Babaie, Hassan A.,
2001, The Brunton® Compass and Geological Objects. Georgia
Geological Society Guidebooks, v. 21, No. 1, October, p.55-60.
Copyright by: H. A. Babaie,
Dept. of Geology, Georgia State Univ., Atlanta, GA 30303
Introduction
The Brunton® compass is used by more geologists for field mapping
of geological objects than other brands. This preference, especially in
North America, is because Brunton provides a precise sighting-clinometer
and hand level capability, and can be used at both waist and eye levels;
advantages that are absent in other brands such as Silva which lacks a
leveling system for sighting bearings (Compton, 1985). Detailed measurement
of geological objects, such as fold hingeline, axial trace, and axial
plane, and geological mapping becomes essentially impractical without
the use of the compass (i.e., Brunton). In this paper, we will review
the application of the Brunton compass in the measurement of a variety
of planar and linear geological features (e.g., structural, sedimentary,
stratigraphic), and discuss the use of the compass in mapping and measurement
of stratigraphic sections, vertical angles, height, etc. Some discussions
are given in the context of the lower hemisphere stereographic projection
of the geological objects for the sake of clarity and practical value.
The Brunton Compass (Pocket Transit)
The Brunton compass was originally designed by a Canadian geologist named
D.W. Brunton, and built by William Ainsworth Company in Denver, Colorado.
Despite its tough design, its delicate mirror and glass components are
vulnerable to shock and moisture (if not water proof), requiring care
and periodic maintenance for proper application. See Compton, 1985 for
maintaining the compass. Since 1972, genuine Bruntons are manufactured
by the Brunton Company in Riverton, Wyoming, which was acquired by Silva
Production, AB of Sweden in 1996. Cheaper Chinese, Japanese, and German
copies of the Brunton design are also available in the market.
The Brunton Compass
Brunton compasses have three main parts, box, sighting arm, and lid. The
box contains most of the components: the needle; bull's eye level (round
level to read horizontal angles); clinometer level (barrel-shaped) and
clinometer scale (for reading vertical angles); damping mechanism (to
more efficiently stabilizing the needle); lift pin (to lock the needle);
side brass screw and index pin (to set and display the declination); graduated
circle or card (to read the bearing). The needle has two ends: the north-seeking
end (commonly white in genuine Brunton compasses, labeled 'N' in others),
and the black, south-seeking end. The north-seeking end of the needle
is pulled down in the northern hemisphere where the magnetic inclination
is downward. An additional small weight attached to the south-seeking
end of the needle provides proper balancing of the needle. The weight
needs to be reversed if using the compass in the southern hemisphere where
the magnetic inclination is upward.
The lid, attached to the box with a hinge, contains the mirror with the
axial line and oval sighting window (for waist- and eye-level sighting),
and the sight. The long sighting arm, attached to the box with a hinge,
has a long, oval rectangular cutout or slot (for reading linear objects),
and a tiltable sighting tip, which is used for aligning the line of sight.
The circle card of the Brunton compass is designed in two traditional
scales. The azimuth scale uses three digits, with north at 000o or 360o,
and south at 180o. The quadrant scale uses an alphanumeric notation (e.g.,
N60oE, S20oW) with the card graduated in four 90o quadrants (NE, SE, SW,
NW); north and south lie at the two upper and lower 0o marks, respectively.
The direction of a line on the ground is given by the bearing of the line,
which is the horizontal angle between the line and a reference, commonly
north in the quadrant scale, or 000o (marked as 0o on the card) in the
azimuth scale. The reference, however, can also be the south (S) in the
quadrant scale, when reading the bearing (i.e., trend) of south-trending
linear objects. The position of 'E' and 'W' are reversed on the circular
card; 'E' lies left of the 0o mark (i.e., at 9 o'clock), and 'W' is to
the right of the 0o (i.e., 3 o'clock) mark on the card. The reversal is
designed to make the correct reading of the bearing possible. To appreciate
this fact, notice that the north-seeking end of the needle always stays
pointing north even when the compass dial is rotated. For example, to
read a bearing of 045o, we level the dial and then turn right of north,
but the north-seeking end of the needle turns to the left of 0o, which
is actually east on the dial; so we read a correct bearing.
