Plane Table Survey: A Step-by-Step Guide

by Wholesomestory Johnson 41 views

Hello there! Let's dive into the plane table survey method! I'll give you a detailed, easy-to-understand explanation, covering everything you need to know. Whether you're a student, a surveyor, or just curious, this guide will break down the plane table survey process step by step, ensuring you grasp every detail.

Correct Answer

The plane table survey method is a graphical surveying technique that involves directly plotting the observations made in the field onto a drawing sheet, using a plane table and other essential surveying instruments.

Detailed Explanation

Alright, let's get into the nitty-gritty of the plane table survey method. This technique is like drawing a map directly in the field. Instead of taking notes and measurements to be plotted later, you create the map as you go. This method is especially useful for small-scale maps and for filling in details where other methods might be less efficient.

Key Concepts

To fully understand the plane table survey, we need to define a few key terms:

  • Plane Table: A drawing board mounted on a tripod. It's the foundation for your map.
  • Alidade: A sighting device used to sight objects and draw lines on the plane table.
  • Leveling: Ensuring the plane table is perfectly horizontal.
  • Orientation: Setting the plane table in the correct direction relative to the features you are surveying.
  • Sighting: Observing and marking the direction of features on the drawing sheet.

Instruments Used in Plane Table Surveying

Before we get into the process, let's list the essential instruments:

  1. Plane Table: The drawing board itself. It should be made of well-seasoned wood to prevent warping.
  2. Tripod: To support the plane table at a convenient height.
  3. Alidade: Usually has two vanes with a sighting slit and a hair or wire for accurate aiming. The alidade can be a plain alidade (just for sighting) or telescopic alidade (which includes a telescope for longer sights and leveling).
  4. Spirit Level: Used to ensure the plane table is perfectly horizontal.
  5. Plumbing Fork with a plumb bob: Used to center the plane table over a ground station accurately.
  6. Drawing Sheet: High-quality paper suitable for drawing.
  7. Trough Compass: Used to align the drawing sheet with the magnetic north.
  8. Drawing Pins or Clips: To fix the drawing sheet onto the plane table.
  9. Measuring Tape: For taking linear measurements if needed for scaling.
  10. Pencils, Erasers, and other drawing tools: For drawing and detailing the map.

The Plane Table Surveying Process: Step-by-Step

Here’s a detailed breakdown of how to carry out a plane table survey:

  1. Setting Up the Plane Table:

    • Setting up: The plane table is fixed on the tripod. The tripod legs are spread and fixed firmly into the ground.
    • Leveling: The plane table is made horizontal using the spirit level. This ensures accurate measurements.
    • Centering: The plane table is positioned directly above a known point on the ground using a plumbing fork and plumb bob.

  2. Orientation:

    • Orienting the table: The plane table must be oriented correctly to match the direction of the survey line on the ground with the corresponding line on the drawing sheet. This is often done using a trough compass to align the drawing sheet with the magnetic north.
    • Back Sighting: if you have previously surveyed points, orient the table using a known point by aligning the line on the drawing sheet with the direction to that point on the ground.

  3. Locating Points by Radiation:

    • Selecting the Station: Choose a strategic station from where you can see as many features as possible.
    • Sighting: Place the alidade along a line drawn on the drawing sheet representing the station point. Sight towards the object, aligning the alidade’s vanes with the object.
    • Drawing the Ray: Draw a ray (a line) along the edge of the alidade towards the object on the drawing sheet.
    • Measuring and Plotting Distances: Measure the distance from the station to the object on the ground (using a tape) and scale it down to the drawing sheet. Mark the location of the object on the ray at the scaled distance.
    • Repeating: Repeat this process for all visible objects from the station.

  4. Locating Points by Intersection:

    • Choosing Two Stations: Select two stations from which you can see the same object.
    • Sighting: Set up the plane table at each station. At each station, orient the plane table and sight the object.
    • Drawing Rays: Draw a ray on the drawing sheet from each station towards the object.
    • Intersection Point: The intersection of these rays on the drawing sheet indicates the location of the object.

  5. Locating Points by Traversing:

    • Setting up: Establish a traverse line (a series of connected lines) across the area.
    • Sighting: Set up the plane table at each station along the traverse line.
    • Orientation and Sighting: Orient the table at each station using back sighting. Sight forward to the next station and draw a line. Measure the distance and scale accordingly.
    • Plotting Details: From each station, survey and plot other features using the radiation or intersection methods.

  6. Leveling and Detailing:

    • Leveling is critical, use the spirit level to ensure the table is perfectly horizontal.
    • Detailing involves adding features like buildings, trees, and other elements to the map. This can be done using the same methods (radiation, intersection, or traversing) as needed.

Advantages of the Plane Table Survey Method

  • Direct Plotting: The map is created in the field, reducing errors and ensuring immediate results.
  • Simple Instruments: The instruments required are relatively simple and inexpensive.
  • Suitable for Unexplored Areas: It's ideal for areas where detailed maps are needed quickly and the ground is relatively flat.
  • Visual Inspection: The surveyor can see the map taking shape, helping to identify any inaccuracies.
  • Adaptability: The method can be easily adapted to the specific requirements of the survey.

Disadvantages of the Plane Table Survey Method

  • Not Suitable for All Weather Conditions: The method is susceptible to rain, wind, and direct sunlight, which can damage the drawing sheet or make sighting difficult.
  • Time-Consuming: Plane table surveys can take a lot of time, particularly for large areas.
  • Requires Skilled Personnel: Accuracy depends on the surveyor's skill and experience.
  • Less Accurate: Generally, it's less accurate than other surveying methods like theodolite surveying.
  • Difficult in Dense Vegetation: Sighting can be difficult in areas with dense vegetation.

Practical Tips for Plane Table Surveying

  • Protect Your Drawing Sheet: Use a waterproof cover or work under a shelter, especially if weather conditions are unpredictable.
  • Choose a Good Location for Stations: Select stations with good visibility and stable ground.
  • Work Systematically: Follow a logical sequence when surveying and plotting points to avoid errors.
  • Check Your Work: Always check the map as you go and compare it with the ground features.
  • Use a Hard Pencil: A hard pencil (e.g., a 4H or 5H) helps to make fine, accurate lines and avoids smudging.

Applications of Plane Table Surveying

  • Mapping Roads and Railways: Create route maps.
  • Topographical Mapping: Producing maps of land features and elevation.
  • Cadastral Surveys: Mapping land boundaries.
  • Forest Surveys: Creating maps of forests and woodlands.
  • Military Surveys: For creating maps in military operations.
  • Urban Planning: Mapping cities and towns for development purposes.

Key Takeaways

  • The plane table survey is a graphical surveying technique where mapping is done directly in the field.
  • Essential instruments include the plane table, alidade, tripod, and spirit level.
  • The method involves leveling, orienting, and sighting to plot features accurately.
  • Methods include radiation, intersection, and traversing to locate points.
  • Advantages include direct plotting and simplicity, while disadvantages include weather dependency and potential for lower accuracy.
  • Plane table surveying is used for various mapping applications, including road, topographic, and cadastral surveys.

I hope this detailed explanation helps you understand the plane table survey method! Feel free to ask any further questions!