XGRIDS Pro Guide™ / Module 4: Positioning

Module 4 Quick Field Guide

Georeferencing method selection, RTK and PPK minimum requirements, GCP placement rules, control point marking procedure, and what happens when any of it is done wrong.

Section 1

Georeferencing Method Selection

This decision must be made before leaving the office. The method cannot be added or changed after the scan is complete. Understand what each method produces before choosing one.

Two of the six methods below do not require surveyed coordinates at all: Relative GCPs constrain SLAM drift without tying the scan to any real-world coordinate system, and None produces a free-floating local model. Absolute coordinates require either RTK, PPK, or GCPs with surveyed positions.

Method

When to Use

Requires

Approx. Added Time

Key Limitation

None

As-builts, virtual tours, internal documentation. No coordinate deliverable required.

Nothing additional

None

No real-world coordinates. Cannot register to GIS or external datasets.

Relative GCPs (no survey)

Large interiors, multi-floor, or multi-session projects where drift control matters but absolute coordinates are not required.

XGRIDS sticker targets. No survey or CSV required.

Minimal. Target placement only.

Constrains internal drift. Does not produce real-world coordinates. Cannot be placed in GIS.

RTK

Outdoor projects with clear sky view and continuous GNSS coverage.

RTK module, NTRIP credentials, Fixed status before scanning

15 min setup

Outdoor only. Loses fix indoors. Corrects coordinate frame but does not constrain SLAM drift on long scans.

PPK

Remote sites, obstructed sky, or anywhere NTRIP is unavailable.

RTK module, base station, RINEX files, base coordinate

4 hr base station operation + post-processing

Base station must be within 3 miles (5 km). Cannot confirm accuracy until after the scan.

GCP with Surveyed Coords

Indoor projects, underground, GPS-denied environments. Any project requiring absolute accuracy without GNSS.

Surveyed point coordinates, physical markers placed before scanning, CSV file

~15 min per point (survey + placement + marking)

Markers must be placed before the scan and visited during it. Cannot be added after the fact.

Hybrid (RTK + Surveyed GCPs)

Large outdoor projects, survey-grade deliverables, projects spanning indoor and outdoor areas.

RTK module + surveyed GCPs at max 330 ft (outdoors) or 165 ft (indoors) intervals

RTK setup + GCP survey time combined

Most demanding to execute but most robust result. RTK corrects coordinate frame; GCPs constrain drift accumulation.

This decision cannot be reversed in post-processing. RTK must be active before recording begins. GCPs must be physically placed and marked during the scan. A completed scan with no georeferencing setup cannot be retroactively given absolute coordinates.

Section 2

RTK: Status and Minimum Requirements

RTK Status Indicators

RTK status is displayed continuously in LixelGO during scanning, alongside satellite count. Monitor both throughout the scan. Status can degrade without warning as you move through obstructions.

Single

No corrections applied. GPS-only position. Not valid for georeferenced scanning under any circumstances.

Float

Receiving corrections but integer ambiguity not resolved. Position accuracy is decimeter-level at best, not centimeter-level. Do not begin scanning with Float status. Wait for Fixed.

Fixed

Integer ambiguity resolved. Centimeter-level corrections applied. This is the only status valid for georeferenced scanning.

RTK Minimum Validity Thresholds

All thresholds must be met. RTK data collected below any threshold may process but with compromised or unverifiable accuracy.

Requirement

Threshold

Notes

Satellite status

Fixed only

Float and Single are not valid. Both will produce a result in LixelStudio but with unverifiable accuracy.

Available satellites

More than 10

GPS, GLONASS, BDS, and Galileo all contribute. Satellite count is displayed in real time in LixelGO during scanning.

Movement while Fixed

More than 33 ft (10 m)

The trajectory bounding box must exceed 33 ft while Fixed. A stationary initialization or short in-place loop does not satisfy this.

Valid RTK data points

More than 100

Displayed in LixelStudio after upload, before processing. Verify this count before running the full process.