The Earth has geographic or true N and S poles, where the rotation axis
intersects the Earth's surface, and magnetic poles, where the magnetic
lines of force emerge (magnetic S) or converge into the Earth (magnetic
N). As a magnetic device, the needle of the Brunton compass (a magnet),
when freely suspended, seeks the magnetic poles, which generally are not
the same as the true north, except in some areas on Earth. A compass needle
is a magnet, and the north pole of any magnet is defined as the side which
points to the magnetic north when the magnet is freely suspended. The
correct name for this end of the needle is "north seeking pole".
Maps label the magnetic pole in the northern hemisphere as the "North
Magnetic Pole".
The angle between the true north and the magnetic north is called magnetic
declination. Declination varies with location, time (secular, diurnal),
local magnetic anomalies, altitude (negligible), and solar magnetic activity
(Goulet, 1999). Declination is therefore the angle between where a compass
needle points and the true North Pole. Magnetic declination is constant
along the so-called isogonic lines. The 0° declination (agonic) line
passes west of Hudson's Bay, Lake Superior, Lake Michigan, and Florida.
The N magnetic pole was positioned in 1999 at 79.8° N, and 107.0°
W, 75 in the Canadian Arctic, 1140 km from the true N. The vertical angle
between the magnetic vectors relative to the level (horizontal) ground
is the magnetic inclination, which varies with latitude; it is 90°
at the north magnetic pole, and 0° at the magnetic equator.
Determining the magnetic declination
If the compass needle points east or west of the true north, the offset
is called east or west declination, respectively. The standard is to use
the magnetic north (MN) as a reference for declination, even in the southern
hemisphere. To determine the magnetic declination in a study area we can
use: (1) Published topographic maps; some maps display an out-of-date
declination indicated by the angle between two arrows pointing to the
magnetic north (MN) and true north (GN). (2) Published or online isogonic
charts, which are available at: http://geomag.usgs.gov/chartsdo.html,
or http://geomag.usgs.gov/models.html. (3) Online calculator to determine
the latest magnetic declination for a given location (latitude and longitude)
and year http://www.geolab.nrcan.gc.ca/geomag/e_cgrf.html
or http://www.resurgentsoftware.com/geomag.html
Setting the declination
Geologists use the compass for mapping and measuring linear and planar
objects. The magnetic declination is set by turning the brass screw on
the side of the compass box. For a west declination of say 16o (i.e.,
declination is 16o west of true north), turn the card west, i.e., counterclockwise
(by turning the screw) so that the index pin points to 16o on the side
of the card marked with 'W' in the quad scale, or 344o in azimuth scale.
For an east declination of 16o, turn the card east (i.e., clockwise),
so that the index pin points to 16o on the side of the card marked with
'E' in the quad scale, or 016o in azimuth scale.
The concept of domain
One of the objectives of studying a complexly deformed area (e.g., refolded
folds) is to identify domains (subareas) within which the fabric data
of, for example, folds, lineations, foliations are homogeneous. This means,
for example, that the hingelines and/or fold axes, and the poles to the
axial planes (or axial planar foliation, if it exists) of all minor folds
define maxima (i.e., cluster distribution), with the mean axis lying on
the mean axial plane. The boundaries of the domains are identified (mapped)
by locating the adjacent stations at which specific fabric data are homogeneous.
The homogeneity in each domain reflects two major facts: (1) Homogeneity
of the strain which results in equal extension in a strain field in which
the axes of the maximum principal extension are parallel at every point,
keeping originally-parallel lines and planes parallel, and straight lines
straight. (2) Homogeneity of the rock, i.e., rock properties is the same
at each point in the rock continuum during the deformation. Geologists
cannot produce a useful map of, or obtain useful information from, moderately-
to highly-deformed areas, without knowing how to use the compass to collect
the fabric data and to delineate the domain boundaries. Thus, we need
to know how to measure linear and planar objects of all kinds, such as
sedimentological and structural fabric elements, and map lithostratigraphic
boundaries such as contacts.
