10.3 Accuracy Reference
What the published specifications actually mean, what conditions produce them, and how to choose the right accuracy method for each AEC project type.
What the Published Specifications Actually Mean
The accuracy figures published by XGRIDS are achievable results under defined conditions. They are not guaranteed outcomes from any scan regardless of technique, environment, or control strategy. Understanding what each number represents prevents misrepresentation to clients and unrealistic expectations in the field. RMSE figures also assume proper field procedure and unbiased error distribution. Real-world results can be affected by atmospheric conditions, antenna geometry, multipath from nearby structures, satellite count, baseline length to the correction source, and CRS or geoid configuration errors. Any of these can introduce systematic error that degrades outcomes below the published specification regardless of Fixed status or GCP count.
L2 Pro
K1
PortalCam accuracy note: The PortalCam produces 3D Gaussian Splat output only, not georeferenced point clouds. Published internal test data shows ranging error below 1% versus a laser ruler across multiple scenarios (2 outdoor, 1 indoor). Absolute coordinate output is not applicable. The PortalCam is designed for visualization and measurement within LCC Studio, not for geospatially anchored deliverables.
Relative Accuracy
Relative accuracy is the internal consistency of the point cloud: whether a wall is planar, whether measured dimensions repeat reliably, whether the geometry holds across the full scan without distortion. A scan with good relative accuracy looks correct and measures correctly within itself, even if it is not registered to any real-world coordinate system.
Relative accuracy is achieved through correct technique and loop closure. It does not require RTK, GCPs, or any external reference. It is the baseline result from any properly executed scan with genuine loop closures throughout the route. In environments with poor scene structure, long linear paths without loops, or technique errors, relative accuracy degrades regardless of which device is used.
Absolute Accuracy
Absolute accuracy is how closely the point cloud registers to a real-world coordinate system. It requires external control: RTK, PPK, or surveyed ground control points. Technique alone cannot produce absolute accuracy, no matter how well the scan is executed.
The 3.0 cm RMSE absolute accuracy figure applies when RTK or GCPs are used correctly. It is not the accuracy of a pure SLAM scan. A scan without any georeferencing has no defined absolute accuracy because it has no connection to a coordinate system.
The Two Sources of Elevation Error
Elevation error in SLAM scans comes from two independent sources that must be addressed separately:
IMU leveling error: The SLAM system uses the IMU to determine level. The average IMU leveling error is approximately 0.07 degrees. Over large areas, this small angular error compounds into meaningful elevation discrepancy. RTK eliminates this error by providing an absolute vertical reference. Surveyed GCPs with Z coordinates also eliminate it.
SLAM accumulated drift: SLAM systems accumulate positional error in X, Y, and Z over distance due to the absence of absolute constraints. Loop closure distributes and reduces this error. Ground control points constrain it. The denser the control point network, the smaller the residual drift error. RTK alone does not fully address SLAM drift across long trajectories.
This is why the hybrid approach (RTK plus GCPs) produces the best results for demanding projects: RTK eliminates IMU leveling error and GCPs constrain accumulated SLAM drift across the dataset. Each addresses a different error source.
What Loop Closure Does and Does Not Fix
Loop closure is the mechanism by which SLAM corrects accumulated drift. When the scanner returns to a previously visited area and the algorithm detects the match, it measures the accumulated error and distributes the correction back across the trajectory. Loop closure requires two conditions: you physically return to an area you have already scanned, and your viewing angle when you return is within approximately 40 degrees of your original angle at that location.
Loop closure improves relative accuracy. It does not provide absolute accuracy. A perfectly looped scan with no georeferencing still has no connection to a real-world coordinate system.
Realistic Accuracy Outcomes by Method
The following table shows the accuracy outcomes achievable with each control method, the conditions required to achieve them, and the factors that degrade each result. These ranges reflect field reality, not idealized test conditions.
Absolute accuracy: undefined
Absolute accuracy: undefined
Method Selection by AEC Project Type
The right accuracy method depends on the deliverable requirement, not the device capability. Choose based on what the client or downstream workflow actually needs, and confirm that requirement before leaving for the field. The control strategy cannot be changed after the scan.
As-Built Documentation for BIM
BIM deliverables are used for design coordination and construction. Dimensional accuracy drives fit and clash detection. Absolute coordinate registration enables multi-session dataset alignment and integration with survey control.
