Understanding Plane Table Temporary Adjustments

Plane table surveying is a graphical method of surveying where field work and plotting are done simultaneously. To get accurate results, the plane table and its accessories need to be set up correctly at each station. This setup involves performing certain temporary adjustments.

These temporary adjustments are crucial for ensuring the plane table is correctly positioned and oriented for taking observations and plotting features accurately on the map sheet.

Sequence of Temporary Adjustments for Plane Table Surveying

After setting up the tripod and placing the plane table on it, there are usually three main temporary adjustments that are carried out in a specific order:

1. Centering
2. Levelling
3. Orientation

Sighting, using an alidade, is done after the temporary adjustments are complete, as part of the observation process to locate points.

Analyzing the Adjustments

Let's briefly understand what each temporary adjustment involves:

• Centering: This involves positioning the table such that the point on the map sheet representing the station occupied is exactly vertically above the station ground point. This is usually done using a plumbing fork and plumb bob.
• Levelling: This involves making the table top perfectly horizontal. This is essential so that the lines drawn on the map truly represent horizontal distances and angles. This is done using a spirit level.
• Orientation: This involves setting the plane table at the new station such that the lines on the map sheet are parallel to the corresponding lines on the ground. This ensures that the map maintains its correct alignment with respect to the ground features and north direction. Orientation can be done by various methods like back sighting or using a trough compass.

Identifying the First Temporary Adjustment

While the exact sequence can sometimes slightly vary depending on the specific method or equipment used (e.g., some might do approximate centering first), the standard and most logical sequence after setting up the tripod and table is:

1. Levelling: It's practical to make the table top horizontal first because both centering (using a plumbing fork hanging from the sheet) and orientation (which relies on accurate alignment) are easier and more accurate when the table is level.
2. Centering: Once the table is level, precise centering over the station mark is performed.
3. Orientation: Finally, the table is oriented correctly.

Sighting is part of the observation process done *after* the table is properly set up and adjusted (leveled, centered, and oriented).

Therefore, the first temporary adjustment typically performed after setting up the plane table at any station is Levelling.

Conclusion on Plane Table Adjustment Sequence

Based on the typical procedure for setting up a plane table, Levelling is the initial temporary adjustment performed to ensure the table's surface is horizontal before precise centering and orientation are carried out. Sighting is an observation step, not a temporary adjustment in this context.

Revision Table: Plane Table Adjustments

Additional Information on Plane Table Surveying

Plane table surveying is a relatively simple and quick method, especially suitable for small-area mapping. It provides a field plot which can be checked immediately. However, it is less accurate than methods using instruments like theodolites or total stations and is affected by weather conditions like rain and wind. The temporary adjustments, particularly Levelling and Centering, are fundamental skills required for any type of surveying instrument that needs to be set up over a point and made horizontal.