Unit 3: Surveying

Introduction to Surveying

Surveying is the science and technique of accurately determining the three-dimensional (3D) position of points and the distances and angles between them.

The primary goal of surveying is to create a map or plan of an area. In geology, it is used to create geological maps, locate outcrops precisely, and measure the thickness and orientation of rock bodies.

Plain Table Survey

This is a graphical method of surveying where the observations are taken and the map is drawn simultaneously in the field.

Key Equipment:

• Plain Table: A drawing board mounted on a tripod.
• Alidade: A ruler with a sighting device (vanes or a telescope).
• Trough Compass: A compass used to orient the table to Magnetic North.

Methods:

• Radiation: For small areas. Set up the table at a central point, sight all visible points (A, B, C), draw rays, measure the distance, and plot them to scale.
• Intersection: The most common method.
  1. Set up a baseline (A-B) and plot it on the paper.
  2. From point A, sight an unknown point (P) and draw a ray (line) on the paper.
  3. Move the table to point B, re-orient it, and sight the same point (P).
  4. Draw a second ray. The intersection of the two rays is the exact location of P.

Pros: Simple, visual, no complex calculations, errors are immediately visible.
Cons: Not highly accurate, slow, difficult in bad weather (wind/rain).

Theodolite

A Theodolite is a precision surveying instrument used for measuring horizontal and vertical angles.

Key Components:

• Telescope: For sighting distant points (targets).
• Horizontal Circle: A graduated scale (0-360°) for measuring horizontal angles.
• Vertical Circle: A graduated scale for measuring vertical angles (elevation or depression).
• Leveling Head & Spirit Levels: To ensure the instrument is perfectly horizontal.
• Vernier Scales (or Micrometers): To read the angles with very high precision (down to seconds of an arc).

Uses:

• Traversing: Creating a network of survey stations by measuring the angles and distances between them.
• Triangulation: Accurately mapping large areas by creating a network of triangles.
• Leveling: Determining the difference in elevation between points.

Pros: Extremely high angular accuracy.
Cons: Requires separate measurement of distance (traditionally with a chain) and complex trigonometric calculations to create the map back in the office.

Chain Survey

This is the simplest and oldest type of surveying. Its key principle is that it measures only linear distances (lengths), not angles.

Principle: Triangulation

The entire area to be surveyed is divided into a network of triangles. A triangle is the only geometric shape whose size and shape are fixed if you know the length of all three sides.

Equipment:

• Chain or Tape: For measuring distances. (e.g., Gunter's Chain, Engineer's Chain, or a steel tape).
• Ranging Rods: Poles used to mark stations and keep measurements in a straight line.
• Arrows: Metal pins used to mark the end of a chain length on the ground.

Process:

1. Establish a main baseline, the longest straight line possible, across the area.
2. Measure the baseline accurately.
3. Identify other points and measure the distances from the ends of the baseline to those points, forming triangles.
4. Measure "offsets" (perpendicular distances) from the chain line to features like buildings or fences.

Pros: Very simple, requires minimal and cheap equipment, no angle calculations.
Cons: Only suitable for small, flat, open areas. Difficult to use in areas with many obstacles. Not very accurate for large areas.

Total Station

A Total Station is a modern, electronic, all-in-one surveying instrument. It is the dominant tool in modern surveying.

It combines three functions:

1. Electronic Theodolite: For measuring horizontal and vertical angles digitally.
2. EDM (Electronic Distance Meter): For measuring distances using a laser or infrared beam. It shoots a beam at a reflector prism held by an assistant, and times how long the beam takes to return, calculating the distance precisely.
3. Microprocessor & Data Logger: An onboard computer that records all measurements (angles, distances) and can instantly perform calculations (e.g., determine X, Y, Z coordinates, horizontal/vertical distances, and elevations).

Pros:

• Extremely fast and accurate for both angles and distances.
• Data is recorded digitally, eliminating human reading/writing errors.
• Instantly calculates coordinates, saving office time.
• "Robotic" total stations can be operated by a single person.

Cons:

• Very expensive.
• Requires battery power.
• More complex to learn and operate than traditional methods.