0 6 -- RTK & Ground Control Points

Applies to: C1 and S20 (unified unless noted)
Related: Master Index | 01-Hardware-Setup | 03-Scanning-Workflows | 07-Troubleshooting

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What RTK Does

RTK (Real-Time Kinematic) provides absolute positioning for your scans -- placing point clouds in real-world coordinates rather than an arbitrary local origin.

Capability	Without RTK	With RTK
Coordinate system	Local Euclidean (origin at start position)	Real-world EPSG projected coordinates
Scan-to-scan alignment	Manual via CloudCompare	Automatic via GNSS Fusion
Absolute accuracy	None	~1-3 cm
Large area scanning	Drift accumulates	Drift minimized by absolute references
GCP recording	N/A	Timestamps only (not coordinates -- see below)

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RTK Hardware

C1 RTK Module

• External add-on (UM980 chipset)
• Attaches to top of device
• Includes positioning plate for ground-level measurements
• Antenna offset from ground: ~44 cm (measured by pierro5261)
• Antenna-to-LiDAR optical center offset: specific to C1 (not publicly documented)

S20 RTK

• S20 RTK antenna offset: ~34.72cm (vertical, antenna to LiDAR optical center -- per Rami Tamimi video)
• Different from C1 -- do not interchange values

NTRIP Providers (US)

• GEODNET: ~$40/month (jyst_mike)
• Rock Robotic base station: Buy hardware (~$700), contribute to network, get free corrections
• NTRIP protocol is standard -- any provider works

NTRIP Setup

1. RTK-enabled device
2. NTRIP subscription (or local base station)
3. Internet connection on phone
4. That's it -- no extra hardware needed

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GNSS Fusion in PointCloud Studio

Enabling GNSS Fusion

1. In Data Calculation -> Mapping Parameters
2. Enable GNSS Fusion
3. Select Horizontal Setting (EPSG code)
4. Enter Height Offset (antenna-to-ground correction)

Horizontal Setting -- CRITICAL

• Select plain EPSG code (e.g., EPSG:32617)
• DO NOT select variant with "(Ft)" suffix -- causes stretched/scaled geometry
• Symptom of wrong setting: known 3-foot door measures 2.98 meters (tool_guy_365)
• Validation: Process same scan without GNSS Fusion to verify geometry

Height Offset

• GNSS Fusion height offset is hardware-specific -- C1 differs from S20's ~34.72cm
• Test against known benchmark: pierro5261 measured +44 cm deviation; tool_guy_365 measured +57 cm
• Currently no option in mobile app to configure antenna offset/pole length/device height

GNSS Fusion Tradeoffs

Setting	Result
GNSS + Undistorted Photos + SFM ON	More noise artifacts (clumps) but cleaner, more photo-realistic surfaces
GNSS + Undistorted + SFM OFF	Less noise, "pretty darn good" baseline quality

Choose based on priority: photorealism vs geometric cleanliness.

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Ground Control Points (GCPs)

Current State (as of May 2026)

IMPORTANT: C1 control points record TIMESTAMPS only -- NOT coordinates.

When you mark a control point on the C1 and hold still for ~10 seconds:
- System records timestamp/marker information
- control_points.csv file has blank X, Y, Z fields (tool_guy_365, confirmed by Luna)
- This is expected behavior in current firmware (Luna, Apr 22)

Luna (Apr 22): "The RTK on our device is mainly used for georeferencing the point cloud and supporting scan alignment, but it does not currently output absolute GNSS coordinates for control points."

Control Point Position

• Recorded position is projected to ground target level -- offset is compensated (Luna, Apr 23)
• Position captured at camera level; software projects down to ground

GCP Calibration Feature (PCS v2.2.3+)

• Available in standard version (not Beta)
• Works WITHOUT RTK module (Frank, Apr 12)
• Video tutorial: https://www.youtube.com/watch?v=qGCNxVh7BSY
• Currently requires manual coordinate entry -- coordinates must be known in advance
• Feature not yet confirmed in any Beta version

GCP Workflow (Manual)

1. Place targets visible in multiple scans
2. Measure target coordinates with professional RTK rover or total station
3. Enter coordinates manually in PCS
4. PCS aligns scans using known GCP positions

DIY GCP Targets

• 3D-printed checkerboard with GNSS pole hole (ser0ka)
• Self-adhesive printed targets: ~40 for 10
• Quick-and-dirty: tape + pen to draw an X

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Multi-Scan Alignment with GCPs

Using CloudCompare (Manual)

1. Place GCP targets in overlapping areas between scans
2. Import both point clouds into CloudCompare
3. Use targets to place alignment markers
4. Apply ICP (Iterative Closest Point) refinement
5. Verified by mr_red1991_63830: "finally learned how to align 4 PC together with checkerboard"

See 08-Third-Party-Tools for detailed steps.

Without External Software

PCS currently does NOT support scan fusion/alignment natively. CloudCompare or Leica Register 360 required.

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RTK Coordinate Issues with 3D Software

Large Coordinate Problem

• RTK data produces 64-bit point clouds with large real-world coordinate values
• Blender: Data appears "divided and incomplete" -- doesn't handle large coordinates
• Fix: Re-center to local coordinates (0,0,0)

Solution: CloudCompare Global Shift

1. Open RTK point cloud in CloudCompare
2. Apply transformation/global shift
3. Export non-georeferenced copy for 3D software
4. Keep one original RTK/georeferenced dataset + one "local coordinates" version for artists (ser0ka)

Alternative: Houdini

• Can automate coordinate transformation
• User preference for those comfortable with Houdini

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RTK Limitations (Current)

Limitation	Detail
No GCP coordinate output	control_points.csv has blank X/Y/Z (confirmed by Luna)
No pole/antenna offset config	Height correction must be done manually in PCS
S20/C1 have different offsets	Values not publicly documented
RTK doesn't guarantee good scans	Scan can still corrupt even with RTK Fixed status (tool_guy_365)
No decimal degree output	Only UTM (Easting/Northing in meters)

Community Requests

• RTK-based GCP coordinate recording (mr_red1991_63830, pierro5261)
• Decimal degree (WGS84) output option (tool_guy_365)
• Pole length/antenna height configuration in mobile app
• Better documentation/tutorial for RTK setup

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RTK Accuracy

Context	Accuracy
Relative (scan-to-scan)	RTK-level (~1-3 cm)
Absolute (world position)	RTK-level when GNSS Fusion enabled
GCP coordinates	NOT provided by C1/S20 -- need separate professional RTK rover

RTK accuracy is for absolute position (where the building/site is located). Relative accuracy (wall heights, window dimensions) depends on the scanner and software you use -- these are different concepts (pierro5261).