Attitude of linear and planar geological objects
Although most geologic structures are generally either curvilinear or
curviplanar, they can be approximated as either linear or planar at specific
scales or domains. For example, a primary linear structure such as the
crest of ripple marks or flute casts on a bed may be folded around the
axis of a fold. At the scale of a large fold, these linear objects are
curved, i.e., have a systematically distributed orientation (e.g., small
circle or great circle distribution). However, within each limb of the
fold (a domain), the orientation of these structures may be homogeneous,
that is, the flute casts or ripple crest are subparallel to parallel.
On each limb, the fabric data of minor folds may have a homogeneous distribution.
The attitude of both linear and planar objects has two general components:
bearing and inclination. Bearing is the horizontal angle between a line
and a specified reference (N or S). The "line" either is the
horizontal projection of an inclined linear object, or a horizontal line
on an inclined plane. Bearing is a scalar feature, i.e., it just is a
number (e.g., 045o or N45oE). Inclination, on the other hand, is the vertical
angle between a linear or planar object and the horizontal. The convention
for direction of inclination is down, i.e., we measure the angle from
the horizontal down (not up), especially when we process the data on the
lower hemisphere, equal area projection (mineralogists also use the upper
hemisphere for crystals). Inclination is a vector, meaning that it has
two components: an amount (angle below the horizontal), and an orientation
specifying the direction to which the planar feature is inclined down
(e.g., 30oNW).
Attitude is too general, and its two components: bearing and inclination,
take different meanings when dealing with linear and planar element. For
planar features such as bedding (the boundaries of a bed), fault, and
foliation, the bearing and inclination become strike and dip. Note that
strike is a scalar, and dip is a vector. Strike is the bearing of a horizontal
line on an inclined plane. Since strike is the bearing of a horizontal
line, we can read the bearing of either of its ends; thus, 000o and 180o
are the same strike. Dip is the inclination of an inclined plane. For
linear fabric such as hingeline, axis, or lineation, we use trend and
plunge to represent the bearing and inclination. Notice that horizontal
planes don't have any strike because they don't intersect the horizontal
along a line.
Trend is the bearing of a linear object measured in the direction to which
the line is inclined down. Plunge is the amount of the inclination of
the linear feature. Thus, both trend and plunge are scalars; together
they define the line vector. For example, a 060o, 30o (also written as
30o, 060o, or 30o, N60oE) is a pair of trend/plunge (direction/magnitude)
or plunge/trend, which means that a line plunges 30o down below the horizontal
in the 060o direction. Linear objects can also be defined by their pitch
on a specific plane. Notice that vertical lines don't have any definable
trend, and that the trend of a non-horizontal linear object must be read
from a reference (e.g., N) to the direction that the line plunges. Thus,
a trend of 000o and 180o are not the same thing (contrast this with strike!).
In practice, it is extremely difficult, if not highly error-prone, to
measure the trend of steeply plunging linear object; in such cases we
use pitch. Notice that the trend of any line on a vertical plane is the
same as the strike of that plane (a useful geometric fact). Pitch is the
acute angle measured on a plane, from the strike of the plane that contains
the line, toward the line (the sense is important!). For example, a pitch
of 40oSW (read as: 40o from SW) means that the line is pitching 40o from
(not to!) the SW end of the strike line of a plane that contains the line.
Notice that pitch generally is not a horizontal or vertical angle, except
for horizontal and vertical planes containing linear features. Pitch is
an alternative to trend and plunge, although, sometimes, it is the only
practical way of measuring a line correctly, especially if the line is
steeply plunging.
