordinate of Station B, as shown in Figure 3-8.
When using trigonometric calculators to compute a traverse, enter the azimuth angle,
and the calculator will provide the correct sign of the function and the dN and the dE. If
the functions are taken from tables, the computer provides the sign of the function
based on the quadrant. Lines going north have positive dNs; lines going south have
negative dNs. Lines going east have positive dEs; lines going west have negative dEs.
The following are examples of how to compute the dN and the dE for different
quadrants:
7.1 TRAVERSE TYPES. There are two basic types of traverses, namely, closed
traverses and open traverses.
7.1.1 CLOSED TRAVERSE. A traverse that starts and terminates at a station of known
position is called a closed traverse. The order of accuracy of a closed traverse depends
upon the accuracy of the starting and ending known positions and the survey methods
used for the field measurements. There are two types of closed traverses.
7.1.1.1 LOOP TRAVERSE. A loop traverse starts on a station of known position and
terminates on the same station--e.g., Station A in Figure 3-9 above. An examination of
the position misclosure in a loop traverse will reveal measurement blunders and internal
loop errors, but will not disclose systematic errors or external inaccuracies in the control
point coordinates. In a loop traverse, the measured angular closure is the summation of
the interior or exterior horizontal angles in the traverse. If there are "n" sides in a loop
traverse, and interior angles were measured, the true angular closure should equal (n-2)
· 180º. If exterior angles were measured when performing a loop traverse, the true
angular closure should equal (n+2) ·180º. In Figure 3-9 above, the starting azimuth from
Station "A" is not shown. This initial azimuth might have been taken from a GPS,
7.1.2 OPEN TRAVERSE. (Figure 3-10 below). An open traverse starts on a station of
known position and terminates on a station of unknown position. With an open traverse,
there are no checks to determine blunders, accidental errors, or systematic errors that
may occur in the measurements. The open traverse is very seldom used in topographic
surveying because a loop traverse can usually be accomplished with little added
expense or effort.
8.1.2 ACCURACY REQUIREMENTS. Control traverses are run for use in connection
with all future surveys to be made in the area of consideration. They may be of Second,
8.1.4 TRAVERSE ROUTE. The specific route of a new traverse should be selected with
care, keeping in mind its primary purpose and the flexibility of its future use. Angle
points should be set in protected locations if possible. Examples of protected locations
include fence lines, under communication or power lines, near poles, or near any
permanent concrete structure. It may be necessary to set critical points below the
ground surface. If this is the case, reference the traverse point relative to permanent
features by a sketch, as buried points are often difficult to recover at future dates. Select
sites for traverse stations as the traverse progresses. Locate the stations in such a way
that, at any one station, both the rear and forward stations are visible. The number of
stations in a traverse should be kept to a minimum to reduce the accumulation of
instrument errors and the amount of computing required. Short traverse legs (courses or
sections) require the establishment and use of a greater number of stations and may
cause excessive errors in the azimuth. Small errors in centering the instrument, in
8.1.5 TEMPORARY HUBS. Temporary station markers are usually 2x2-inch wooden
hubs, 6 inches or more in length. These hubs should be driven flush with the ground,
especially in maintained areas or where the hubs could present a hazard. The center of
the top of the hub is marked with a surveyor’s tack or an "X" to designate the exact point
of reference for angular and linear measurements. To assist in recovering a station, a
reference stake (e.g., a flagged 1 x 2 inch wood stake) may be set near the hub. The
reference stake should be marked with the traverse station designation, stationing,
offset, etc.—as applicable.
8.1.7 FIELD DATA REDUCTIONS. All survey field notes should be carefully and
completely reduced; with the mean angle calculated in the field and recorded along with
the sketch. All traverse adjustments should be made in the office unless this capability
is available on the data collector in the field. A sketch of the permanent monument
locations should be made in the field and a detailed description on how to recover them
should be recorded in writing. This information can be used for making subsequent
record of the survey monument and survey report. Temporary monuments need only be
briefly described in the field notes
9.1 CRANDALL RULE. The Crandall rule is used when the angular measurements
(directions) are believed to have greater precision than the linear measurements
(distances). This method allows for the weighting of measurements and has properties
similar to the method of least squares adjustment. Although the technique provides
adequate results, it is seldom utilized because of its complexity. In addition, modern
distance measuring equipment and electronic total stations provide distance and
angular measurements with roughly equal precision. Also, a standard Least Squares
adjustment can be performed with the same amount of effort.
