Bench Marks
A benchmark is a permanent reference point with a known elevation used in surveying and construction. Benchmarks are crucial for establishing accurate vertical measurements and ensuring consistency in leveling operations across large areas. They serve as the foundation for determining elevations of other points in a survey or construction project.
Benchmarks are used for various purposes, including:
- Establishing a consistent vertical datum for surveys and construction projects
- Providing reference points for leveling operations
- Ensuring accuracy in elevation measurements over time
- Facilitating the creation of topographic maps and engineering designs
Based on their permanency and precision, benchmarks are classified into four main types:
- G.T.S. Bench Marks (Great Trigonometrical Survey Bench Marks)
- Permanent Bench Marks
- Arbitrary Bench Marks
- Temporary Bench Marks
1. G.T.S. Bench Marks
The Great Trigonometrical Survey (G.T.S.) Bench Marks are the most accurate and permanent type, established by the Survey of India. These points are spaced at intervals of about 100 kilometers across the country. Their elevation is determined relative to the mean sea level (MSL), which is calculated based on tide readings taken at Mumbai port over a span of 19 years. The elevation data for G.T.S. Bench Marks, precise to two decimal places in meters, is published in leveling pamphlets and depicted on topographical sheets, providing a critical reference for large-scale projects across India. These points are used as foundational references for geodetic surveys and large infrastructure projects due to their high precision.
Additional important information about G.T.S. Bench Marks
- G.T.S. benchmarks form the primary vertical control network for the entire country
- They are typically marked with permanent monuments or plaques for easy identification
- Regular maintenance and verification of these benchmarks ensure long-term reliability
- The 19-year observation period accounts for various tidal cycles, including the 18.6-year lunar nodal cycle, providing a highly accurate mean sea level reference
- G.T.S. benchmarks are crucial for large-scale engineering projects, such as canal systems, railways, and highways, that span across multiple regions
- G.T.S. benchmarks within 1° Latitude and 1° Longitude areas are documented in leveling pamphlets
2. Permanent Bench Marks
Permanent Bench Marks (PBM) are established between G.T.S. Bench Marks by various government agencies, such as the Survey of India and the Public Works Department (P.W.D.). These marks are set on clearly defined and enduring natural or man-made features, ensuring long-term availability. Examples of locations include isolated rock formations, culverts, kilometer stones, railway platforms, and gate pillars of inspection houses. The primary purpose of these marks is to serve as intermediate reference points between the highly precise G.T.S. Bench Marks.
Permanent Bench Marks created by the Survey of India are engraved with inscriptions like “G.T.S.O B.M. ,” indicating their origin and type. Their elevations are documented in leveling pamphlets, making them accessible for surveyors and engineers in need of reference data. Bench marks established by the P.W.D. are marked differently, typically on flat surfaces by etching a rectangle, below or above which the letters “B.M.” (Bench Mark) and the Reduced Level (R.L.) of the mark are inscribed. The rectangle is often filled in Japan black to make the marking stand out.
These Bench Marks play a vital role in engineering projects, especially in ensuring the correct elevation is maintained for precise construction tasks. For instance, in irrigation projects, G.T.S. or permanent Bench Marks are crucial in determining the required slope of canal beds to ensure that water flows efficiently under the force of gravity, preventing issues like water stagnation or inefficient drainage.
Additional Important Information about Permanent Bench Marks
- Permanent Bench Marks bridge the gap between widely spaced G.T.S. Bench Marks, allowing for more precise local measurements
- They play a vital role in urban development, road construction, and local flood control projects
- The use of durable materials and protected locations helps ensure the longevity of these bench marks
- In modern practice, many Permanent Bench Marks are now being integrated with GPS coordinates for enhanced precision and ease of location
3. Arbitrary Bench Marks
Arbitrary Bench Marks (ABM) are reference points used for smaller, localized leveling tasks where high precision is not necessary. Unlike G.T.S. or Permanent Bench Marks, which are tied to a specific datum like the mean sea level, Arbitrary Bench Marks are assigned an elevation that is assumed for the convenience of a particular project. These marks do not have any formal relationship to established datums and are often used when precise elevation data is not critical, such as in minor construction or temporary surveying operations.
For example, during small-scale land development or short-term construction projects, the elevation of a particular point, such as a stone or building corner, might be designated as zero or any arbitrary value. This allows the project team to perform leveling without needing a connection to national or permanent benchmarks. While not suitable for projects requiring long-term accuracy, Arbitrary Bench Marks are practical and efficient for day-to-day leveling tasks.
These benchmarks offer flexibility but are limited in scope and are typically abandoned after the project or task is completed, as they hold no relevance outside their immediate context.
Additional Important Information about Arbitrary Bench Marks
- When using Arbitrary Bench Marks, it’s crucial to clearly document their arbitrary nature in all project records
- If the project later needs to be tied into an official datum, a conversion factor can be established by connecting to a known bench mark.(This factor is the difference between the arbitrary elevation assigned to a point and its true elevation as determined by connecting to an official bench mark. For example, if an Arbitrary Bench Mark was assigned an elevation of 100.00 m but its true elevation is found to be 543.27 m, the conversion factor of 443.27 m would be added to all project elevations to align them with the official datum.)
- Care should be taken to avoid confusion between Arbitrary Bench Marks and official bench marks in the same area
- For projects that may expand in the future, it’s often wise to relate Arbitrary Bench Marks to a nearby official bench mark, even if not immediately necessary.
Best Practices While Using Arbitrary Bench Marks
- Clearly mark Arbitrary Bench Marks as such to prevent future misunderstandings
- Document the assumed elevation and any relevant local references
- Consider the potential future needs of the project when deciding whether to use Arbitrary Bench Marks
- When possible, establish a relationship to at least one official bench mark for potential future use
4. Temporary Bench Marks
Temporary Bench Marks (TBM) are short-term reference points used in leveling operations when a permanent Bench Mark is unavailable. These points mark the end of a day’s work and serve as the starting point for the next day’s measurements. Unlike permanent or arbitrary benchmarks, Temporary Bench Marks are intended for use over a brief period, typically the duration of a specific surveying or construction task.
The elevation of a Temporary Bench Mark is referred to as its Reduced Level (R.L.), which is crucial for continuing accurate leveling from the previous day. Careful selection is important when establishing a TBM, and it is usually placed on stable, well-defined, and easily identifiable features such as kilometer stones, parapet walls, the floor of verandahs, or the roots of old trees. This ensures the mark remains undisturbed and reliable until the work is completed.
To maintain accuracy and avoid confusion, the exact location and description of the TBM must be documented meticulously in level books, including details like the feature it is placed on and the corresponding Reduced Level. These descriptions are essential for resuming work accurately, ensuring consistency in elevation measurements across different days.
Additional Important Information about Temporary Bench Marks
- When selecting a TBM, consider potential disturbances or changes that might occur overnight
- In areas prone to vandalism or accidental disturbance, it’s advisable to establish multiple TBMs
- For projects spanning weeks or months, it’s crucial to periodically check TBMs against known permanent bench marks to ensure accuracy
- In some cases, well-established TBMs may be upgraded to semi-permanent status for longer projects
Best Practices While Using Temporary Bench Marks
- Use durable markers or paint to clearly identify the exact point of the TBM
- Take photographs of the TBM and its surroundings for easy relocation
- When possible, use natural features unlikely to be disturbed by human activity
- Establish TBMs at regular intervals in linear projects to minimize cumulative errors
- Inform all team members about the location and importance of TBMs to prevent accidental disturbance