Measuring the attitude of linear objects
Measuring trend and plunge: If the linear object is below our line
of sight, open the sighting arm and the lid, and align the open, long
slot of the arm parallel to the linear feature. If the linear feature
is above our head (e.g., on a bedding above us), stand under the object
and align the linear feature with the black axial line on the mirror on
the lid of the compass. In either case, level the bull's eye, round level
while aligning. If the linear object is plunging, only one of the needles
(the north-seeking or the south-seeking) indicates the true trend of the
linear feature. This is a case where many inexperienced geologists can
make a common, critical mistake! Some people have the habit of only reading
the north-seeking (white) needle of the compass, or vice versa, which
is an error-prone practice. When using the Brunton compass we should be
color-blind, and only read the direction of the needle that is correctly
indicating the direction to which the line is plunging (down, not up!).
Thus, the trend of only one of the needles is correct when reading a line.
To figure out which one, we should be aware of the local geographic directions,
that is, know the direction of north or south in the field at all times.
For example, if we are measuring a linear object plunging down to the
south (S or somewhere in the SE or SW quadrants), we must read the trend
indicating any one of these southern directions (e.g., 120 o or S60 o
E), and not the diametrically opposite northern directions (i.e., 300
o or N60oW) indicated by the opposite end of the needle. For a plunging
line, 120 o and 300 o are not equivalent; only one is the true down direction
(120 o or S60 o E in this case). The true direction of the trend may be
indicated by the white or the black needle; the color depends on how we
hold the compass (sighting arm away or toward our body), and on which
way we are facing in the field (looking north or south). Therefore to
avoid a common mistake, no matter how you are holding the compass or which
way you are facing; just know where the geographic N or S is in the field,
and ask yourself this question: Which way is the line going down (i.e.,
plunging)? If it is plunging to around the N, then read the needle (white
or black) that points to N or in the NE or NW quadrants and not the opposite
directions. This is the easiest and most practical way of correctly measuring
a line. Of course, if a linear feature is non-plunging (a special case),
we have the freedom of reading either the white or the black needle, because
the line is horizontal (both ends are the same).
Example: We are measuring the crest of a ripple mark which is roughly
trending somewhere around north (we know which way is N in the field because
we have the compass!). The crest of the ripple mark is plunging, and lies
on a bedding, which is dipping. Align the compass's sighting arm with
the crest and then read either the direction indicated by the white or
the black needle that points somewhere to the north. Thus, if the black
needle points to N20oW and the black needle points to the S20oE, we must
read the black needle. Don't wrongly assume that the white needle gives
you the north readings; a common misconception.
Measuring vertical angles, height, and distance
To measure vertical angles, fold the lid and use the compass as was described
for measuring the plunge of lines, i.e., with the clinometer. The vertical
angle (q) can then be used to calculate the height (h) of an object (e.g.,
wall, tower, mountain peak) using the equation h = x tanq, if we know
the distance (x) to the object. We can also use the trigonometric functions
to calculate the horizontal distance (x) from point A to an object located
at point B as follows. Walk from point A to another point C such that
AC is perpendicular to line AB. This is done by taking a bearing at 90o
to the bearing of AB with the compass. Use a tape or a measured pace (if
pace spacing is known). In practice, we define AC to be 10 meters; or
walk from A to C by 10 meters with our pace. Read a bearing from point
C to point B. Subtracting the two bearings gives the angle b between AB
and CB. Now we have a right angle triangle ABC with AC = 10 m, AB = x,
and a known angle b. Use the equation tanb = AC/AB = 10m/x, and calculate
x in meters.
If the linear object of interest is steeply plunging, it is better to use pitch instead of the trend and plunge. Measuring pitch is only possible if the linear feature lies on a physical plane. For example, if a set of slickenlines (striations) plunges (e.g., around S) on a fault, we measure the striations as follows. First, measure the plane (i.e., fault) that contains the linear features (see next section for this). Next, measure the pitch of the striations on the fault plane as follows. The Brunton compass has a circular, high relief ring on its back, which is designed for measuring pitch. Open the compass (the arm and lid opened completely) and align the edge of the lid and box with the line while the whole ring on the back of the compass touches the fault. If the clinometer, barrel-shaped level is not centered in this position, gently move the box off the plane and slightly turn the clinometer, and lay the box back on the plane while aligning the edge with the line. If the clinometer is not centered, repeat these steps several times until the clinometer is leveled while the edge of the box is parallel to the line, and the circle behind the compass is completely lying on the plane. This is a trial and error process that requires some practice to master.