Accuracy verification route

330 ft minimum, L-shaped or curved

Walk at least 330 ft (100 m) in an L-shaped or curved path while maintaining Fixed throughout. A straight-line path of 330 ft confirms distance but not angular stability. The direction change is what verifies the fix is reliable, not just momentary.

RTK corrects the coordinate origin. It does not constrain SLAM drift. RTK eliminates IMU leveling error, which is a tilt introduced at initialization. It does not prevent drift from accumulating across the scan trajectory over distance. On scans exceeding a few hundred feet, use GCPs at 330 ft (100 m) intervals outdoors in addition to RTK. Indoors, where RTK signal is unavailable, use GCPs at 165 ft (50 m) intervals.

Section 3

PPK: Minimum Requirements

PPK Minimum Thresholds

Base station to rover distance: less than 3 miles (5 km). Keep baselines under 1.2 miles (2 km) for optimal accuracy. Longer baselines degrade the PPK solution quality even with clean data.
Scan area coverage must exceed 33 ft (10 m) of open ground visible to the sky. A fully GPS-denied scan with no outdoor segment cannot produce PPK output.
Valid PPK data points after processing: more than 100. Displayed in LixelStudio after upload. Check this before processing. Below 100 means the PPK solution is unreliable.
Base station RINEX: continuous, gap-free logging at 1-second intervals or finer throughout the entire scan window. Gaps in the base station record cannot be patched in post-processing. Any gap during the scan window degrades or invalidates the solution for that period.
Antenna tilt: within 20 degrees of vertical throughout the scan. The PPK antenna on the L2 Pro must remain near vertical during field collection.

PPK is not inherently less accurate than RTK. Both process the same GNSS observations. RTK does it in real time, PPK does it afterward with the full dataset available. In practice, PPK can achieve equivalent or better accuracy than RTK when baseline distances are short. The critical requirement is clean base station data. A base station with logging gaps, antenna movement, or poor sky view produces a degraded PPK solution regardless of rover data quality.

Section 4

GCP Placement Rules

GCPs come in two fundamentally different types. The same physical marker can serve either purpose. The distinction is whether a surveyed coordinate is attached to it during processing.

GCP Type Reference

GCP1 (Absolute): Carries known, surveyed real-world coordinates. Applied in LixelStudio, it aligns the scan to a real-world coordinate system and corrects IMU leveling error. Required when the deliverable needs absolute coordinates for GIS, BIM, or survey-grade comparison work.
GCP2 (Relative): Uses XGRIDS sticker targets with no surveyed coordinates. Functions as an internal SLAM anchor: it constrains drift and improves internal consistency without tying the scan to any real-world system. Use when relative accuracy matters but georeferencing is not required — large interior scans, multi-floor documentation, Map Fusion projects.
The same physical sticker target can be either type. Place it, mark it in LixelGO, and the decision of whether to supply a coordinate for it is made in LixelStudio during processing.

Spacing Limits

L2 Pro: maximum 330 ft (100 m) between any two GCPs. Reduce spacing in feature-poor or complex environments.
K1: maximum 165 ft (50 m) between any two GCPs. The K1's shorter range makes it more sensitive to drift accumulation between anchors.
PortalCam (Map Fusion): minimum 3 points, at least 33 ft (10 m) apart, in an L-shaped distribution. PortalCam uses Multi-SLAM (LiDAR, visual, IMU) but has no GNSS capability. All control points for PortalCam are relative unless you supply surveyed coordinates for them.

Placement Distribution Rules

Points cannot be collinear. Three or more points on a single straight line provide constraint in only one axis. Distribute points so they form a three-dimensional network, not a line or a flat plane.
Points must be evenly distributed, not clustered. A cluster of control points at one end of the site with none elsewhere provides poor constraint across the scan extent.
Minimum quantity: 3 to 4 points for small sites, 6 to 8 for large sites. More points mean shorter intervals between corrections and smaller residual errors across the dataset.
Place points at scan boundaries and at zone transitions: floor changes, building entries, and locations where coverage paths cross.
Each point must be visitable during the scan in a path that allows the scanner to approach, circle, and continue without disrupting the route plan.

