In surveying, BM stands for Bench Mark, which is a fundamental reference point used to establish and maintain vertical control. A Bench Mark (BM) is a relatively permanent object, natural or artificial, bearing a marked point whose elevation is known. These points serve as crucial reference datum for various surveying, mapping, and engineering projects, ensuring consistency and accuracy in elevation measurements.
Purpose and Significance of Bench Marks
The primary purpose of a Bench Mark is to provide a reliable and precisely known elevation reference for a given area. They are indispensable for:
- Vertical Control: Establishing a stable vertical datum for all elevation measurements.
- Consistency: Ensuring that all surveys and construction projects within a region refer to the same elevation system.
- Project Design and Construction: Providing a fixed height reference for building foundations, roads, bridges, and other infrastructure.
- Mapping and GIS: Creating accurate topographical maps and digital elevation models.
- Hydrology and Flood Plain Management: Monitoring water levels and delineating flood zones.
Types of Bench Marks
Bench Marks can be categorized based on their permanence and method of establishment:
Permanent Bench Marks (PBMs)
Permanent Bench Marks are durable markers installed for long-term use, typically by government agencies like the National Geodetic Survey (NGS) in the United States. They are designed to withstand environmental changes and human disturbance.
- Characteristics:
- Usually brass or bronze disks embedded in concrete monuments, rock outcrops, or sturdy structures.
- Precise elevations are determined through rigorous geodetic leveling tied to a national vertical datum (e.g., North American Vertical Datum of 1988 - NAVD88).
- Often include identification numbers, the surveying agency, and the year of establishment.
Temporary Bench Marks (TBMs)
Temporary Bench Marks are less formal and are established for the duration of a specific project. Their elevations are typically derived from a nearby PBM.
- Characteristics:
- Can be a nail in a utility pole, a chiseled mark on a curb, a paint mark on a fire hydrant, or a bolt on a bridge abutment.
- Their stability is less certain than PBMs, and they are prone to being disturbed or destroyed.
- Used for immediate, localized elevation control within a project area.
| Feature | Permanent Bench Mark (PBM) | Temporary Bench Mark (TBM) |
|---|---|---|
| Durability | High; designed for long-term stability | Lower; used for project duration |
| Material | Brass/bronze disks, concrete monuments | Nails, chisel marks, paint marks, existing features |
| Elevation | Highly precise; tied to national datum | Less precise; derived from PBMs or local control |
| Installation | Professional, rigorous geodetic methods | Simple, quick, project-specific |
| Use Case | Regional vertical control, large-scale projects | Localized project control, daily site operations |
Characteristics of Effective Bench Marks
An effective Bench Mark should possess several key attributes to ensure its reliability and utility:
- Stability: Placed in geologically stable areas, free from subsidence, frost heave, or erosion.
- Accessibility: Easily locatable and accessible for surveyors, even if partially obscured.
- Protection: Protected from vehicle traffic, construction activities, and vandalism.
- Clear Marking: Clearly identifiable with a unique designation or number.
Establishment of Bench Marks
Bench Marks are established through precise leveling techniques, most commonly differential leveling. This process involves using a level instrument and a graduated rod to determine the precise height difference between points. All Bench Mark elevations are referenced to a specific vertical datum, such as mean sea level (MSL) or a more refined geodetic datum. The accuracy of a Bench Mark's elevation depends on the precision of the leveling survey, which can range from general project control to high-order geodetic networks.
Practical Applications of BMs
Bench Marks are fundamental to a wide array of activities:
- Civil Engineering Projects: Setting foundation depths, calculating earthwork volumes for roads and dams, and designing drainage systems.
- Land Development: Grading land for subdivisions, ensuring proper slope for utilities and stormwater runoff.
- Infrastructure Management: Monitoring the settlement of bridges and large structures, planning utility installations.
- Environmental Studies: Tracking sea-level rise, monitoring land subsidence or uplift, and delineating flood plains for insurance maps.
- Scientific Research: Providing baseline elevation data for geological studies, glaciology, and environmental monitoring.
By providing a fixed and known elevation, Bench Marks allow surveyors, engineers, and planners to work with consistent and accurate height data across various projects and over time.
