Photogrammetry for Archaeology and Documentation

Tim Trombley

Educational Technologist

Media Center for Art History

Art History and Archaeology

Media Center for Art History

  • learn.columbia.edu
  • Projects from 20yrs+
  • Resources for students
  • Databases of resources and external projects

What is Photogrammetry

  • Structure from Motion
  • Create 3D models from photographs
  • More specfic meanings out there, but this is what most people mean when they say photogrammetry

Comparison with other documentation approaches

  • Total Station
  • High-end Laser Scanning
  • Handheld Laser Scanning
  • Structured Light Scanning (SLS)
  • Cheap!
  • Quick
  • Relatively Easy
  • Flexible
  • Active software development increasing accuracy and detail

Photogrammetry

  • Cannot see results until post-processing
  • Accuracy concerns and human error
  • Computing requirements - RAM, GPU compute, CPU

Pros

Cons

Workflow

  • Imaging
  • Image Processing
  • Photogrammetry Software
  • Deployment

Imaging

  • Camera Settings
  • Coverage
  • Use-case examples for different objects/settings
  • Hope for an overcast day

Camera Settings

  • Lens choices
  • Consistent focal length (prime lenses) - software relies on EXIF data and lens profiles
  • Sharpest focus possible (tripod highly recommended)
  • Consistent exposure levels (can be adjusted in post to a point)
  • Lowest ISO possible

Imaging Coverage

  • Software can only process what it can see.
    Omissions = interpolation
  • No such thing as too many photos (provided you can process them)

Imaging Use Cases

  • Small object - Dome coverage
  • Large object - Coverage/Height
  • Small exterior/Trench - Coverage
  • Large exterior/Site - Coverage, speed
  • Drone: N/A - permits almost always required

Imaging Use Cases

Small object: Macchiozzo Inscription

Imaging Use Cases

Large object - Stronach Togate Torso

Imaging Use Cases

Small exterior/Trench: Onchestos Trench

Imaging Use Cases

Large Exterior: Macchiozzo

Imaging Use Cases

Large Interior: N/A

Will Attempt in Future likely with Fisheye Lens

Image Processing

  • Consistent Lighting levels/exposure
  • Masking - in Lightroom/Photoshop or Photogrammetry software
  • High Quality Jpeg export - can use tiffs, but they are overly heavy in most cases
  • Recommend working locally, not on network drive for reasons of speed

Photogrammetry Software

  • Agisoft - popular, great community, relatively cheap, actively being developed
  • Workflow:
    • align photos
    • refine/optimize cameras
    • generate dense cloud > export?
    • mesh
    • refine mesh
    • texture
    • export
  • Quality settings can be adjusted at each step
  • Higher quality = longer processing time and greater computing resources required, higher chance of crashes

Align Photos

  • Basic placement of cameras relative to each other
  • Looks for points in common
  • Creates basic point cloud
  • Will likely need straightened
  • Lower quality settings may not align all photos
  • Difficult photos may require manual point registration
  • Some aligned photos may be erroneous and detrimental to larger model

Refinement and Optimization

  • Gradual selection to remove garbage data
    • Reprojection error, Reconstruction uncertainty, Image count,  Projection accuracy
    • All values on sliders
  • Optimize camera alignment after each change. Tighter alignment of cameras, less error
  • Rinse and repeat

Generate Dense Cloud

  • Most computationally intense step - looks for points in common between each photo
  • Quality you choose plays a big role: 105 photos at the highest quality on my high-end workstation - 50hrs
  • May result in 100s of millions of points
  • Will almost always require cleaning of fly-away points
  • Can register scale bars at this point for real world dimensions
  • Could export points from this step to deployment with Potree or others or import into other software packages

Generate Mesh

  • Inherently Interpolative
  • Quality choice determines analytical viability and interoperability
  • Can be exported and reimported for decimation and optimization
  • Almost always a middle-man step

Create Texture

  • Number of images used for texture
  • Resolution of images
  • UVW Wrapping

Deployment

  • Audience
  • Purpose
  • Intended Devices

Deployment - Points

  • Potree
    • Open-source (new release 3/12)
    • Octree loading - mobile friendly (to a point)
    • measurable, take sections, etc.
  • Sketchfab - a bit awkward with points
  • Other commercial solutions tend to be very expensive

Deployment - Meshes

  • Sketchfab - huge library/community, VR/AR, strong support for cultural heritage, we have secured pro accounts for all CU emails
  • Other commercial hosts and libraries
  • 3D PDF - small models best, but viable in may situations, includes good tools
  • Import into other software for compositing or analysis
  • more flexible than points
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