Point names are case-sensitive and must match exactly. The name in your CSV coordinate file must match character-for-character what you type in LixelGO when marking the point during the scan. A single uppercase-to-lowercase difference is enough to cause the point to be silently discarded in LixelStudio. You will not receive an error message.

Section 5

Control Point Marking Procedure

K1 and L2 Pro — Marking in LixelGO

Write all point names on paper before going to site. Type them from that written list into both the coordinate CSV file and LixelGO. Do not invent names in the field while scanning.
Place the steel collection plate directly on the surveyed coordinate point. The plate is the physical reference. It goes on the spot, not near it. The device then approaches the plate closely enough for a clean view — typically 3 to 6 ft away — not on top of it.
Tap the control point button in LixelGO and enter the exact point name.
Hold the device stationary for at least 5 seconds after submitting the mark. Moving immediately after submission risks a poor-quality mark.
Circle the control point once or twice after marking. Walking around the point at close range after marking provides additional geometry around the anchor and improves the quality of the SLAM constraint at that location. This is best practice at every control point.
Resume scanning after completing the circle.
After the last control point, continue scanning for at least 15 seconds and walk a small loop before ending the session. This gives SLAM additional constraint context around the final control point.

PortalCam — Marking in LCC Scan

PortalCam does not use a steel collection plate. Instead of a physical marker, the PortalCam uses its surrounding visual environment to record the control point. The device must be placed on its tripod at the control point location and held stationary during marking. The visual cameras record the surrounding geometry, which becomes the anchor reference.
Deploy the PortalCam tripod at the control point location. Place the tripod on a stable, level surface. The device must not rock or shift during the marking procedure.
Choose locations with good ambient lighting and clear surface texture. Avoid featureless walls, dark corners, and areas with direct harsh lighting on the camera. The surrounding geometry is the anchor — it needs to be distinctive and well-lit.
Tap the control point icon in LCC Scan and enter the point name. Select the appropriate type: Map Fusion or Aerial-Ground Map Fusion. Names are case-sensitive.
Tap Add and wait for the on-screen confirmation. Do not power off within 5 seconds of adding the point. The device needs time to write the data to the project file.
Position consistency across sessions matters. For shared control points used across multiple scan sessions, the device must be placed within 20 inches (0.5 m) of the same position, within 20 degrees of the same orientation, and within about 4 inches (10 cm) of the same height as the original session. These are the tolerances within which Map Fusion can reliably match the points.

Shared Points for Map Fusion — All Devices

For any multi-session Map Fusion project, shared control points must be physically visited and marked in every scan session that needs to connect to others. This applies to K1, L2 Pro, and PortalCam equally.
Use identical point names across all sessions. The fusion algorithm matches shared points by name. A name difference between sessions means the algorithm treats them as different points and the connection fails.
At K1/L2 Pro shared points, the collection plate must be placed at the same physical location in each session. At PortalCam shared points, the tripod must be placed within the tolerances described above.

Section 6

Coordinate System Quick Reference

You do not need a surveying background to configure coordinate systems in LixelStudio, but you need to understand these four terms and what they mean for your processing setup. Your surveyor or CORS provider will tell you which values to use. This section tells you what those values actually are.

Term

What It Means for XGRIDS Processing

Ellipsoid

The mathematical model of Earth's shape used as the reference surface for coordinates. WGS84 is the global GPS standard used by the XGRIDS RTK module. GRS80 is used by most modern national systems including NAD83 and ETRS89. The two ellipsoids are numerically nearly identical, but you must still specify which one your CORS network transmits. The RTK module records in whatever ellipsoid your CORS provider uses. Confirm this before processing.

Datum

A specific realization of a coordinate system, defined by an ellipsoid and a set of ground control points that anchor the ellipsoid to the actual Earth surface. WGS84 is a global datum used by GPS satellites worldwide — it is not a US system. NAD83 is the US horizontal datum. ETRS89 is the European equivalent. WGS84 and NAD83 share the same ellipsoid (GRS80) but are anchored differently, so coordinates in the same location can differ by up to approximately 3 ft (1 m) between them depending on the region. You must know which datum your CORS network transmits and which datum your deliverable requires.

