XGRIDS Pro Guide™ / Module 4: Positioning

4.4 Ground Control Points

Ground control points are the correct georeferencing method indoors, in GPS-denied spaces, and anywhere GNSS cannot be relied on. Getting the spacing, distribution, naming, and coordinate file right determines whether LixelStudio can apply them at all.

Section 1

Two Types of Control Points

The distinction matters for what you prepare before the scan.

GCP1: Absolute

Carries known coordinates from a GPS survey or total station. Applied in LixelStudio, it aligns the scan to a real-world system and corrects the IMU leveling error that RTK would otherwise handle. It anchors the coordinate frame and constrains SLAM drift. Denser points mean smaller residual error between them.

GCP2: Relative

A marked target with no surveyed coordinate and no external survey requirement. It constrains SLAM drift and improves internal consistency without tying the scan to a real-world system. Use it when relative accuracy matters but georeferencing is not required.

Both are called "control points" in LixelGO. The only difference is whether a surveyed coordinate is attached in LixelStudio. The same marked point becomes GCP1 with a coordinate, or GCP2 without one.

Section 2

Placement and Distribution

Placement determines how well the SLAM trajectory is constrained across the scan. Four rules govern where points go.

At least 3 control point pairs. LixelStudio rejects the coordinate transformation with fewer than 3. Plan 4 to 5 to leave margin for any that fail matching
Not collinear. Points on a single straight line correct only one dimension. Distribute them so they are not all on the same floor, corridor, or wall line
Even, not clustered. A cluster in one corner constrains that corner and little else. Space points across the full extent
On stable, permanent surfaces. Avoid portable equipment, furniture, or compressible carpet that may shift between survey and scan

Two different minimums, do not confuse them. The 3-pair floor above is for coordinate transformation in LixelStudio. For Map Fusion, each input scan that does not use RTK needs a minimum of 2 ground control points, 3 ideal.

Transformation Points vs. Checkpoints

In LixelStudio's control point editor, each matched pair can be toggled on or off:

Selected pairs are transformation points. They drive the coordinate transformation and trajectory correction
Unselected matched pairs are checkpoints. They are not used to compute the transformation; the GCP report shows the residual error at each, an independent measure of accuracy at points the transformation did not see

For maximum accuracy, use all points as transformation points. For a formal accuracy report, reserve at least one as a checkpoint.

Section 3

Spacing

Maximum spacing is the outer boundary. Feature-poor zones, complex geometry, and high-accuracy specs all call for tighter spacing. The relationship is direct: halving the spacing roughly halves the maximum drift between points.

Device

Maximum Spacing

Rationale

L2 Pro

100 m (330 ft)

Up to 300 m LiDAR range supports wider spacing in open environments

50 m (165 ft)

100 m LiDAR range; closer spacing limits drift accumulation between points

In feature-poor environments (data center aisles, tunnels, large featureless interiors), use the tighter end. More closely spaced points always produce a tighter result.

Section 4

Physical Markers

XGRIDS reflective sticker targets are one option, not the only one. A checkerboard target or any clear, stable floor marker works too. What matters is that the marker sits on a flat, floor-level surface where the scanner can rest on top of it and stay vertical, never on a wall, and that you mark the exact same spot with the same name every time.

Use good ambient lighting. LixelStudio's automatic target detection relies on the scan's camera imagery; dark or shadowed markers are harder to detect
Place markers in the open where the scanner can approach from multiple angles, not in corners or against walls
Survey the center of each marker before scanning. The surveyed position is what LixelStudio applies; any survey error propagates straight into the cloud
If you use existing features as control (floor survey nails, column bases), make sure the feature is clearly visible in the cloud with unambiguous geometry for manual identification

Section 5

Coordinate File Format

LixelStudio accepts control point coordinates in CSV or TXT. The structure depends on whether you provide absolute coordinates only or both source geographic coordinates and projected plane coordinates for a 7-parameter transformation.

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

In the parameter calculation interface, the columns are: name, Source B, Source L, Source H, Target N, Target E, Target Z. "Source B" is latitude (from the geodetic abbreviation Breite), "Source L" is longitude (Laenge).

Point names are case-sensitive and must match exactly. The file name must match character-for-character what you typed in LixelGO, including case. A single case difference drops that point. For Map Fusion, every occurrence of the same physical location must use the exact same name across all segment files. At least three valid points are required for the 7-parameter calculation.

Section 6

Indoor GCP Workflows

For interiors beyond the reach of an RTK carry-in, surveyed absolute GCPs are the method. Survey every marker before the scan so it has a known coordinate before you mark it in LixelGO.

Near building entries with sky visibility, the scanner with an active RTK fix from the outdoor approach is itself the positioning instrument. Scan from outside to inside with Fixed status, mark those threshold GCPs during the scan, and their positions record with the benefit of the carried-in frame. No separate survey instrument is needed there
Deeper inside, beyond the carry-in distance from any entry (about 100 m / 330 ft for the L2 Pro, about 50 m / 165 ft for the K2), use a total station (a surveyor's instrument that measures precise positions) referenced to an exterior control network to survey those markers before the scan
Place markers at their surveyed positions before the scan. Do not survey marker positions after placing them without a fixed reference
Verify the coordinate file is complete and correctly named before leaving for site. A naming error found in the office means a return visit
On multi-floor buildings, place control on every floor scanned. Control on one floor does not constrain drift on another
Where a total station survey is not feasible, relative control points (GCP2) still constrain internal drift without external alignment

Markers placed, coordinates surveyed, file prepared. Next: 4.5 Control Point Marking, the field procedure for marking control points during the scan.

Previous 4.3 PPK Positioning Next 4.5 Control Point Marking