Analyzing Visual Evidence for 3D Reconstruction
Introduction​
When working with visual evidence such as video footage or images, we approach the material through the specific lens of spatial reconstruction. Rather than viewing the evidence purely as documentation of an event, we analyze it as source material for creating an accurate 3D model of the scene. This reconstruction-focused perspective shapes how we evaluate every element in the footage—from fixed architectural features to dynamic objects and actors.
Our analysis asks: What spatial information does this evidence contain? What can we measure, verify, and model? What additional data sources do we need to fill gaps in our understanding of the three-dimensional space?
This guide uses the example of analyzing footage of an incident in a city square, but the methodology applies to any scenario requiring spatial reconstruction from visual evidence.
Ground Truth
Establish the fixed spatial framework using satellite data and structural measurements.
Key Objects
Identify and analyze dynamic elements like vehicles and people within the scene.
Documentation
Maintain clear records and chain of custody for all identified elements.
Phase 1: Establishing Ground Truth​
Ground truth refers to the baseline spatial and structural reality of the scene—the fixed environmental elements that provide the framework for our reconstruction.
Gathering Satellite and Geospatial Data​
Begin by collecting all available overhead and geospatial data for the location:
- Digital Elevation Models (DEMs): Check for available elevation data that provides terrain height information
- Historical satellite imagery: Use Google Earth's timeline feature to view the location at different dates. This helps identify changes in the built environment over time
- Temporal analysis: Note the dates of available imagery and compare them to the date of your evidence to identify any discrepancies
Identifying Key Structural Elements​
Review your footage carefully and identify which environmental elements are most important for establishing spatial relationships:
- Buildings and structures: Which buildings appear prominently in the footage? Which ones are essential for understanding spatial relationships and sight lines?
- Perspective analysis: Look for structures in your evidence that show clear perspective lines—edges of buildings, rooflines, window patterns. These perspective cues are invaluable for photomatching and camera alignment in later reconstruction phases
- Temporal verification: Have these structures changed over time? Check historical satellite imagery to confirm whether buildings have been renovated, replaced, or demolished between the imagery date and your evidence date
- Prioritization: Focus on structures that appear in multiple frames or from multiple angles in your evidence
Extracting 3D Data from Google Earth​
Google Earth and Google Maps offer varying levels of 3D data depending on the location. Explore what's available for your area of interest:
Photogrammetry-based 3D Models
- Some locations in Google Maps and Google Earth feature 3D models generated through photogrammetry
- These models capture the actual geometry of buildings and terrain
- Check if your location has 3D coverage by switching to 3D view in Google Earth
- Quality and detail vary significantly by location
Google Earth Pro 3D Buildings
- For certain major cities and landmarks, Google Earth Pro includes purpose-built 3D models of key buildings
- These models often have cleaner geometry than photogrammetry-based reconstructions
- More suitable for architectural analysis and precise measurements
Extracting 3D Models with RenderDoc
- RenderDoc is a graphics debugging tool that can capture 3D geometry rendered by applications like Google Earth
- This allows you to extract 3D building models for use in your reconstruction workflow
- The extracted models retain their spatial relationships and can be imported into 3D software
- Process: Run Google Earth through RenderDoc, navigate to your location, capture a frame, and extract the mesh geometry
Determining Structural Dimensions​
Once you've identified key structures, gather dimensional information through multiple methods:
Width and Depth (Plan View)
- Use QGIS to export scaled satellite imagery. When imported into Blender, these images maintain real-world scale, providing accurate horizontal measurements
- Google Earth's ruler tool allows direct measurement of distances and building footprints
- Cross-reference multiple sources when possible to verify measurements
Height (Elevation)
- Online resources: Search for building specifications, architectural records, or municipal databases that may list building heights
- Reference images: Collect additional photographs of the structures from various angles and times
- Sun analysis: Use tools like SunCalc in conjunction with shadow analysis to estimate heights (detailed in photomatching section)
- Reference objects: Use objects of known dimensions visible in the footage as measurement references
- Photomatching techniques: Align camera perspectives with known geometry to derive height information (covered in detail in photomatching documentation)
Note: Ground truth at this stage is primarily structural—we're establishing the fixed architectural and topographical elements that form the spatial framework of our scene.
Phase 2: Key Objects of Interest​
Beyond the structural ground truth, we need to identify and analyze dynamic or incident-specific objects that play a central role in the event being documented.
Vehicle Identification and Modeling​
If vehicles appear in the footage:
- Identification: Determine the make, model, and if possible, the year of each vehicle
- Dimensions: Once identified, standard specifications provide accurate real-world dimensions
- 3D assets: Search for existing 3D models of the specific vehicle. Many common vehicles have models available in online repositories
- Blueprint sourcing: If 3D models aren't available, look for technical drawings or blueprints that can serve as reference for modeling
- Role in incident: Document how each vehicle relates to the incident—positioning, movement, interaction with actors
Human Actors​
Document the people involved in the scene:
- Count and tracking: How many individuals are visible? Can they be tracked across multiple frames?
- Identification scheme: Establish a consistent naming or numbering system for each actor visible in the evidence (e.g., "Actor 1," "Person A," "Individual in red jacket")
- Spatial relationships: Note each actor's position relative to fixed structures and other objects of interest
- Movement patterns: If dealing with video, track actor movements through the scene
Vegetation and Environmental Details​
Natural elements can provide additional context and spatial reference:
- Species identification: If possible, identify the types of trees or plants visible. Different species have characteristic heights and growth patterns
- Seasonal context: Vegetation appearance can help establish the time of year
- Growth patterns: Trees and plants can sometimes help date imagery or confirm temporal consistency
- Spatial markers: Significant trees or plantings serve as additional fixed reference points in your reconstruction
Other Objects of Interest​
Catalog any other elements relevant to the incident:
- Street furniture (benches, lamp posts, signs)
- Temporary structures (tents, barriers, scaffolding)
- Ground surface features (pavement patterns, markings, curbs)
For each object, gather the same types of information: identification, dimensions, availability of 3D models or blueprints, and relevance to the incident.
Documentation and Organization​
As you gather this information, maintain clear records:
- Create a database or spreadsheet listing all identified elements
- Document the source of each measurement or specification
- Note confidence levels for measurements and identifications
- Cross-reference objects between different sources of evidence
- Maintain a clear distinction between verified ground truth elements and objects requiring further validation