Plane Table Surveying Explained: A Comprehensive Guide

by Wholesomestory Johnson 55 views

Hello there! I'm excited to break down the concept of plane table surveying for you. You asked a great question, and I'm here to provide a clear, detailed, and accurate explanation. Let's dive in!

Correct Answer

Plane table surveying is a graphical method of surveying where field observations and plotting are done simultaneously on the field using a plane table.

Detailed Explanation

Plane table surveying is a unique surveying technique because it combines field observations and map creation into a single step. Instead of taking measurements in the field and then drawing a map later in the office (like in methods such as theodolite surveying or leveling), with plane table surveying, you observe the features directly in the field and draw them on the map at the same time.

This method is particularly useful for creating topographic maps and cadastral maps of areas that aren't too large. Its main advantage lies in its simplicity and directness, making it a valuable technique for many surveying applications.

Key Concepts

Before we get deeper into the process, let's clarify some essential concepts:

  • Plane Table: The main instrument. It's a drawing board mounted on a tripod, providing a stable and level surface for plotting.
  • Alidade: A sighting device with two vanes (sight) used to align the plane table with specific points in the field. It has a ruler with beveled edges.
  • Leveling: Ensuring the plane table is perfectly horizontal.
  • Centering: Positioning the plane table directly over a known point on the ground.
  • Orientation: Aligning the map on the plane table with the corresponding features in the field.

Instruments Used in Plane Table Surveying

Let's take a look at the key instruments needed:

  1. Plane Table: A drawing board, usually made of well-seasoned wood, mounted on a tripod. Its size varies depending on the map size needed. A good size for general use is about 750 mm x 600 mm.
  2. Alidade: This sighting device comes in two primary types:
    • Plain Alidade: This consists of a metal or wooden ruler with two vanes (sight). One vane has a narrow slit, and the other has a wider slit with a hair or wire. It's used for sighting distant objects.
    • Telescopic Alidade: Contains a small telescope with a leveling bubble. This allows for more accurate sighting and is also useful for measuring differences in elevation.
  3. Spirit Level: Used to ensure the plane table is perfectly horizontal.
  4. Plumbing Fork: Or U-frame, used for centering the plane table directly over a ground station. It has a plumb bob hanging from the center, which is aligned over the ground point.
  5. Drawing Sheet: High-quality paper is used to draw the map.
  6. Other Accessories: These include a drawing pencil, eraser, and drawing pins to fix the drawing sheet to the plane table.

Procedure for Plane Table Surveying

Here's the typical procedure for carrying out plane table surveying:

  1. Setting Up the Plane Table:

    • Centering: Place the plane table over the station point (e.g., a point whose position is known). Use the plumbing fork to center the plane table accurately over the point. The plumb bob should hang directly above the ground station.
    • Leveling: Use the spirit level to ensure the plane table is perfectly horizontal.

  2. Orientation:

    • Orientation means aligning the plane table so that the lines on the map are parallel to the corresponding lines on the ground. There are two main methods:
      • Using a Compass: Place a drawing sheet on the plane table. Locate magnetic north on the drawing sheet using a trough compass. Then, align the sheet so that the north direction on the map corresponds to the magnetic north in the field.
      • Back-Sighting: If a line has already been plotted on the map (e.g., from a previous setup), orient the plane table by aligning the plotted line with the corresponding feature in the field (using the alidade). This is generally more precise.

  3. Plotting and Sighting:

    • Choose a suitable scale for your map. Mark the station point on the drawing sheet. This is the point over which the plane table is set up.
    • Sighting: Place the alidade along a line on the drawing sheet that radiates from the point on the sheet that represents the ground station. Sight the object through the alidade. Draw a ray or line of sight on the drawing sheet extending from the station point, using the edge of the alidade.
    • Measuring Distances (Optional): Measure the distance to the object (e.g., using a tape or tacheometry if using a telescopic alidade). Scale the distance and mark the plotted position of the object on the line of sight. If using a plain alidade, distances can be estimated or obtained from previous surveying data.