9.2 COMPASS RULE. The Compass Rule adjustment (also called the Bowditch
Method) is used when the angular and linear measurements are of equal precision. This
is the most widely used traverse adjustment method. Since the angular and linear
precision are considered equivalent, the angular error is distributed equally throughout
the traverse. For example, the sum of the interior angles of a five-sided traverse should
equal 540º 00' 00".0, but if the sum of the measured angles equals 540° 01' 00".0, a
value of 12".0 must be subtracted from each observed angle to balance the angles
within traverse. After balancing the angular error, the linear error is computed by
determining the sums of the north-south latitudes and east-west departures. The
misclosure in latitude and departure is applied proportional to the distance of each line
in the traverse.
9.3 LEAST SQUARES. The method of least squares is the procedure of adjusting a set
of observations that constitute an over-determined model (redundancy > 0). A least
squares adjustment relates the mathematical (functional model) and stochastic
(stochastic model) processes that influence or affect the observations. Stochastic refers
to the statistical nature of observations or measurements. The least squares principle
relies on the condition that the sum of the squares of the residuals approaches a
minimum.
9.3.2 STOCHASTIC MODEL. The stochastic model is the greatest advantage of the
least squares procedure. In least squares adjustment, the surveyor can assign weights,
variances, and covariance information to individual observations. The traditional
traverse balancing techniques do not allow for this variability. Since observations are
affected by various errors, it is essential that the proper statistical estimates be applied.
9.3.3 OBSERVATIONS. Observations in least squares are the measurements that are
to be adjusted. An adjustment is not warranted if the model is not over-determined
(redundancy = 0). Observations vary due to blunders and random and systematic
errors. When all blunders and systematic errors are removed from the observations, the
adjustment provides the user an estimate of the “true” observation.
9.3.4 BLUNDERS. Blunders are the result of mistakes by the user or inadvertent
equipment failure. For example, an observer may misread a level rod by a tenth of a
foot or a malfunctioning data recorder may cause erroneous data storage. All blunders
9.3.7 REFERENCES. Many field data collectors are capable of performing Least
Squares traverse adjustments; thus, simple traverses are more frequently being
adjusted by this method.
10.1 GENERAL. Traverse computations and adjustments require the following steps:
10.2.2 AZIMUTH ADJUSTMENT. The Compass Rule is based on the assumption that
angular errors have accumulated gradually and systematically throughout the traverse.
The angular correction is then distributed systematically (equally) among the angles in
the traverse. Refer to the "Balanced Angle" column in the example at Figure 3-11 below
where a 4-second misclosure was distributed equally.
The position misclosure (after azimuth adjustment) can then be distributed among the
intermediate traverse station based on the adjustment rule being applied. For the
Compass Rule, the latitude and departure misclosures are adjusted in proportion to the
length of each traverse course divided by the overall traverse length. For any traverse
leg with length dX (departure) and dY (latitude) in each coordinate, and with a final
misclosure after azimuth adjustment of " ΔX " and " ΔY ", the corrections to the dX or dY
lengths are adjusted by:
Once the above corrections are applied to the latitudes and departures in each traverse
course, the adjusted length and direction of each course can be computed, along with
the final adjusted coordinates of each intermediate point. (These final computations are
not shown in Figure 3-11).
10.2.4 ADJUSTMENT TECHNIQUES. In the past, the above adjustment was performed
using a tabular form that was laid out to facilitate hand calculation of the angular and
Figure 3-11
Tabular computation format for a Compass Rule traverse adjustment
11.2 NETWORKS. When practicable, all triangulation and trilateration networks should
originate from and tie into existing coordinate control of equal or higher accuracy than
the work to be performed. An exception to this would be when performing triangulation
or trilateration across a river or some obstacle as part of a chained traverse. In this
case, a local baseline should be set. Triangulation and trilateration surveys should have
adequate redundancy and are usually adjusted using least squares methods.
11.3 ACCURACY. Point closure standards listed in Chapter 4 must be met for the
appropriate accuracy classification to be achieved. If project requirements are higher-
order, refer also to the FGCS "Standards and Specifications for Geodetic Control
Networks" (FGCS 1984).
11.4.1 LOCATION. Points for observation should be selected to give strong geometric
figures, such as with angles between 60 and 120 degrees of arc.
11.4.2 REDUNDANCY. If it is possible to sight more than three control points, the extra
points should be included in the figure. If possible, occupy one of the control stations as
a check on the computations and to increase the positioning accuracy. Occupation of a
control station is especially important if it serves as a control of the bearing or direction
of a line for a traverse that originates from this same point.
11.4.3 MEASUREMENTS. Both the interior and exterior angles should be observed and
recorded. The sum of these angles should not vary by more than three (3) arc-seconds
per angle from 360 degrees. Each angle should be turned not less than 2-4 times (in
direct and inverted positions).