‘Structure-from-Motion’ photogrammetry: A low-cost, effective tool for geoscience applications
Summary (3 min read)
1.1. Photogrammetric Survey Methods
- Similarly, improvements in the cost and quality of compact and single lens reflex (SLR) cameras, and methods for the calibration of such non-metric cameras (Clarke and Fryer, 1998; Chandler et al., 2005) have democratized access to photogrammetric modelling and encouraged a wide range of uses in geomorphology.
- Digital photogrammetry has also been applied to a number of geological problems, including discontinuity characterization (e.g. Krosley et al., 2006; Sturzenegger and Stead, 2009) and rock slope stability analysis (e.g. Haneberg, 2008) .
- Close-range applications have also included direct quantification of soil erosion and the morphodynamics of laboratory-scale landscape evolution models (e.g. Stojic et al., 1998; Brasington and Smart, 2003; Lane et al., 2001; Hancock and Willgoose, 2001; Rieke-Zapp and Nearing, 2005; Heng et al., 2010) .
1.2. Structure-from-Motion
- Developed in the 1990s, this technique has its origins in the computer vision community (e.g. Spetsakis and Aloimonos, 1991; Boufama et al., 1993; Szeliski and Kang, 1994) and the development of automatic feature-matching algorithms in the previous decade (e.g. Förstner, 1986; Harris and Stephens, 1988) .
- The approach has been popularized through a range of cloud-processing engines, most notably Microsoft® Photosynth™ (Microsoft, 2010), which uses SfM approaches documented in Snavely (2008) and Snavely et al. (2008) .
- These tools can make direct use of user-uploaded and crowd-sourced photography to generated the necessary coverage of a target scene, and can automatically generate sparse 3-D point clouds from these photosets.
- The possibilities of SfM appear boundless, however, to date, the technique has rarely been used within the geosciences (e.g. Niethammer et al., 2012) and there exist few quantitative assessments of the quality of terrain products derived from this approach.
1.3. The First Principles of SfM
- GCPs can be derived post-hoc, identifying candidate features clearly visible in both the resulting point cloud and in the field, and obtaining their coordinates by ground survey (i.e., by GPS).
- In practice, however, it is often easier to deploy physical targets with a high contrast and clearly defined centroid in the field before acquiring images.
- This approach simplifies the unambiguous co-location of image and object space targets and also ensures a reliable, well-distributed network of targets across the area of interest, enabling an assessment of any non-linear structural errors in the SfM reconstruction.
1.4. Goals of this Article
- Applications of SfM to a range of contrasting landscapes and landforms are described, including coastal cliffs, a moraine-dammed lake, and a smaller scale glacially-sculpted bedrock ridge.
- Importantly, the authors also undertake a detailed assessment of the quality of a derived topographic model, in this case a c. 300 x 300 m cliff section in Aberystwyth, Wales, through comparison with a high resolution terrain model derived from a precision terrestrial laser scan survey.
2.1.1. Image acquisition and keypoint extraction
- A wide variety of imaging sensors can be used for SfM, from video stills, through to low grade compact digital cameras.
- The primary requirement is well-exposed photographs of the feature(s) of interest.
- From their experience, 'bigger' is not necessarily 'better'.
- Whereas image quality and resolution are improved by using increasingly expensive digital SLR models, images captured at the highest resolutions (e.g. >12 megapixel) will almost inevitably need to be re-sized (with the consequent loss of image detail) to avoid lengthy processing times.
- If operating in remote regions, specific consideration should be given to robustness and battery life, including methods for charging and performance in extreme temperatures.
2.1.3. Post-processing and digital elevation model generation
- When combined, SfM and point-cloud decimation potentially offer a powerful tool for geomorphological analysis.
- This model may be visualized effectively by draping the orthophoto derived from the SfM processing over this surface.
- The final result is a fully georeferenced, high-resolution, photo-realistic DEM.
3.1. Data acquisition and processing
- The extensive photoset was decomposed into three 'batches' to reduce computational demand, and input photographs re-scaled to 55% of their original resolution to reduce computational demand.
- The processing steps outlined in section 2 were employed, producing unreferenced sparse and dense point-clouds as output (Table 2 ).
- The SfM data were transformed to the TLS co-ordinate system through manual identification of matching GCP centroids in both datasets (Fig. 4b-d ).
- The three SfM batches were registered individually, with no significant difference in the quality of the three transformation models, and average transformation residuals of 0.124 m, 0.058 m and 0.031 m for xyz.
4.1 Dig Tsho moraine complex
- Background photographic information was sufficient to reconstruct the entire lake basin, including the 2 km long northern lateral moraine.
- As in the previous example, significant topographic detail (sub-metre scale) has been resolved.
- The entire breach was successfully reconstructed, and notable morphological features captured by the model include the narrow central section and expansive exit, as well as two abandoned spillways.
- A number of interpolation artefacts are present across the scene, but are largely confined towards the south and correspond to an extensive area of snow cover.
5. Discussion
- The example applications presented in section 4 were ideally suited to the application of the SfM technique.
