Surveying plays a vital role in the planning and mapping of geographical areas. The aim of surveying is to create plans and maps that accurately represent objects on the ground, showing their relative positions at a suitable scale. The process involves both fieldwork and office work. This article delves into the various steps required to transform field data into a detailed map.
Mapping Methods in Surveying
Surveying involves several methods, each tailored to specific requirements. Chain and compass surveys demand significant office work to process the data collected in the field. Conversely, plane table surveys require less office work due to their real-time plotting nature. Regardless of the method used, the preparation of a comprehensive plan requires meticulous attention to detail during office work.
Steps in Map Creation
Creating a map from survey data involves a series of methodical steps. These steps ensure the accuracy and usability of the final plan.
1. Applying Necessary Corrections to Measurements
Before plotting the map, corrections must be applied to account for errors in the measurements. This includes tape and chain corrections and adjustments for local attraction in compass surveys. These corrections ensure the accuracy of survey lines and measurements.
2. Drawing the Index Plan
The index plan, or key plan, is a rough sketch that outlines the surveyed area. While it does not need to be to scale, it should approximately represent the distances and directions of the survey lines. The index plan is crucial for visualizing the area to be plotted.
3. Selecting the Scale
The scale of the map depends on the type of survey conducted. A larger scale is preferable whenever possible to provide more detail. The size of the paper and the largest linear measurement in the field also influence the choice of scale.
4. Selecting Orientation
The orientation of the map is determined based on the index plan. The map is positioned so that it fits neatly within the drawing sheet, with its larger dimension aligned along the length of the paper. The North direction is selected and marked to ensure proper alignment.
5. Drawing the Network of Survey Lines
Using the index map as a guide, the network of survey lines is drawn to scale. The first station point is marked, and subsequent survey lines are plotted based on their direction and scale. Adjustments to scale or orientation may be made if necessary.
6. Distributing Closing Errors
In closed traverses, discrepancies may arise where the plotted endpoint does not coincide with the starting point. This is known as a closing error. Before correcting this error, plotting errors should be ruled out. Small errors can be distributed across the network graphically or mathematically, but larger errors may require re-surveying.
7. Filling in the Details
Once the survey lines are plotted, details such as buildings, roads, trees, and poles are added based on field notes. Using the scale, salient points are marked, and the respective lines are joined to represent objects. Standard conventions are followed to ensure consistency and clarity.
8. Colouring the Map
If the map requires colour coding, standard shades as specified in IS 962-1989 are applied. Light washes are typically used to distinguish features clearly without obscuring the details.
9. Drawing the Graphical Scale
To prevent discrepancies due to shrinking or stretching of the drawing sheet, a graphical scale is added. This scale, typically 150 to 270 mm long, is positioned in the lower right-hand corner, just above the index space.
10. Writing the Index
The index provides essential details about the map, such as the description of the area, the scale used, and the names of the survey leader and drafter. It is written in the lower right-hand corner of the sheet, with the North direction neatly shown in the upper right corner.
Conclusion
Surveying and map making involve meticulous planning, precision, and attention to detail. From applying corrections to measurements to writing the index, each step contributes to creating an accurate and usable map. The process not only reflects the skills of the surveyor but also underscores the importance of standardization in mapping practices. An accurate map is an invaluable tool in planning, development, and decision-making.