1.17. Plane Tabling

Plane Table is called the father of 'Mapping'. It is used to prepare various maps in the field. It is a graphical method, where observations, mapping and contouring, proceed simultaneously. It is a time-consuming and laborious process to create a map, but is a more accurate approach. The plane table consists of a wooden drawing board with arrangement of fixing it on a tripod. With the availability of Total Station, GPS, satellite images and LiDAR data, Plane tabling methods of surveying are almost obsolete now-a-days.

Plane tabling is a method used for mapping where the map is created in the field instead of in an office setting. It is termed the 'father of mapping' due to its foundational role in the practice of surveying and cartography. This technique involves using a plane table, which is a simple wooden drawing board that can be set up on a tripod. The advantage of this method is that observations, mapping, and contouring happen simultaneously. However, it can be quite labor-intensive, taking a lot of time to create maps accurately. Despite its accuracy, modern technologies like Total Stations and GPS have made plane tabling less common today.

Imagine trying to create a detailed scrapbook while documenting a family vacation in real-time. You would take photos, write notes, and sketch memories as you experience them. Similar to this, plane tabling allows surveyors to capture precise landscape details as they survey, helping create a more accurate and up-to-date ‘map’ of the area.

Advantages: (a) The map is prepared in the field; therefore, the office work is minimal. (b) The observations and plotting are done simultaneously, and the surveyor can see the terrain before him/her, so chances of losing the details are rare. (c) Errors in plotting the details on plane table can be checked by drawing the check lines using several methods. (d) The angles and distance measurements can be obtained graphically, and hence there is no need to carry out measurements in the field. (e) Many far-off objects/details can be plotted accurately using intersection method to prepare the map in shortest time. (f) It is advantageous in magnetic areas where compass survey is not reliable.

The advantages of plane table surveying include a reduction in office work since maps are made directly in the field. The simultaneous observation and plotting allow surveyors to accurately capture the landscape without the risk of missing important details. Additionally, any plotting errors can be checked using check lines drawn from different locations. The graphical measurement of angles and distances means that actual measurements may not need to be taken in the field. The intersection method enables precise plotting of distant objects quickly, and this technique is also beneficial in locations where magnetic compasses do not function well.

Think of a chef preparing a meal in a busy kitchen while also plating the dishes simultaneously. This allows the chef to see how the ingredients interact and adjust flavors on the spot without needing to remember everything for later, which parallels how surveyors can make immediate adjustments and observations while mapping.

Disadvantages: (a) Plane tabling work is dependent on weather conditions (e.g., rains, high winds). (b) The plane table is not very much suitable in a dense forest area or urban area. (c) If survey map is to be replotted at some different scale, entire work has to be re-done. (d) Its accessories are heavy to carry in the field from one location to another. (e) The paper maps are subject to shrinkage or expansion while working in the field for long time. (f) It is laborious and time-consuming to prepare map of a large area. (g) It requires large manpower for doing various activities from observations to plotting.

While plane table surveying has its advantages, it also has several disadvantages. Weather conditions greatly affect the feasibility of surveying; for example, strong winds or rain can hinder operations. This method is less practical in areas with dense vegetation or urban settings where visibility is limited. Additionally, if a map needs to be altered in scale, the entire mapping process must be repeated. The equipment can be cumbersome to carry, and paper maps can change size with temperature or humidity, leading to inaccuracies. Furthermore, creating maps over large areas requires considerable time and manpower.

Imagine trying to set up a picnic but it starts to rain heavily. You have to pack everything up and wait for the weather to clear before you can enjoy your day. Similarly, surveyors may have to halt their work due to unfavorable weather conditions, which can delay their mapping project.

There are three steps to work with plane table: (i) Levelling the plane table, (ii) Centering the plane table, and (iii) Orienting the plane table. Levelling is done using the sprit bubble tube. Centering is done with the help of a plumb bob. Orientation is done either with a magnetic compass or with a back ray method.

To effectively use a plane table, surveyors must follow three key steps: levelling, centering, and orienting. Levelling involves ensuring the plane table is perfectly flat, which can be checked using a spirit level. Centering ensures that the table is positioned directly over a specific point of interest, typically using a plumb bob. Orientation sets the table to face a known direction, which can be accomplished with a magnetic compass or a back ray method to align with reference points on a map.

Think of setting up a camera for a group photo. First, you need to ensure it's level so everyone looks good. Then, you center it on the group, and finally, you aim it in the right direction to capture everyone's smiles. Surveyors do similar things with their plane tables to ensure accurate maps.

There are four methods of plane table surveying: (i) Radiation, (ii) Intersection, (iii) Traversing, and (iv) Resection. The radiation method uses of a distance and a direction to locate the object on the map. The plane table is set up at a station and various points in the vicinity are located on the map by measuring the distances to the different stations using a tape (or tacheometry), and subsequently radiating (drawing) the rays from the instrument station to those points which are to be located on the map. The intersection method is used extensively to locate an object by two directions from two stations at the end of a base line of known length. The method is widely used for plotting the far-off details on the maps. The traversing method is used in areas where visibility is obstructed by high-rise building/forest or topography. The resection method is used to accurately locate the position of ground station (unknown point) on the map, occupied by plane table. It is done by drawing the rays from two or three already known and plotted point on the map. Once the position of ground station is plotted on the plane table, any of the method for plotting the details could be used.

There are four primary methods of plane table surveying each suited for different scenarios. The Radiation method involves measuring distances from a central point to various locations, then drawing rays from this point on the map to accurately depict the terrain. The Intersection method helps locate an object by surveying it from two different stations along a known baseline, effectively providing a precise spot based on where the two observation lines cross. The Traversing method is useful when visibility is obstructed, allowing surveyors to plot known points and work from there. Finally, the Resection method allows surveyors to pinpoint an unknown location by referencing known points already mapped. This versatility is what has historically made plane tabling a valuable surveying tool.

Consider a treasure hunt where you have a map with various land markers. Radiation would be like moving straight towards a marker from your treasure map, while the Intersection method would resemble triangulating the treasure's location with clues from two different landmarks on the map. Traversing is like going through a dense forest, plotting signs as you navigate your way through, and Resection could be viewed as using landmarks you pass to verify that you're still headed in the right direction.

Now-a-days, plane tabling is considered as obsolete, and therefore is not carried out in the field. The field data is directly collected and stored using either Digital Levels, GPS or Total Station, and mapping is carried out using the capabilities of the software in a computer. Alternatively, photogrammetry and remote sensing images are used to create various types of maps required in various civil engineering projects.

In contemporary surveying practices, the plane tabling method has largely been phased out due to the availability of advanced technologies. Digital levels, GPS devices, and Total Stations enable surveyors to gather and record data more efficiently and accurately. This data is typically then processed through sophisticated software applications, which can generate detailed maps much faster than the traditional plane table methods. Moreover, innovations such as photogrammetry and remote sensing now offer extensive capabilities to create maps using aerial imagery, further minimizing the need for ground-based methods like plane tabling.

Think of how many people used to write letters by hand, but now most communication happens through emails and messaging apps. Similarly, academia and various industries have moved on to digital tools for mapping and surveying, rendering the traditional plane tabling methods outdated in comparison to the quicker and more efficient modern techniques.

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