Comprehensive Guide to Plane Table Surveying Techniques and Tools

Plane table surveying, a traditional land measurement technique, offers unique advantages for field surveys. It allows for immediate visualization of the surveyed area directly on paper, which is beneficial when quick and accurate mapping is needed. This method combines simplicity with efficiency, making it a valuable skill for surveyors interested in capturing detailed site layouts without relying heavily on digital tools.

Equipment Used in Plane Table Surveying

The effectiveness of plane table surveying relies on the precise use of its equipment. Central to this setup is the plane table itself, a flat, stable surface typically mounted on a tripod, serving as the drawing board for on-site mapping. The table’s stability is crucial, as any movement can lead to inaccuracies.

The alidade, a sighting device, is used for taking precise angular measurements. Modern alidades often include telescopic sights, enhancing accuracy over longer distances. This tool aligns the plane table with the features being surveyed, ensuring the drawn map accurately reflects the terrain. A smoothly pivoting alidade maintains the integrity of survey lines.

A plumb bob ensures the plane table is perfectly horizontal. This simple device, consisting of a weight suspended from a string, helps level the table by indicating any tilt. A level plane table is necessary for accurate plotting, as even slight inclinations can distort results.

Setting Up the Plane Table

Establishing the plane table in an optimal location is essential for survey accuracy. The site should offer a clear line of sight to all relevant landscape features. Surveyors often choose elevated positions to minimize obstructions. Once a location is selected, the plane table is anchored firmly to ensure stability, even in windy conditions.

Positioning the table involves strategic orientation. Aligning it with a known reference point, such as a landmark, establishes a consistent framework for the survey. This orientation is often achieved using a compass, ensuring the table’s alignment is true to the cardinal directions. This sets a baseline for accurately capturing the terrain’s nuances.

After securing and orienting the table, achieving a level surface is the next focus. Adjusting the tripod legs compensates for uneven ground, ensuring the table is horizontal. A spirit level assists in this process, providing a visual guide to verify levelness. A stable and level setup directly impacts the accuracy of the plotted data.

Methods of Plane Table Surveying

Plane table surveying includes several methods, each suited to specific needs and site conditions. These techniques allow surveyors to adapt to various terrains and project requirements, ensuring comprehensive mapping.

Radiation Method

The radiation method involves plotting points directly from a single station. The surveyor uses the alidade to sight various features from a central point and draws lines on the paper to represent these features. Distances to these points are measured and scaled directly onto the map. This method is useful for small areas where features are visible from one location. Its simplicity makes it popular for quick surveys, though it may not suit larger or more complex sites requiring multiple stations.

Intersection Method

The intersection method uses two or more stations to determine a point’s position by intersecting lines drawn from these stations. This technique is advantageous when direct distance measurement is impractical, such as across a river. By sighting the same feature from different locations, surveyors can accurately plot its position through line intersections. This method enhances accuracy in challenging terrains and is often used with other methods for comprehensive surveys.

Traversing Method

In the traversing method, surveyors move the plane table from one station to another, plotting a series of connected lines representing the survey path. This technique is ideal for mapping linear features like roads or property boundaries. At each station, the surveyor aligns the table with the previous station and plots the next point, creating a continuous chain of measurements. This method provides detailed representation and is effective for large-scale projects.

Resection Method

The resection method determines the surveyor’s position on the map by sighting known points in the landscape. This technique is useful when the surveyor’s location is unknown or when starting a survey from a new point. By sighting at least three known points and drawing lines back to the plane table, the surveyor can pinpoint their exact location through line intersections. This method is effective in complex terrains where direct measurement is challenging.