4.4 Ground Control Points

Two Types of Control Points

XGRIDS documentation distinguishes between two types, and the distinction matters for what you need to prepare before the scan.

GCP1, Absolute ground control points carry known, absolute coordinates obtained by GPS survey or total station. When applied in LixelStudio, they align the scan to a real-world coordinate system and simultaneously correct the IMU leveling error that RTK would otherwise address. They both anchor the coordinate frame and constrain SLAM drift. The denser they are, the smaller the residual error between them.

GCP2, XGRIDS relative control points use XGRIDS sticker targets and carry no absolute coordinates. They have no external survey requirement. When marked in LixelGO and applied in LixelStudio, they function as internal anchors that constrain SLAM drift and improve internal consistency without tying the scan to any real-world coordinate system. Use these when relative accuracy matters but absolute georeferencing is not required.

Placement and Distribution

Control point placement determines how well the SLAM trajectory is constrained across the full scan. Four rules govern where points must go.

• A minimum of 3 control point pairs is required. LixelStudio will reject the coordinate transformation if fewer than 3 pairs are provided. This is a hard software limit. Plan at least 4 to 5 pairs to give yourself margin for any that fail matching validation
• Points cannot all be collinear. Three or more points on a single straight line provide correction in only 1 dimension. Points need to be distributed in three dimensions across the scan area, meaning they must not all be on the same floor, same corridor, or same wall alignment. At minimum, distribute them so that any two-dimensional cross-section of the scan contains at least one point
• Points must be evenly distributed, not clustered. A cluster of four points in one corner and nothing elsewhere provides good constraint in the corner and little constraint everywhere else. Space them evenly throughout the full extent of the scan area
• Place points at stable, permanent surfaces. Floor-mounted sticker targets are the most common placement. Avoid placing markers on portable equipment, furniture, carpet that compresses, or any surface that might shift between surveying and scanning
• For accuracy verification, the target stickers must be scanned properly, circle around each one before placing the device on the ground to mark it. This ensures enough geometry surrounds the marker for LixelStudio to detect it automatically during the accuracy verification process

Transformation Points vs. Checkpoints

In LixelStudio's control point editing window, each matched control point pair can be toggled on or off with a checkbox. This toggle determines how the point is used during processing:

• Selected (checked) pairs function as transformation points. They actively participate in the coordinate transformation and trajectory refinement that corrects the point cloud position
• Unselected but matched pairs function as checkpoints. They are not used to compute the transformation. Instead, the GCP report uses them as independent accuracy verification: the report shows the residual error at each checkpoint, giving you a measure of how accurate the transformation is at locations that were not used to create it

Using all points as transformation points maximizes constraint. Reserving 1 or 2 points as checkpoints provides an independent accuracy assessment but reduces the number of active constraints. For projects requiring a formal accuracy report, designate at least 1 point as a checkpoint. For maximum spatial accuracy, use all points as transformation points.

Spacing Requirements

Maximum spacing defines the outer boundary of acceptable placement. In practice, environments with long feature-poor zones, complex geometry, or very high accuracy requirements benefit from tighter spacing. The relationship between spacing and error is direct: halving the spacing between control points roughly halves the maximum SLAM drift that can accumulate between them.

Physical Markers

XGRIDS provides reflective sticker targets for use as control point markers. These are designed to be detectable by LixelStudio's automatic target recognition during accuracy verification. Place them on flat, stable, floor-level surfaces at the positions you intend to survey.

• Place markers at locations with good ambient lighting, LixelStudio's target detection relies on the visual camera imagery from the scan, and dark or shadowed markers are harder to detect automatically
• Place markers in open areas where the scanner can circle them freely, not in corners, against walls, or behind obstructions where coverage from multiple angles is restricted
• Survey the center of each marker to the required accuracy before scanning begins. The surveyed position is the position that will be applied in LixelStudio, any error in the survey propagates directly into the final point cloud
• For projects without physical marker placement (using existing features as control, such as floor survey nails or column bases), ensure that the feature used is clearly visible in the point cloud and has unambiguous geometry for manual identification in LixelStudio

Coordinate File Format

LixelStudio accepts control point coordinates in CSV or TXT format. The file structure depends on whether you are providing absolute coordinates only (for GCP2-style relative use or simple absolute application) or both source geographic coordinates and projected plane coordinates (for 7-parameter coordinate transformation).

For 7-parameter coordinate transformation, required when the source ellipsoid of your RTK CORS differs from the target coordinate system, the file must include both the measured geographic coordinates (latitude, longitude, height in source datum) and the target projected coordinates (Northing, Easting, Elevation in the target system).

PointName, SourceLat(D:M:S), SourceLon(D:M:S), SourceHeight, TargetNorthing, TargetEasting, TargetElevation

GCP01, 37:23:14.512, -122:01:45.221, 42.183, 4136822.441, 573948.112, 42.051
GCP02, 37:23:22.118, -122:01:38.904, 41.997, 4137071.223, 574187.339, 41.864
GCP03, 37:23:18.741, -122:01:52.663, 42.344, 4136944.882, 573712.004, 42.211

In the LixelStudio parameter calculation interface, these columns appear as: name, Source B, Source L, Source H, Target N, Target E, Target Z. "Source B" is latitude (from the geodetic abbreviation Breite) and "Source L" is longitude (Laenge). Source coordinates are geographic in the CORS datum. Target coordinates are projected in your output system.

Indoor GCP Workflows

For purely indoor areas beyond the reach of an RTK carry-in, ground control points with surveyed absolute coordinates are the georeferencing method. The workflow requires a survey step before scanning begins so that every marker has a known coordinate before it is marked in LixelGO.

• For GCP positions near building entries with sky visibility, the L2 Pro with its RTK module active from the outdoor approach is itself the GNSS positioning instrument. Scan from outside to inside with Fixed status active, mark those threshold GCPs during the scan, and their positions are recorded with the benefit of the carried-in RTK coordinate frame. No separate survey instrument is required for these positions
• For GCP positions deeper inside the facility, beyond approximately 100 meters of scan path from any entry, use a total station referencing to an established exterior control network to survey those marker positions before the scan begins
• Place markers at the surveyed positions before the scan. Do not attempt to survey marker positions after placing them without a fixed reference
• Verify the coordinate file is complete and named correctly before leaving for site. A naming error or missing point discovered in the office means a return visit
• On multi-floor buildings, distribute control points on every floor included in the scan. Control points on 1 floor do not constrain drift on other floors
• For projects where a total station survey is not feasible, XGRIDS relative control points (GCP2) with no absolute coordinates are still beneficial. They constrain internal drift even without external coordinate alignment