Lesson 3: Horizontal Distance Measurement Flashcards
distance between any two
random points in 3D space
Spatial Distance
- linear distance between two points on any surface
- if points are at different elevations, distance is the horizontal length between plumb lines at the points
Horizontal Distance
Factors to be Considered in Choosing
Methods of Determining Horizontal Distances
- Purpose of the Measurement
- Required Accuracy
* Rough reconnaissance ( 1 part in 100 )
* First-order surveys (1 part in 1,000,000) - Cost
Methods and Instruments Used to
Determine Horizontal Distances
- Pacing
- Tacheometry
- Graphical and Mathematical Methods
- Mechanical Devices
- Photogrammetry
- Taping
- EDM
- Global Positioning Systems
- Consists of counting the number of steps in the required distance.
- Furnishes a rapid means of approximately checking a more precise measurement of distance.
Pacing
Pacing is used on:
- reconnaissance survey
- small-scale mapping
- locating details
- defined as the length of a step
in walking
– 2 is equivalent to a stride
or a double-step
– Relative Precision: 1/200 to
1/100 is seldom less than 1/300
Pace
How to Calibrate One’s Pace?
– walk at a uniform step along a line of known length on a level
ground and count the number of paces to cover the whole stretch
– compute the pace factor (in m/pace)
Factors Affecting Length of Pace:
- Speed of pacing
- Roughness of the ground
- Weight of clothing and shoes
- Fatigue on the part of the pacer
- Slope of the terrain
- Age and sex of the individual
Mechanical Pace Counters
- Pedometer
- Passometer
a pager-size device worn on the belt
that simply records the number of steps
made based on the body’s movement (hips)
Pedometer
registers the number of paces and
distances
Pedometer
registers a pace by impact
each time a foot touches the
ground
Passometer
about the size of a watch
Passometer
- an indirect method of measuring horizontal
distances - it is based on the optical geometry of the
instruments used - uses subtended intervals and angles
observed with a transit or a theodolite on a
graduated rod or scale - relative accuracy is 1 part in 300 to 400
Tacheometry
Methods in Tacheometry
- Stadia Method
- Subtense Bar Method
- uses a telescope with 2 horizontal cross hairs and a graduated rod.
Stadia Method
Precision of the stadia method depends on the ff:
- Refinement with which
instrument was manufactured - Skill of the observer
- Length of measurement
- Effects of refraction
is set up at a distant station while a
theodolite at the end of the line
measures the horizontal angle
subtended by the distance
between the two targets
Subtense Bar Method
distances are determined geometrically
Graphical and Mathematical Methods
Mechanical Devices
- Odometer
- Measuring Wheel
- Optical Rangefinder
- Mileage Recorder
attached to a wheel for
purposes of roughly measuring
surface distances (speed)
Relative Precision:
1/200 on fairly level ground
Uses:
Route Location Surveys
Preliminary Surveys
Small-scale Mapping
Odometer
an odometer except that it is a more portable and self-contained device
Use:
Determining distance along
irregular or curved lines and
boundaries
Measuring Wheel
determines distances approximately by focusing
Optical Rangefinder
attached to ordinary automobile speedometer
Mileage Recorder
- Measuring of images on a photograph
- Uses scales to obtain ground distances
Photogrammetry
Measurement of distance is based on the invariant speed of light or electromagnetic waves in a vacuum
Electronic Distance Measuring (EDM)
Equipment
EDM Classification:
- Electro-optical
e.g. Geodimeter - Electromagnetic (microwave)
e.g. Tellurometer
Advantages of EDM
- speed and accuracy
- lightweight and portable
- low power requirement
- easy to operate
- ease in measurement over inaccessible terrain
- applicable to the measurement of both short and long lengths
- automatically measures, displays and records needed data
- slope measurements are internally reduced to horizontal and
vertical components - automatically accumulates and averages readings for slope range, horizontal and vertical angles
- a combination electronic transit and
electronic distance measuring device
(EDM) - angles and distances from the
instrument to points to be surveyed are
determined - using trigonometry, angles and
distances may be used to calculate the
actual positions (x, y, and z or northing,
easting and elevation) of surveyed
points in absolute terms
Total Station
- involves direct measurement of distances with
steel tapes - most common method of measuring or laying
out horizontal distances - consists of stretching a calibrated tape between two points and reading the distance indicated on the tape
Taping
- by Sir Edmund Gunter (1620)
- 66 ft with 100 links
- 1 link = 0.66ft = 7.92 inches
Gunter’s Chain
Types of Measuring Tapes
- Steel tape
- Metallic tape
- Non-metallic tape
- Invar tape
- Lovar tape
- Fiberglass tape
- Wires
- Builder’s tape
- Phosphor-bronze tape
- Nylon-coated steel tape
ribbon of steel
0.5-1.0cm in width; weighs 0.8-1.5kg / 30 meters
for most conventional measurements in surveying and
engineering works
Steel tape
aka woven tapes
made of water-proof linen fabric
woven longitudinally with small brass, copper or bronze
reduce stretching, increase strength
for short distances
unsuitable for precise measurements and around electrical installations
Metallic tape
coated with plastic material
reduce effects of moisture, humidity and abrasion
safe to use when working around electrical
installations
Non-metallic tape
made of an alloy of nickel (35%) and steel (65%)
with a very low coefficient of thermal expansion
(1/30 to 1/60 of steel tape)
“Invariable”
used for precise measurements
10 times as expensive as steel
easily bent and damaged
Invar tape
properties and costs between steel and invar tapes
Lovar tape
woven with fiberglass (longitudinal and transverse pattern)
strong and flexible
best used in vicinity of electrical equipment
Fiberglass tape
used before thin flat steel tapes
still practical in hydrographic surveys
Wires
smaller cross sections and lighter than engineer’s tape
graduated to suit commonly used dimensions in building constructions
Builder’s tape
rust-proof tape
used in vicinity of salt water
Phosphor-bronze tape
flexible and crack-resistant
steel core tape
with coating of permanently bonded non-conducting
nylon
designed to be resistant to corrosion and rust
Nylon-coated steel tape
Taping Accessories
- Tension Handle
- Chaining Pins
- Leather Thong
- Range Pole
- Clinometer
- Plumb Bob
- Wooden Stake
Operations in Taping
- Breaking Tape
- Slope Taping
measurement of shorter distances which are accumulated to total a full tape length
Breaking Tape
taped measurements are made directly along the slopes when the ground is of uniform inclination and fairly smooth
Slope Taping
