Quick Answer
How do you use a reflectorless total station for as-built survey?
A reflectorless total station measures distance by bouncing a laser off the target surface without a prism. For as-built survey, set up on a known control point, aim the EDM laser at the feature to measure (pipe invert, wall face, finished surface), and record the shot. Accuracy in reflectorless mode is typically +/-3-5mm at short range but degrades at long range and on surfaces at shallow angles. Confirm distances with a tape check when accuracy is critical.
How to Use a Reflectorless Total Station for As-Built Survey
Applies to: Topcon GT-503 series, Trimble S5/S7 DR, Leica TS15/TS16 R, Sokkia SRX Series — all with built-in reflectorless EDM
Reflectorless measurement capability transforms as-built survey efficiency. Instead of sending a rodperson with a prism to every point, one person on the instrument can shoot elevations on pipe inverts in open manholes, structure faces, guardrail posts, curb faces, and other features that can be aimed at but not physically occupied. The key is knowing when reflectorless accuracy is sufficient and when a conventional prism shot is needed.
Step 1: Set Up on Known Control and Verify Orientation
Set up the total station on a known control point using a tribrach and optical plummet. Level carefully and orient to a backsight control point. Confirm orientation with a check shot on a third independent point — residuals should be under 10mm horizontally. As-built surveys typically have looser tolerances than stakeout, but the setup must still be within the project specification.
Enter the instrument height precisely. As-built elevations are computed from the instrument height above the control point — an error in HI (Height of Instrument) shifts every elevation measurement by that amount.
Step 2: Understand Reflectorless vs. Prism Mode
Modern total stations offer two non-prism modes:
Reflectorless (DR — Direct Reflex): The EDM laser reflects off any surface — concrete, asphalt, pipe, metal. Accuracy is typically +/-3mm at 50m on a perpendicular surface. Accuracy degrades on surfaces at shallow angles (oblique incidence), on dark or transparent materials, and at distances over 100m.
Prism mode: The EDM locks to a retroreflective prism for maximum accuracy and range. Use prism mode for precise control work or when reflectorless accuracy is insufficient.
For as-built survey, reflectorless mode is appropriate for: pipe inverts in manholes, structure face positions, road surface elevations, finished floor elevations, and similar features where sub-centimeter accuracy is not required. For precise benchmark ties or structural dimensions, use prism mode.
Step 3: Targeting Technique for Accurate Shots
Aim the EDM laser precisely at the point of interest. The visible red or green laser dot shows exactly where the measurement will be taken — place the dot on the intended point, not near it. On pipe inverts, aim for the lowest point of the pipe interior (6 o'clock position). On concrete surfaces, aim for a clean surface away from joints, cracks, or loose material that could scatter the beam.
Watch for the EDM "good measurement" indicator before recording. Some surfaces (glass, polished metal, wet surfaces at steep angles) cause the EDM to return inconsistent readings. If the distance display is unstable or flashing, reposition the aim point slightly or use a reflective target sticker for a stable return.
Step 4: Record As-Built Points
In the data collector, configure the point description code system to match the as-built feature types on the project (PIPE_INV, MH_RIM, EP_CURB, BLDG_CORNER, FINISH_GRADE, etc.). Record each point with the appropriate code — this allows automatic feature extraction and drafting in CAD or GIS.
For pipe as-builts: record the manhole rim elevation (prism or reflectorless on the rim casting), the pipe invert elevation (reflectorless aiming down at the pipe invert), and the pipe diameter from field measurement. These three values define the pipe's as-built horizontal position, elevation, and size.
Step 5: Check Accuracy with Physical Measurements
For critical as-built features, spot-check reflectorless measurements against physical tape measurements. Measure a known dimension (wall-to-wall, manhole diameter, or pipe offset) with a tape and compare to the reflectorless data. Discrepancies over 25mm on short distances indicate aiming error or an EDM calibration issue.
Document the physical check measurements in the field notes as verification of reflectorless accuracy. For as-built surveys submitted to the owner or engineer of record, check measurements demonstrate due diligence in the data quality.
Frequently Asked Questions
What is the accuracy of reflectorless total station measurements?
Typical reflectorless accuracy is +/-3-5mm at distances under 50m on a perpendicular white or light-colored surface. Accuracy degrades to +/-10-20mm at 100-200m and on dark, wet, or oblique surfaces. Always check specifications for the specific instrument model — high-end instruments (Leica TS16, Trimble S7) achieve better reflectorless accuracy than budget models.
Can reflectorless total stations measure into manholes?
Yes — this is one of the most common uses. The EDM laser can measure the invert elevation at the bottom of a manhole without anyone descending. Aim carefully at the pipe invert (not the manhole floor), confirm the dot is on the target surface, and verify with a tape check on the first few manholes of a project to calibrate your technique.
When should I use prism mode instead of reflectorless mode?
Use prism mode when: distance exceeds 100m, the target surface is oblique, accuracy better than +/-5mm is required, or the surface material gives inconsistent EDM returns (glass, polished metal, dark surfaces). For control surveying and precise benchmarks, always use a prism or forced-centering system.
How do I prevent false readings with a reflectorless total station?
False readings occur when the EDM laser hits an unintended surface behind or beside the target — a common problem in cluttered areas like pipe yards or utility corridors. Verify the displayed distance is plausible for the target (roughly matches the estimated distance to the point). On modern instruments, watch for the "multiple targets" warning indicator that signals the laser is hitting more than one surface.
Capture as-built data, pipe invert records, and structure documentation with Gradelog. As-built field records organized and shareable for owner delivery. Free to start at gradelog.com.