  4. Radiation Method (for details):

    • The radiation method is the simplest. It works for points that are visible from the plane table station.
    • With the plane table set up and oriented, sight each point to be surveyed with the alidade. Draw a ray toward the points. Measure and plot the distances to the points along the rays.
    • Draw the map features by connecting the points.

  5. Intersection Method:

    • Used when it's not possible to directly measure the distance to a point (e.g., a distant inaccessible object like a tower or a peak). At least two stations are required.
    • Set up the plane table at two different stations. Sight and draw rays to the inaccessible point from each station. The intersection of these rays on the map sheet determines the position of the point.

  6. Traversing Method:

    • Useful for establishing a network of survey lines (traverse) and plotting the features.
    • The plane table is set up at consecutive stations. The orientation is done by back-sighting to a previously plotted point. The method involves taking observations and plotting the details.
    • The traverse lines are plotted, and other features are located using the radiation method.

  7. Resection Method:

    • Used when the plane table's position needs to be determined on the map sheet, when the position of three or more well-defined points (control points) are already known on the map sheet.
    • This method involves setting up the plane table at an unknown station and orienting it by sighting at the known points. The rays are drawn to the known points from the unknown station. The intersection of these rays determines the position of the plane table on the map.

  8. Levelling with the Telescopic Alidade:

    • The telescopic alidade is used for measuring elevations of points.
    • Set up the plane table and level it. Sight the point of interest through the telescope and take a reading on the staff. Calculate the elevation of the point using the staff reading and the known elevation of the plane table station.

Advantages of Plane Table Surveying

  • Simplicity: The method is straightforward to understand and apply.
  • Directness: Maps are created directly in the field, which saves time and reduces the chance of errors that might occur when transferring data from field books to a map in the office.
  • Suitability for Complex Terrains: It's well-suited for mapping irregular terrains and areas with many details.
  • Visual Aid: Plane table surveying allows the surveyor to see the map take shape in front of them, allowing to better understand the surveyed terrain.
  • Cost-Effective: The equipment is relatively inexpensive compared to some other methods.

Disadvantages of Plane Table Surveying

  • Weather Dependence: It is best suited for use in clear weather conditions because rain, wind, and sunlight can interfere with the work.
  • Accuracy: The accuracy depends on the skills of the surveyor and the precision of the instruments. It is generally less accurate than methods using theodolites.
  • Terrain: The method is suited to relatively small areas and relatively flat, open areas. Rough terrain may reduce its effectiveness.
  • Difficulties in Dense Vegetation: Sight lines can be obstructed by vegetation, making it challenging to observe distant objects or plot details.
  • Bulky Equipment: The plane table and tripod can be somewhat bulky to transport and set up, particularly over long distances or difficult terrain.

Errors in Plane Table Surveying

Errors can occur during plane table surveying, affecting the accuracy of the map. These errors can be classified into:

  • Instrumental Errors: These arise from imperfections in the instruments used.
  • Personal Errors: These are due to the surveyor's limitations, such as errors in sighting, leveling, or plotting.
  • Natural Errors: These are caused by environmental factors like temperature changes or wind.

To minimize errors, use high-quality instruments and follow the procedures correctly, with meticulous attention to detail.

Key Takeaways

  • Plane table surveying is a graphical method where observation and plotting are done simultaneously in the field.
  • Instruments: This method uses plane tables, alidades, spirit levels, plumbing forks, and drawing sheets.
  • Methods: The methods used in plane table surveying include radiation, intersection, traversing, and resection.
  • Advantages: Plane table surveying's simplicity, directness, and suitability for complex terrains make it valuable.
  • Disadvantages: Limitations include weather dependency and potential for lower accuracy than other methods.
  • Accuracy is key: Minimize errors by using quality instruments and following proper procedures.

I hope this detailed explanation of plane table surveying is helpful! Let me know if you have any more questions. Happy surveying!