Surveying: Linear Measurements

• There are two principal measurements in surveying: linear and angular.

Linear Measurements

• Linear measurements are distances between two points.
• These can be horizontal, slope, or vertical distances.
• The basic unit is the meter, including decimal fractions where accuracy is needed.
• Other units of measure include the foot, inch, yard and mile.
• 1 foot = 12 inches.
• 1 inch = 2.5cm.
• 1 foot = 0.3048 meter.
• 1 mile = 5280 feet = 1600 meters.
• 1 yard = 3 feet.
• Distance can be measured by mechanical or electronic devices.
• Horizontal distance can be measured using tapes, chains, tacheometers, EDM, pacing, or an odometer.
• Vertical distance can be measured using levels and tacheometers.
• Slope distance can be measured using a total station or calculations.
• Pacing helps conduct rough surveys quickly, mostly in preliminary surveys and explorations.
• The number of paces between two points of a line are counted.
• Knowing the average length of a pace helps estimate the length of a line.
• Pace length depends on the person measuring, the ground, speed and slope.
• A passometer is a watch-like instrument carried in a pocket or tied to a leg.
• Passometers record paces when a person moves, and the distance is calculated by multiplying paces by the average pace length.
• A pedometer is similar but automatically measures distance.

Tapes

• Cloth or linen tapes are used for subsidiary measurements due to being lightweight and easy to handle, but they are affected by moisture.
• Metric steel tapes are made of steel, have a ring for holding, and are for shorter distances.
• Invar tapes are used for high-precision work and are made of alloy steel.
• Laser distance meters are portable devices utilizing laser technology for precise distance measurements.
• They’re indispensable in building, surveying, and other fields requiring high precision.

Tape Accessories

• Ranging poles are used to clear the line of tapping.
• Channing pins mark tape ends on the ground.
• Plumb bobs locate points directly below or above another point.
• Tension handles apply appropriate pull to the tape.
• Pegs are wooden and mark station positions, often made of hard timber and tapped red at one end.
• Spray paint is one type of ground marking accessory used by surveyors.
• Odometers measure the number of revolutions of a wheel; the value of the distance covered is given by the number of revolutions multiplied by the wheel's circumference.
• Odometers do not provide accurate results on undulating surfaces, but speedometers can be used when the surface is smooth.
• Chaining is a method to determine distance using a tape or a chain.
• It is one of the accurate mechanical methods to determine linear measurements
• Chains are used for ordinary precision, while tape and special bars are used for high precision.

Electronic Distance Measuring (EDM)

• EDMs are a method of determining the length between two points using phase changes that occur as electromagnetic energy waves travel between the points.
• EDMs use electromagnetic waves to measure distances with high accuracy.
• Levels measure differences in elevation and are optical instruments used to establish or verify points in the same horizontal plane, known as leveling.
• Levels are used in conjunction with a leveling staff to establish relative heights or levels of objects or marks.
• Leveling rods are graduated wooden or aluminum rods used with a leveling instrument to determine the difference in height between points or heights of points above a vertical datum.

Tacheometry

• Tacheometry is a type of theodolite used for rapid measurements.
• Modern forms determine the distance to a target electronically or electro-optically.
• It's a system of rapid surveying by which the horizontal and vertical positions of points on the Earth's surface relative to one another are determined.

Angular Measurements

• Two types of angular measurement are horizontal and vertical angles.
• Horizontal angles are measured by theodolites and compasses.
• Vertical angles are measured by theodolites and total stations.
• Though the basic unit is the radian, the degree is used in surveying fieldwork.
• Subdivisions of the degree are available for accuracy, via the minute and the second.
• 1 degree = 60 minutes.
• 1 minute = 60 seconds.
• 2π radians = 360 degrees.

Compass Survey

• A compass survey measures angles with a magnetic compass.
• Chain and compass surveys make linear measurements with chain or tape and angular measurements with compass.
• Theodolites have a mobile telescope installed to offer angular readouts while rotating around horizontal and vertical axes.
• Theodolites connect the first point seen via the telescope to future sightings of additional points from the same position.
• Theodolites measure horizontal and vertical angles and establish straight and curved lines.
• Total stations are surveying tools consisting of a theodolite, an EDM, and a control panel, and they are used for accurate measurement.
• They measure both distance and angle for both horizontal and vertical measurements.
• Data is stored and processed electronically.
• GPS is the Global Positioning System.
• It is a satellite-based navigation system that allows professionals to collect reliable and accurate coordinates of latitudes and longitudes without needing to measure distances and angles.
• Surveying Instrument GPS RTK (Real Time Kinematic) with Global Navigation Satellite Systems (GNSS) has become a common practice in geomatics.
• RTK surveying is a relative positioning technique that measures positions in real time using two GNSS antennas.
• When surveying on flat terrain at a short distance, the working distance, d, should be less than the length of the tape, ensuring the tape can keep the same line.
• On flat terrain at a long distance, d is more than L.

Setting Out a Straight Line

• Poles A, B, C, and D are in line if the observer, standing one or two meters behind Pole A, sees only Pole A, with the other poles hidden behind it.
• Ranging is the making of three or more poles at a straight line.
• When surveying on a slope, use ranging with a plumb bob.
• Setting out straight lines over a ridge or hill involves similar techniques.
• It may be necessary to break the tape around the obstacle.
• Horizontal angles are used to transform resulting horizontal lengths to an equivalent horizontal length along the measurement line.
• Horizontal angles are measured using a compass or calculated using the cosine law.

Map Scales

• Map scales are a basic requirement for plan or map creation.
• Scales represent large distances on paper.
• A scale is the ratio by which the actual length of an object is reduced or increased in a drawing.
• If 1 cm on a map represents 10 meters on the ground, the map scale is 1 cm = 10 m.