Minimum requirement: GCPs with surveyed coordinates at maximum spacing (L2 Pro: 100 m, K1: 50 m). Verify GCP residuals in LixelStudio before delivery.
When to use hybrid: Large facilities where the scan spans multiple floors or sections. RTK constrains the elevation across floors; GCPs constrain drift within each floor.
Pure SLAM is insufficient if the BIM model will be used for construction coordination or clash detection. Dimensional errors compound across large datasets.
Legal Survey or Cadastral Work
XGRIDS mobile SLAM is not a replacement for licensed survey instruments for legal boundary work. The system can provide supporting documentation and area measurements, but legal survey requirements vary by jurisdiction.
If XGRIDS data is used as supporting evidence: Use hybrid RTK plus GCPs, verify all residuals, and retain the processing report as documentation.
Accuracy context: The 3.0 cm RMSE absolute specification can support many legal documentation requirements, but confirm with the licensed surveyor of record before committing the data for legal purposes.
Do not misrepresent mobile SLAM data as survey-grade without independent verification.
Construction Progress Monitoring
Progress monitoring requires repeatable registration between scan sessions more than it requires the highest possible absolute accuracy. What matters is that each session aligns to the same coordinate reference so that differences represent actual construction change, not scan-to-scan registration error.
Key principle: Use the same control strategy and the same physical GCP locations across every session. Changing the control network between sessions breaks the registration consistency.
Permanent GCP monuments are worth installing on long-term projects. They eliminate re-survey costs and guarantee consistent registration across dozens of scan sessions.
RTK alone is acceptable for open-air construction sites where Fixed status is reliable and consistent.
Visualization and Client Delivery (3DGS)
3D Gaussian Splat deliverables via LCC Studio are primarily visual. Clients viewing the model in a browser or the LixelWeb viewer are not measuring dimensions against a coordinate system. Absolute georeferencing adds cost and complexity that typically does not improve the deliverable for visualization purposes.
When absolute coordinates do matter for 3DGS: Map Fusion across multiple sessions requires a shared coordinate reference. If the project involves stitching multiple PortalCam or LCC-pipeline scans together, RTK or GCP data is required for the fusion to align correctly.
PortalCam-only projects: The PortalCam does not produce a point cloud or support GCP workflows. Relative accuracy within the LCC model is typically within 1% of true dimensions for small to medium spaces. This is adequate for most visualization, space planning, and client presentation use cases.
Industrial Facility or Large Indoor Site
Large indoor facilities are the most demanding accuracy scenario for mobile SLAM. RTK signal is unreliable indoors. Long trajectories accumulate drift. Multi-floor coverage introduces vertical error from IMU leveling that compounds across levels.
GCP spacing guidance for large indoor facilities: The published maximums (100 m for L2 Pro, 50 m for K1) are upper limits, not targets. For facilities where high dimensional accuracy matters, space control points at 50 m or less with the L2 Pro, and 25 to 30 m with the K1.
Multi-floor projects: Place GCPs on each floor and include points visible from stairwells or open voids that span floors. This constrains vertical drift across the building height.
Walk-and-stop technique at control points improves accuracy. Set the scanner down gently at the marker, allow it to rest for several seconds, then continue.
Outdoor Site with RTK Coverage
Open outdoor sites with unobstructed sky view are where RTK performs most reliably. If the environment supports a sustained Fixed satellite lock throughout the scan, RTK alone can achieve the published 3.0 cm absolute accuracy specification.
Verify Fixed status before beginning: Confirm more than 10 satellites and a Fixed status (not Float) in LixelGO. Walk at least 10 meters while Fixed before starting the scan route. Do not trust RTK data captured during the initial fix period.
When to add GCPs to outdoor RTK scans: If the site includes areas under tree canopy, near tall structures, or in urban canyons where RTK signal may drop or degrade, add GCPs in those zones. Do not assume RTK is maintaining Fixed throughout the entire scan.
Antenna tilt: Keep the RTK antenna within 20 degrees of vertical throughout the scan. Exceeding this invalidates the fix record even when the LED remains green.
The accuracy method must be chosen before the scan begins. RTK must be configured and confirmed as Fixed before scanning starts. GCPs must be physically placed and marked in LixelGO during the scan. There is no way to apply georeferencing to a scan after the session without control that was captured during it. A scan collected without any georeferencing cannot be retrospectively tied to a real-world coordinate system.