Using the compass as hand level on a Jacob Staff
The compass can be used as hand level, mounted on a Jacob Staff to measure
the true stratigraphic thickness of a lithostratigraphic unit (e.g., member,
formation) as follows. Measure the true dip of the layers and set the
clinometer at that angle. Mount the compass vertically (as in reading
the plunge) on the Jacob Staff with the lid half closed; making sure that
the clinometer is set at the measured dip angle. Start at the lower contact
of a stratigraphic unit. Tilt the staff in the direction of the dip of
the beds, and look inside the mirror until the clinometer level is centered.
At this position, look through the sighting tip and through the sighting
window. Identify a point (e.g., a brush, a piece of rock) on the ground
where the line of sight intersects the ground. Move the base of the Jacob
staff to that point. Use a counter and register the number of times (n)
it takes to go from the basal contact of the lithostratigraphic unit to
its top. At the upper contact, multiply the length of the Jacob Staff,
which is 1.5 meter, by 'n', to get the true stratigraphic thickness of
the unit.
Measuring the attitude of planes
If the plane is flat, smooth, and non-magnetic, the easiest way to measure
the strike and dip of the plane is to touch the edge of the box (not all
the rectangular side!) with the plane while centering the circular, bull's
eye level. This will generate a horizontal line parallel to the edge of
the box on the plane of interest. We have the freedom of reading either
of the two needles; it does not matter which one we read! (e.g., 140o
and 320o are the same strikes). This is the strike of the plane; we can
mark the strike line on the plane with a pencil drawn parallel to the
edge of the box. In special cases such as when taking oriented samples
of rock for structural analysis, we need to distinguish and mark only
one of these two ends of the horizontal line with an arrow (preferably
with half arrow tip).
After the strike is measured, the magnitude of the dip of the plane is
measured by putting the entire rectangular side of the box perpendicular
to the strike line and centering the clinometer level. The general, dominant
direction of the dip is identified geographically by checking the down-dip
direction (by centering the bull's eye level and finding out where the
principal geographic directions are). In North America, the format is
strike, dip and dip direction (e.g., 050 o, 30 o NW), because that is
the sequence of measuring the attitude of a plane with the Brunton compass.
Silva and other similar compasses allow easier measurement of the dip
direction before or without identifying the strike direction. Thus, in
Europe and other places, the format may be dip amount, dip direction,
which is a vector (e.g., 30o, 320o).
If the plane of interest is not flat, and lies in front of us at the level
of our line of sight, we must use eye-level sighting as follows. Stretch
the sighting arm and bend the sighting tip. Close one of the eyes, and
move sideways while looking onto the edge of the inclined plane. Stop
moving if further moving exposes the surface of the plane. In this position,
we are looking edgewise along the plane. Fold the compass lid until we
see the edge of the inclined plane in the sighting window through the
lid. Hold the compass as follows: Put the two thumbs under the sighting
arm on the box; the two index fingers on the edge of the lid, and the
center fingers behind the horizontally positioned box. Adjust the lid
with the index fingers until the bull's eye level is apparent in the mirror.
Center the bull's eye level, and intersect the edge of the plane with
the black line in the sighting window (don't try to align the black line
with the edge (because it tilts the box) unless the plane is vertical).
Hold your breath, and read the bearing indicated by the black or white
needle (whichever is apparent in the mirror), that is, the strike of the
plane in the mirror without moving the box or going off level. While in
the same position, read the amount of dip by aligning the flat edge of
the box with the edge of the plane. Determine the direction of the dip
by inspection; give the principal direction of inclination from the geographic
space as described above.
If the plane of interest is vertical and the terrain is horizontal (a
special case), stand directly above the edge of the plane and read the
trend of the edge of the plane as the strike. Some inexperienced geologists
assume that they can determine the strike of an inclined, non-vertical
plane in this way. The technique of standing above the edge of a plane
does not work if the plane is non-vertical and/or the top surface is not
horizontal. This is because the intersection of a non-vertical plane and
non-horizontal plane is not a horizontal line, and thus cannot be a strike!
