ANGULAR MEASUREMENT
Points on the ground or on a map are related to each other
through a horizontal distance and a horizontal angle (or direction.)
Horizontal angular measurements are made between survey lines to
determine the angle between the lines. A horizontal angle is the
difference between two measured directions. Horizontal angles are
measured on a plane perpendicular to the vertical axis (plumb line).
Vertical angular measurements are measured to determine slope of
survey lines from the horizontal plane (level line). When the vertical angle
is applied to the slope distance, the horizontal and vertical distances may
be calculated. Vertical angles are measured on a plane passing through
the vertical axis perpendicular to the horizontal plane. In order to facilitate
the trigonometric calculations of horizontal and vertical distance, the
reference or zero angle is on the vertical axis directly above the
instrument, which is termed the zenith angle.
In the United States, the sexagesimal system of angular measurement is
used. In the sexagesimal system, there are 360° in the circumference of a
circle. The basic unit is the degree (°), which is further divided into 60
minutes (60'), and the minute is subdivided into 60 seconds (60"), and
decimals thereof. `
Historically, horizontal angles were measured by using a compass to
provide a magnetic bearing. The deflection from the bearing was
recorded.
Later, more precise scribed discs later improved better angular
resolution. Mounting telescopes with reticles atop the disc allowed
more precise sighting. (see theodolite).
Levels and calibrated circles allowed measurement of vertical
angles. verniers allowed measurement to a fraction of a degree,
such as with a turn-of-the-century transit.
1
SURVEYING
,Direct measurement of angles and line direction by total station,
theodolite, compass, or transit is familiar to all surveyors. However, many
surveyors are not completely familiar with specific procedures that will
achieve specified results.
Compass
Compass
surveyor uses a compass to determine the direction of a line. the
compass needle points to the MAGNETIC NORTH POLE and by
turning the compass in the direction of the line being surveyed, the
direction of the line can be observed.
Although there are many varieties of compasses, they all fall into
two main categories: either a "plain" compass or a "vernier"
compass.
A plain compass has no adjustment and always reads magnetic
north. A vernier compass has an adjustable scale that allows for the
"setting off" of the magnetic declination and the compass can then
directly read true north
Theodolite
2
SURVEYING
, Theodolite
The transit and theodolite are used by the surveyor to measure both
horizontal and vertical angles.
While the purpose of the two is similar, as a general rule a
theodolite is more accurate than a transit. However, there is no
specific rule as to when one definition ends and the other begins.
Generally, these instruments have a minimum accuracy of one
minute of angle and some very precise theodolites will measure
angles to an accuracy of one-tenth of a second of angle.
To put these accuracies into perspective, at a distance of one mile,
one minute of angle covers about 1.5 feet. At a distance of one
mile, one-tenth of a second of angle covers about 0.003 feet.
Meridian and bearing
3
SURVEYING
Points on the ground or on a map are related to each other
through a horizontal distance and a horizontal angle (or direction.)
Horizontal angular measurements are made between survey lines to
determine the angle between the lines. A horizontal angle is the
difference between two measured directions. Horizontal angles are
measured on a plane perpendicular to the vertical axis (plumb line).
Vertical angular measurements are measured to determine slope of
survey lines from the horizontal plane (level line). When the vertical angle
is applied to the slope distance, the horizontal and vertical distances may
be calculated. Vertical angles are measured on a plane passing through
the vertical axis perpendicular to the horizontal plane. In order to facilitate
the trigonometric calculations of horizontal and vertical distance, the
reference or zero angle is on the vertical axis directly above the
instrument, which is termed the zenith angle.
In the United States, the sexagesimal system of angular measurement is
used. In the sexagesimal system, there are 360° in the circumference of a
circle. The basic unit is the degree (°), which is further divided into 60
minutes (60'), and the minute is subdivided into 60 seconds (60"), and
decimals thereof. `
Historically, horizontal angles were measured by using a compass to
provide a magnetic bearing. The deflection from the bearing was
recorded.
Later, more precise scribed discs later improved better angular
resolution. Mounting telescopes with reticles atop the disc allowed
more precise sighting. (see theodolite).
Levels and calibrated circles allowed measurement of vertical
angles. verniers allowed measurement to a fraction of a degree,
such as with a turn-of-the-century transit.
1
SURVEYING
,Direct measurement of angles and line direction by total station,
theodolite, compass, or transit is familiar to all surveyors. However, many
surveyors are not completely familiar with specific procedures that will
achieve specified results.
Compass
Compass
surveyor uses a compass to determine the direction of a line. the
compass needle points to the MAGNETIC NORTH POLE and by
turning the compass in the direction of the line being surveyed, the
direction of the line can be observed.
Although there are many varieties of compasses, they all fall into
two main categories: either a "plain" compass or a "vernier"
compass.
A plain compass has no adjustment and always reads magnetic
north. A vernier compass has an adjustable scale that allows for the
"setting off" of the magnetic declination and the compass can then
directly read true north
Theodolite
2
SURVEYING
, Theodolite
The transit and theodolite are used by the surveyor to measure both
horizontal and vertical angles.
While the purpose of the two is similar, as a general rule a
theodolite is more accurate than a transit. However, there is no
specific rule as to when one definition ends and the other begins.
Generally, these instruments have a minimum accuracy of one
minute of angle and some very precise theodolites will measure
angles to an accuracy of one-tenth of a second of angle.
To put these accuracies into perspective, at a distance of one mile,
one minute of angle covers about 1.5 feet. At a distance of one
mile, one-tenth of a second of angle covers about 0.003 feet.
Meridian and bearing
3
SURVEYING
