Understanding the Align to Control Report: A Complete Guide

Introduction

The Align to Control Report is generated after completing the Align to Control wizard and documents the transformation results after aligning your capture to ground control. This report provides a comprehensive overview of how your model was aligned to real-world control points, including transformation quality metrics and accuracy assessments.

This report is essential for:

• Surveyors and mapping professionals validating project accuracy
• Project stakeholders confirming deliverables meet specifications

Report Structure Overview

The report contains four main sections that document the transformation process from start to finish:

• Project Information
• Ground Control Point Coordinates and Residuals
• Accuracy Assessment
• Notes

Downloading the Report

After running the Align to Control wizard on your capture, the system automatically generates this report. To download the report for your records or to share with stakeholders:

1. Navigate to the Align to Control toolbar in your capture workspace
2. Click the Download Report button (document icon) located in the toolbar
3. The report will be saved as a PDF to your local downloads folder

Section 1: Project Information

The Project Information section provides essential metadata about your capture and the coordinate system configuration.

Key Fields:

Capture Name: The name of the capture that has been aligned to ground control.

Site Name: The name of the site that the capture is in.

Coordinate System: The complete coordinate reference system definition of the site.

This coordinate system definition is critical because all transformed coordinates and residual measurements will be expressed in these units and reference frame.

Report Date: The date when the report was generated.

Section 2: Ground Control Point Coordinates and Residuals

This section is the core of the report, detailing the transformation results for each ground control point and check point in your project.

Understanding Point Types

The report differentiates between two critical point types:

Control Points: Ground control points used to calculate the rigid-body transformation. These points directly influence the mathematical transformation model that aligns your looq data to your field measurements. 

Check Points: Imported points deliberately excluded from the transformation calculation. These serve as independent validation points to verify the transformation accuracy. Check points provide an unbiased assessment of how well the transformation performs on data that wasn't used to create it.

Point Coordinates

For each point, the report displays three coordinate types:

Control (Original Coordinates): The surveyed ground coordinates for each point that are brought in during the “import” process. These represent the "true" real-world position as measured with field equipment such as RTK GNSS or total stations.

Transformed (Model Coordinates): The expected position of each point as calculated by applying the transformation to your looq data. These coordinates show where the system predicts each point will be based on the transformation model.

Coordinates Format: All coordinates are displayed in the units of your specified coordinate system:

Residuals Explained

The Residuals section contains five columns that quantify the differences between surveyed control coordinates and transformed model coordinates. These are divided into two groups:

Coordinate Deltas (dE, dN, dZ)

The first three columns show the component-wise differences between control and transformed coordinates:

dE (delta Easting): The difference between the control Easting coordinate and the transformed Easting coordinate, calculated as: Control Easting - Transformed Easting. This shows the east-west directional error.

dN (Delta Northing): The difference between the control Northing coordinate and the transformed Northing coordinate, calculated as: Control Northing - Transformed Northing. This shows the north-south directional error.

dZ (Delta Elevation): The difference between the control Elevation and the transformed Elevation, calculated as: Control Elevation - Transformed Elevation. This shows the vertical directional error.

These delta values are simple subtractions that preserve sign (positive or negative), indicating the direction of the error. They help diagnose systematic shifts or biases in the transformation.

Total Residuals (2D, 3D)

The last two columns show the magnitude of total error, regardless of direction:

2D Residual: The total horizontal distance between control and transformed coordinates.

3D Residual: The total spatial distance between points in three dimensions.

The 2D and 3D residuals are always positive values representing error magnitude, while the deltas (dE, dN, dZ) can be positive or negative, indicating error direction.

N/A Values: Points that were skipped or ignored during the Align to Control wizard appear as "N/A" in both the Transformed coordinates and all Residuals columns. These points exist in your dataset but were not used in any capacity for the transformation.

Section 3: Accuracy Assessment

The Accuracy Assessment section provides statistical summaries of transformation quality, separately calculated for control points and check points.

Control & Check Point Statistics

RMSE (Root Mean Square Error): The standard statistical measure of transformation accuracy for points used in the transformation calculation.

95% Confidence: The estimated accuracy at the 95% confidence level, calculated as approximately 1.96 × RMSE (based on normal distribution assumptions). This value represents the threshold within which 95% of your points should fall.

