SURVEYING CH 4 Flashcards
is one of the fundamental measurements in surveying.
Distance
Distance is required to…?
to give scale to a network of control points
to fix the position of topographic detail
by offsets or polar coordinates
to set out the position of a point
in constructon work
The basic methods of measuring distance are
by taping or by electromagnetic distance measurement
Methods for Making Linear/Horizontal Measurements
- Pacing
- Odometer readings
- Tacheometry (stadia)
- Subtense bar
- Taping
consists of counting the number of steps or paces in a
required distance.
Pacing
defined as the length of a step in
walking. It may be measured from heel to heel or from toe to toe,
a pace
means moving with measured steps
pacing
A stride is equivalent to
two paces or a double step
the length of
one’s pace
pace factor
is defined as the procedure of obtaining
horizontal distances and difference in elevation based on the optical
geometry of the instrument
Tachymetry or tacheometry
is a method of measuring distances rapidly with a telescope
(usually engineer’s transit or theodolite) and a graduated rod
stadia method
is a 2 m long rounded steel tube through which runs a thin
invar.
subtense bar
There are three fundamental sources of error in taping:
Instrumental errors
Natural errors
Personal errors
a tape may differ in actual length from its nominal graduated
length because of a defect in manufacture or repair, or as a result of kinks.
Instrumental error
the horizontal distance between end graduations of a tape varies
because of the effects of temperature, wind, and weight of the tape itself.
natural errors
tape persons may be careless in setting pins, reading the tape, or
manipulating the equipment.
personal error
can be one of the most important errors
incorrect length of tape
types of tape correction
due to temperature
due to its own weight
due to tension
is an instrument constant (usually equal to 100) which will depends on the spacing between stadia hairs.
Stadia interval factor
is the apparent distance between the ponts on the rod where the upper and lower stadia hairs intersects the rod.
stadia intercept
It is determined by subtracting the lower stadia hair reading from the upper stadia hair reading.
stadia intercept
the distance from the center of the instrument to the principal focus.
stadia constant
may be used for general use, where precision is not a prime consideration.
Linen or glass fiber tapes
are standardized so that they measure their nominal flength at a designated temperature usually 20 °C and at a designated applied tension usually between 50 N to 80 N.
steel bands
they have a negligible coefficient of expansion compared with steel, and hence temperature variations are not critical.
invar tapes
is roughly, midway between steel and invar.
lovar tape
are made of wood or steel, 2 m long and 25 mm in diameter, painted alternately red and white and have pointed metal shoes to allow them to be thrust into the ground. They are generally used to align a straight line between two points.
range poles
made from No. 12 steel wire are also used to mark the tape lengths
chaining arrows
are generally used with roller-grips or tapeclamps to grip the tape firmly when the standard tension is applied.
spring balances
are also necessary to record the tape temperature at the time of measurement, thereby permitting the computation of tape corrections when the temperature varies from standard.
field thermometer
may be used to ensure that the tape is horizontal. This is basically a hand-held tube incorporating a spirit bubble to ensure a horizontal line of sight.
hand levels
may be necessary if stepped taping is used.
plumb bobs
are necessary in rough ground, to afford a mark against which the tape may be read.
measuring plates
is the process of breaking the overall distance down into manageable short sections, each much less than a whole tape length.
step measurement
