SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 ‘Structure-from-Motion’: A high resolution, low cost photogrammetric tool for geoscience applications Matt Westoby mjw08@aber.ac.uk Supervisors: Prof N F Glasser1, Prof J Brasington2, Prof M J Hambrey1, Prof J M Reynolds3 1Centre for Glaciology, IGES, Aberystwyth University 2University of Canterbury, Christchurch, NZ 3Reynolds International Ltd, Mold

∘ Passive: stereo imagery ∘ Active: SAR, LiDAR Total station, TLS SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Introduction - Basic Principles - Workflow - Example applications - Comparison with TLS - Discussion & summary • Traditional / contemporary methods for DTM/DEM generation: ∘ Passive: stereo imagery ∘ Active: SAR, LiDAR Total station, TLS • Often prohibitively expensive • Portability issues in remote areas Thus an identifiable need for an effective, portable and affordable alternative for obtaining high-resolution topographic data, at a range of scales..... Aerial or spaceborne platforms Ground-based

‘Structure-from-Motion’ (SfM) photogrammetry SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example applications - Comparison with TLS - Discussion & summary ‘Structure-from-Motion’ (SfM) photogrammetry ‘Structure’ - structure to be reconstructed ‘Motion’ - multiple camera baselines Key problem addressed is the fully-automated estimation of camera pose, and determination of 3D location of matching features in multiple photographs, taken from different angles/perspectives, WITHOUT the need for manual control point identification.

SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 2 - Workflow - Example applications - Comparison with TLS - Discussion & summary Popularised in past ~2-3 years through Microsoft’s web-based PhotoSynth application: 3D Panoramic Sparse point cloud

Clustering views for MVS (CMVS) (Furukawa et al., 2010) SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example applications - Comparison with TLS - Discussion & summary Key software / algorithms: Scale Invariant Feature Transform (SIFT) (Lowe, 2004) ‘Bundler’ (Snavely, 2004) - sparse PC reconstruction Clustering views for MVS (CMVS) (Furukawa et al., 2010) Patch-based MVS (PMVS2) (Furukawa & Ponce, 2009)

Dig Tsho glacial lake / moraine dam complex SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary Dig Tsho glacial lake / moraine dam complex Khumbu Himal

Dig Tsho glacial lake / moraine dam complex SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary Dig Tsho glacial lake / moraine dam complex Photo: ICIMOD (2010) Photo: TJ Peters (1982) Photo: M. Westoby (2010)

GCP network established (35 yellow targets) - dGPS SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary GCP network established (35 yellow targets) - dGPS 1649 (ground-based) photographs taken across the moraine using consumer-grade camera (Panasonic DMC-G10) and used as input to Bundler, CMVS, and PMVS2 Bundler output (sparse point cloud) n = 223,920 PMVS2 output (dense point cloud) n = 22,635,855 Transformation from relative to real-world (UTM) co-ordinate system performed by manual identification of GCPs in point cloud and shift-scale-rotate using centroid locations

SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary

Dam breach profiling A B A B C C Lake Flow SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary Dam breach profiling SRTM SfM A Lake B A B C Flow C Image courtesy of GeoEye®

Quantification of moraine volume removed by outburst flood: SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary Quantification of moraine volume removed by outburst flood: Post-flood geometry Estimate of volume eroded by Vuichard and Zimmerman (1987) - ~900,000 m3 Estimate of volume eroded using SfM + DEM interpolation - 581,380 m3 Reconstructed

Estimate of water volume released (SfM) - >6.2 x 106 m3 SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic principles 1 - Workflow - Example application: Dig Tsho - Comparison with TLS - Discussion & summary Post-flood shoreline Pre-flood shoreline Estimate of total volume of lake (Vuichard + Zimmerman, 1987) - 5-6.5 x 106 m3 Estimate of water volume released (SfM) - >6.2 x 106 m3

Constitution Hill, Aberystwyth, UK SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic Principles - Workflow - Example applications - Comparison with TLS - Discussion & summary Constitution Hill, Aberystwyth, UK Photograph Sparse data (Bundler) Dense data (PMVS2) Photograph SfM

SfM vs. TLS – Constitution Hill, Aberystwyth, UK SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic Principles - Workflow - Example applications - Comparison with TLS - Discussion & summary SfM vs. TLS – Constitution Hill, Aberystwyth, UK TLS SfM 11.5 x 106 points 11.3 x 106 points TLS SfM

SfM vs. TLS – Constitution Hill, Aberystwyth, UK SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic Principles - Workflow - Example applications - Comparison with TLS - Discussion & summary SfM vs. TLS – Constitution Hill, Aberystwyth, UK 92,500 Aerial photograph Point density (m2)

SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic Principles - Workflow - Example applications - Comparison with TLS - Discussion & summary zdiff (m)

SfM: a high resolution, low cost photogrammetric tool for geoscience applications AGU Fall Meeting 2011 Overview - Basic Principles - Workflow - Example applications - Comparison with TLS - Discussion & summary Discussion / Summary ∘ Operates under same basic principles as stereoscopic photogrammetry, though multiple photos (1000+) used as input ∘ Software/algorithms freely available – no need for expensive photogrammetry packages (+training) ∘ Extremely portable – consumer-grade digital camera, GPS, and GCP targets …thus well suited for rapid, high-resolution micro- and meso-scale terrain reconstruction in remote areas – e.g. moraines, debris-covered glacier surfaces ∘ Comparison with TLS data reveals minimal differences across majority of test site ∘ Main disadvantage – struggles with homogeneous terrain – e.g. snow, grass, sand ∘ Flexibility – ground-based data collection presented, though photo acquisition from low-altitude aerial platforms (UAVs, blimps, kites) far more effective for covering large areas – e.g. Debris-covered glacier surface, floodplains Thank you. Acknowledgements: Field assistance: John Balfour Pippa Cowley Sam Doyle James Hickman Heidi Sevestre Mark Smith Colin Souness Summit Trekking Ltd, Kathmandu Financial support: NERC Open CASE Award Aberystywth University Reynolds International Ltd.