Using the Report for Quality Control

A systematic approach to reviewing your Align to Control Report:

Step 1: Verify Project Configuration

• Confirm capture name matches your intended dataset
• Verify site name and coordinate system are correct
• Check that coordinate system matches project specifications
• Ensure report date aligns with project timeline

Step 2: Review Control Point Distribution

• Examine which points were used as control vs. check points
• Verify adequate spatial distribution of control points across project area
• Confirm sufficient check points for independent validation (typically 20-30% of total points)
• Look for gaps in coverage that might affect transformation quality

Step 3: Analyze Individual Residuals

• Scan for unusually large residuals on control points (outliers)
• Investigate any control point with residuals >3× the average
• Check that residuals are consistent across the project area
• Verify check point residuals align with control point performance

Step 4: Evaluate Accuracy Statistics

• Compare RMSE values against project requirements
• Ensure 95% confidence intervals meet specifications
• Confirm control and check point statistics are comparable
• Document any discrepancies for further investigation

Step 5: Review Transformation Quality Indicators

• Look for systematic patterns in residuals (e.g., all points shifted in one direction)
• Check if residuals increase with distance from control points
• Verify transformation quality is uniform across elevation ranges
• Assess if additional control points might improve results

Red Flags to Investigate

Control point residuals >0.1m: May indicate point identification errors, coordinate system issues, or survey measurement problems. Investigate points individually.

Check point statistics significantly worse than control points (>2× RMSE): Suggests the transformation may not generalize well across the project area. Consider adding more control points or redistributing existing points.

Systematic residual patterns: If all residuals trend in the same direction or magnitude increases with distance, the transformation model may not adequately capture the geometric relationship. This could indicate:

• Scale errors
• Rotation misalignment
• Coordinate system definition issues
• Lens distortion not properly calibrated

N/A values on critical points: Ensure points marked as N/A were intentionally skipped. Accidentally skipped control points can compromise transformation quality.

Interpreting Results for Different Applications

High-Precision Surveying (e.g., boundary surveys, engineering design)

• Target: 95% confidence <0.05m horizontally, <0.10m vertically
• Focus: Individual point residuals and check point validation
• Requirements: ASPRS standards or local survey regulations

Aerial Mapping and GIS (e.g., base maps, asset management)

• Target: 95% confidence <0.15m horizontally for most applications
• Focus: Overall accuracy statistics and spatial consistency
• Requirements: Project-specific accuracy requirements

Volumetric Analysis (e.g., stockpiles, cut/fill calculations)

• Target: Varies by application; typically <0.10m for 3D residuals
• Focus: Elevation accuracy and vertical residuals
• Requirements: Volume measurement uncertainty specifications

General Documentation (e.g., progress monitoring, visual reference)

• Target: 95% confidence <0.50m may be acceptable
• Focus: Overall transformation success and gross error detection
• Requirements: Client communication needs

Best Practices

Document everything: Save the Align to Control Report as part of your project deliverables. Include it in quality assurance documentation and client deliveries.

Compare across captures: If processing multiple captures for the same site, compare accuracy statistics across reports to ensure consistent quality.

Maintain control point survey data: Keep detailed records of how control points were surveyed, including GNSS observation times, instrument calibration, and any known measurement limitations.

Use appropriate check point ratios: Reserve 20-30% of control points as check points for robust validation, distributed throughout the project area.

Iterate if necessary: If accuracy statistics don't meet requirements, consider:

• Adding more control points
• Redistributing control points for better coverage
• Re-identifying points in the photogrammetric model
• Verifying control point survey accuracy
• Checking coordinate system definitions

Communicate results: Share the report with stakeholders and explain what the accuracy metrics mean for project deliverables and intended use cases.

Conclusion

The Align to Control Report is a critical quality assurance document that validates the geometric accuracy of your photogrammetric model. By understanding each section and systematically reviewing the data, you can confidently assess whether your capture meets project specifications and accuracy requirements.

Key takeaways:

• Project Information establishes the coordinate reference framework
• Ground Control Point Coordinates document the transformation for each point
• Residuals quantify the difference between surveyed and transformed positions
• Accuracy Assessment provides statistical summaries for control and check points
• Check points offer independent validation of transformation quality
• Systematic review helps identify and resolve accuracy issues

For questions about specific accuracy requirements or interpretation of unusual results, consult with your surveying team or photogrammetry specialists. This report provides the quantitative foundation for quality-assured geospatial deliverables.