Projection

The mathematical transformation that converts curved geographic coordinates (latitude and longitude) into flat planar coordinates (Easting and Northing). LixelStudio outputs projected coordinates. You must specify the target projection. LixelStudio currently supports four projection types: UTM, Transverse Mercator, Gauss-Kruger (3-degree zones), and Gauss-Kruger (6-degree zones). State Plane and other national systems must be configured using the Transverse Mercator option with the appropriate parameters. UTM is the most practical choice for most AEC work where a single zone covers the project area.

Helmert transformation

Required when your RTK source datum differs from your output target datum — for example, when your CORS network transmits WGS84 but your deliverable requires NAD83 State Plane coordinates. The Helmert transformation uses 7 parameters: 3 translations, 3 rotations, and 1 scale factor. These are loaded in LixelStudio via the RTK settings panel. Parameters are region-specific and must come from an authoritative geodetic authority. Do not estimate these values. For WGS84 to GRS80, the parameters are all zeros (the systems are numerically equivalent at that level), but for WGS84 to NAD83 or regional datums, the values are non-trivial.

Practical setup in LixelStudio: In the RTK settings panel, set Source Coordinate to WGS84, then set Target Coordinate and Target Ellipsoid to match your deliverable requirement. Select your Projection Type and enter the projection parameters. If a Helmert transformation is needed, load the 7-parameter file using the Load Parameters button. Your surveyor should provide the parameter file for your region. For most US projects using NAD83 UTM output, the configuration is: Source WGS84, Target NAD83, Projection UTM, zone per project location, height fitting GEOID18 if orthometric heights are required.

Section 7

Critical Warnings

No recovery Georeferencing cannot be added after the scan. RTK must be configured and active before recording begins. GCPs must be placed in the physical space and marked during the scan. A project delivered without georeferencing cannot be corrected without rescanning the site.
Processing failure Mismatched GCP names cause silent processing failure. The name in your CSV file must match exactly what you typed in LixelGO, including case. A mismatched point is silently discarded in LixelStudio. You will not get an error. You will get a result without that control point applied.
Warning Float is not an acceptable substitute for Fixed. Scanning with RTK in Float status embeds unreliable position data into the scan file. The coordinate conversion in LixelStudio will produce a result, but the accuracy of that result is not centimeter-level and cannot be verified after the fact.
Warning RTK corrects the coordinate origin. It does not prevent SLAM drift. RTK eliminates IMU leveling error, which causes a systematic tilt across the entire dataset. It does not constrain the geometric drift that accumulates along the scan trajectory over distance. On large projects, a scan with RTK and no GCPs will have an accurate coordinate frame but may still accumulate drift between the start and end of long trajectory segments. Supplement RTK with GCPs for high-accuracy deliverables on projects covering more than a few hundred feet.
Warning RINEX gaps in the base station record cannot be recovered in post-processing. If the base station stopped logging at any point during the scan window, the PPK solution for that time period is degraded or absent. Monitor base station status during the scan. A gap means that portion of the trajectory has no valid PPK correction.
Warning Neither RTK nor GCPs compensate for poor scanning technique. A hybrid-georeferenced scan with no loop closures and excessive speed will still produce a drift-affected result, just one with absolute coordinates attached to it. Georeferencing and scanning technique are separate concerns. Both must be correct.
Warning GCPs on a single straight line provide correction in only one axis. Three collinear points are not a valid control network. Distribute points to form a three-dimensional network across the scan extent.
Warning GCP residuals over 2 inches (5 cm) in LixelStudio indicate a problem. After applying GCPs in processing, LixelStudio reports residuals for each point. A residual over 2 in (5 cm) means either the surveyed coordinate contains an error, the physical marker was disturbed between survey and scan, or the point was marked incorrectly in LixelGO. Recheck the survey, inspect the marker placement, and add additional control points if needed before delivering the dataset.

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