In such cases, we need to directly measure the strike by either eye-level
sighting or by touching as described above.
Measuring the bearing of a line between two points
Commonly we want to measure the trend and plunge of a line connecting
two points, e.g., a line connecting a person and another person, or another
landmark (e.g., house, tower, smoke stack). To do this we can either use
the eye-level or waist-level sighting. The eye-level sighting was described
above. For the waist-level sighting, we put the lid against our body,
and tilt the lid while holding the box horizontally by centering the bull's
eye level. Position the target on the black line on the mirror, and after
centering the round level, read the trend.
Measure the plunge of this line as follows. Flip the compass (box is vertical)
while the lid and sighting arm are folded. Look through the hole in the
sighting tip and through the sighting window, and then center the clinometer
level while shooting to a specific point on the target. If the two persons
have the same height, intersect the other person's eyes with the black
line on the sighting window. If we are sighting (shooting) to another
person who is shorter than we are, say by 5 cm, then we shoot 5 cm above
that person's eye level (at forehead or head level). If the other person
is taller by 5 cm, then we shoot to the mouth level of that person.
Measuring the attitude of a plane with the two-line technique
The two-line technique is a very useful and precise method of measuring
subhorizontal and gently dipping planes. Such low-dip planes are very
common, and cannot accurately be measured by measuring the strike and
dip. If the exposed surface of a plane is small, we drop two sharpened
pencils on the plane at high angles to each other. Point the sharp ends
of the pencils down-plunge. Measure the trend and plunge of the two lines
(l1 and l2). If the subhorizontal plane is large and extensive (e.g.,
a basalt layer), we define two long lines with two persons. The two persons
stand at two points and shoot at each other to determine the trend and
plunge of the line connecting them (l1). Take the average of the two readings.
They repeat this for a second line by constructing a second line (l2).
To determine the orientation of the plane that contains the two lines,
plot the lines as two points on the stereonet, and align them on the same
great circle. Read the strike and dip of the great circle.
Although the two-line technique is the best way to determine the attitude
of subhorizontal or gently dipping layers, the attitude of small, gently
dipping planes (e.g., bedding at the hinge zone of a mesoscopic folds)
can be determined by trial and error as follows. Measure the dip of the
layer where we think is near the maximum inclination (true dip); center
the clinometer level. Remember the dip value. While the rectangular side
of the box is still completely touching the layer, slowly turn the compass
and reread the dip. If the dip is less than the previous reading, then
we are going away from the maximum inclination, and our previous reading
was closer to the true dip. Move toward the first position and go to the
opposite direction. Repeat the process until we identify the maximum inclination
which is the true dip. When the true dip orientation and magnitude is
registered, measure the strike of the plane perpendicular to this line.
Using the compass for the two-point problem
Sometimes we may be located at a contact of a horizontal layer (e.g.,
a basalt layer or a bed) on a hill, and be interested to locate a point
of the same elevation on an adjacent hill. To do this, we set the clinometer
at the 0o mark, and flip the compass sideways (vertically) as described
for measuring the plunge. Look through the hole of the sighting tip, through
the sighting window, and center the clinometer level (which is set to
0o) by moving the box up or down and looking in the mirror (without turning
the clinometer). When the level is centered, locate a point on the other
hill at the intersection of your line of sight and the ground. That point
has the same elevation as the point of our position.
This technique is also handy in determining the strike of a layer. Just
set the clinometer at the 0o mark; stand on the layer, look along the
layer, and center the clinometer without turning the clinometer. After
the level is centered, locate a point on the sloping layer along your
horizontal line of sight. Now that we know the strike line (the horizontal
line), we need to read its bearing either by eye-level or waist-level
sighting. While in the same position, read the dip of the layer across
the strike line using the clinometer as described for measuring the dip.
References:
Compton, R. R., 1985. Geology in the Field. John Wiley & Sons, New
York, 398p.
Goulet, Chris, M. 1999. At: http://www.cam.org/~gouletc/decl_faq.html
