Before the first building corner is staked, the first utility is laid out, or the first anchor bolt is set, the survey crew needs reliable horizontal and vertical control. When control is established correctly at the beginning of a project, every crew that follows has a consistent reference to work from. When it isn’t, even a small discrepancy can carry through an entire site.
At Eagle Surveying, establishing and verifying control is one of the most important steps we take before construction layout begins.
What Should Be Used for Control Points?
Control points need to be stable, repeatable, and placed somewhere they are unlikely to be disturbed during construction.
Depending on the site, control may consist of:
- Rebar or iron rods set firmly in stable ground
- Survey nails or PK nails in pavement or concrete
- Mag nails with washers
- Concrete monuments or other permanent marks
- Existing project control that has been independently verified
A control point is only useful if it can be trusted. Whenever possible, control should be set in stable locations outside active excavation, grading, haul routes, material storage areas, and future construction.
Control should also be distributed around the project rather than concentrated in one location. This gives survey crews multiple points from which to work and allows them to independently check their setup.
How Many Benchmarks Should a Construction Site Have?
A construction project should never depend on a single benchmark.
At a minimum, we generally want two reliable vertical benchmarks, with additional benchmarks established as the size and complexity of the project increases. On a large or long site, several benchmarks distributed throughout the project may be appropriate.
Having multiple benchmarks allows crews to verify elevations instead of simply assuming that one benchmark has remained undisturbed.
If two benchmarks do not agree, work should stop until the discrepancy is resolved.
Redundancy is one of the simplest forms of quality control in surveying.
Grid or Surface? Know Before You Stake
One of the most important questions at the beginning of a project is:
Are the project coordinates grid or surface?
Grid coordinates are based on a mapping coordinate system or projection. Surface coordinates represent distances closer to actual ground distances at the project location. The difference may appear small, but over a large construction site it can become significant.
Before layout begins, the surveyor should review the project coordinate system, control information, civil drawings, survey files, and any provided scale factor.
The field crew should know:
- What coordinate system the project is using
- Whether coordinates are grid or surface
- What scale factor, if any, is being applied
- Where the project origin or scaling point is located
- Whether the CAD files and field data are using the same basis
This information should be documented and communicated clearly so that every crew working on the project uses the same system.
Should You Perform a Site Calibration?
Not automatically.
A site calibration can be a useful surveying tool, but it should not be treated as a routine step on every project.
If the project already has reliable published control and the survey equipment can work directly within the project’s coordinate system, a calibration may not be necessary.
Performing an unnecessary calibration can introduce another transformation into the project and potentially create differences between the field coordinates and the engineering or CAD design.
When a calibration is necessary, it should be based on multiple well-distributed control points—not one or two points clustered together.
After a calibration is created, independent control points should be checked to confirm that the calibration represents the entire project accurately.
The goal is not simply to make the equipment “fit” the control. The goal is to understand the coordinate system and maintain consistency throughout construction.
How Many Check Shots Should a Crew Take?
Checking control should not be a one-time event.
Every survey setup should include independent verification.
A good practice is to occupy or initialize from known control and then check to at least two additional independent control points before beginning layout. Whenever practical, checking more than one point provides even stronger verification, particularly on larger sites or when the work involves tight tolerances.
Crews should also perform check shots periodically during the day and after anything that could affect the setup, including:
- Moving the instrument
- Reinitializing GNSS equipment
- Changing base stations or network corrections
- Discovering disturbed control
- Receiving unexpected staking results
- Beginning precision layout
At the end of critical layout, another check provides additional confidence that the setup remained good throughout the work.
A check shot takes very little time compared with the cost of discovering an error after construction has already taken place.
Where Should Control Be Located?
Good control is both stable and usable.
Control should generally be placed outside areas likely to be disturbed but close enough that crews can efficiently use it.
Good locations may include:
- Corners or edges of the project outside construction limits
- Existing pavement that will remain throughout construction
- Stable concrete structures
- Areas behind curb or outside proposed grading
- Locations with good visibility across the site
- Areas accessible to both robotic total stations and GNSS equipment
- Opposite sides or different ends of the project to provide good geometry
Avoid placing primary control:
- Inside future building pads
- Near excavation limits
- In haul roads
- Along heavily traveled equipment routes
- In material staging areas
- Where future utilities will be installed
- In areas scheduled for demolition or grading
For large projects, control should be spread throughout the site so crews do not have to rely on long setups or continually transfer control from one end of the project to another.
Protecting Control Is Everyone’s Responsibility
Construction sites change quickly.
A control point that was in a safe location during initial grading may eventually be surrounded by excavation, paving, utilities, or equipment traffic.
Control should be clearly identified, documented, and periodically verified throughout the project. If primary control is likely to be disturbed, replacement control should be established and checked before the original point is lost.
If a control point is hit, moved, removed, or suspected of being disturbed, it should never simply be reset from memory or assumed to still be correct.
It needs to be verified from independent control.
Control Should Agree With the Design
Before construction staking begins, survey control should be compared with the project’s design information.
That means confirming that:
- The civil CAD matches the supplied control
- Horizontal coordinates agree
- Published benchmark elevations agree
- The coordinate basis is understood
- Grid-to-surface relationships are accounted for
- Any provided survey or engineering control information is consistent
Finding a discrepancy during the initial setup is relatively inexpensive.
Finding the same discrepancy after concrete has been poured is not.
Good Control Creates Good Construction Layout
Survey control may not be the most visible part of a construction project, but nearly everything built on the site ultimately depends on it.
Building corners, utilities, paving, walls, columns, anchor bolts, foundations, and finished elevations all trace back to the same horizontal and vertical reference system.
That is why Eagle Surveying believes construction layout should begin with a deliberate control process:
Establish it. Verify it. Document it. Check it. Protect it.
Taking a little more time at the beginning of a project can prevent costly mistakes later—and gives contractors, engineers, and survey crews confidence that everyone is building from the same starting point.
Eagle Surveying — Building Confidence From the Ground Up