- Minimal vegetation coverage and relatively complex, heterogeneous topography at both the meso-and micro-scales facilitate the extraction of suitable numbers of keypoint descriptors for consistent, dense point cloud coverage.
- Similarly, the method is ideally suited for application in (semi)arid environments.
- In contrast, the method's suitability for topographic reconstruction of, for example, riparian landscapes may be limited, given that, at present, only waterfree surfaces would be suitable for reconstruction, and point density is likely to be limited, and of questionable accuracy, in areas of dense vegetation.
6. Conclusions
- This paper has outlined a novel low-cost, ground-based, close-range terrestrial photogrammetry and computer vision approach to obtaining high-resolution spatial data suitable for modelling meso-and micro-scale landforms.
- The nature of the SfM method eliminates the requirement for manual identification of image control prior to processing, instead employing automatic camera pose estimation algorithms to simultaneously resolve 3-D camera location and scene geometry; this is an extremely significant advantage of the technique over traditional digital photogrammetric methods.
- As the raw SfM output is fixed into a relative co-ordinate system, particular time and attention should be taken in the establishment of a GCP network to facilitate transformation to an absolute coordinate system and the extraction of metric data.
- Taking the hypothesised effectiveness of an aerial approach into account, the terrestrial data collection method presented herein nevertheless represents an effective, financially viable alternative to traditional manual topographic surveying and photogrammetric techniques, particularly for practical application in remote or inaccessible regions.
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Citations
606 citations
Cites methods from "‘Structure-from-Motion’ photogramme..."
...Westoby et al. (2012) used SfM techniques to map the 3D structure of a steep alpine hill slope and demonstrated that the SfM-derived elevation measurements were within 0.1 m of a TLS scan....
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573 citations
Cites background or methods from "‘Structure-from-Motion’ photogramme..."
...…SfM–MVS has produced terrain models with centimeter precision and point cloud resolutions that fall between LiDAR and TLS (Doneus et al., 2011; Fonstad et al., 2013) andhas beenutilized to accuratelymodel objects on the centimeter to kilometer scale (James and Robson, 2012; Westoby et al., 2012)....
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...While the study reach highlights the capabilities of the described workflow, the reasonable errors of the extended reach illustrate the potential for thisworkflow to produce qualitatively convincingDEMs from retrofitted data thus, significantly increasing the topographic detail and research opportunities of pre-existing or limited datasets....
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...While it is recognized that PhotoSynth and SFMToolkit are both capable of producing quality DEMs (e.g. James and Robson, 2012; Westoby et al., 2012), this research utilized PhotoScan (version 0....
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...Traditional photogrammetric DEMs were typically less accurate and precise than airborne LiDAR (Baltsavias, 1999); however, SfM–MVS has produced terrain models with centimeter precision and point cloud resolutions that fall between LiDAR and TLS (Doneus et al., 2011; Fonstad et al., 2013) andhas beenutilized to accuratelymodel objects on the centimeter to kilometer scale (James and Robson, 2012; Westoby et al., 2012)....
[...]
..., 2013) andhas beenutilized to accuratelymodel objects on the centimeter to kilometer scale (James and Robson, 2012; Westoby et al., 2012)....
[...]
565 citations
Cites background or methods from "‘Structure-from-Motion’ photogramme..."
...Advice on SfM-MVS image acquisition for specific applications is given in several papers, including Favalli et al. (2012); James and Robson (2012); Westoby et al. (2012); Bemis et al. (2014); Micheletti et al. (2014); Smith et al., (2014) and Stumpf et al. (2015)....
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...Full 360 coverage is ideal (Westoby et al., 2012), though not always necessary so long as all surfaces of interest are visible in multiple photographs....
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...Scenes devoid of distinct features (e.g. smooth ice surfaces) will be challenging and often produce fewer keypoint correspondences and lower point densities (Westoby et al., 2012)....
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..., 2014); however, large images may need to be re-sized to reduce processing times (Westoby et al., 2012)....
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...smooth ice surfaces) will be challenging and often produce fewer keypoint correspondences and lower point densities (Westoby et al., 2012)....
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548 citations
504 citations
Cites background from "‘Structure-from-Motion’ photogramme..."
...), coupled with imaging, ranging, and positioning sensors, are able to collect multispectral imagery at cm-level resolution and offer great possibilities in the precision farming domain [7–10], agriculture and forestry management [11,12], and geosciences [13]....
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"‘Structure-from-Motion’ photogramme..." refers methods in this paper
...Keypoints in multiple images are matched using approximate nearest neighbour (Arya et al., 1998) and Random Sample Consensus (RANSAC; Fischler and Bolles, 1987) algorithms, and ‘tracks’ linking specific keypoints in a set of pictures are established....
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"‘Structure-from-Motion’ photogramme..." refers methods in this paper
...This package contains a number of open-source applications including, in order of execution, SiftGPU (Lowe, 1999, 2004), Bundler (Snavely et al., 2008), CMVS and PMVS2 (Furukawa and Ponce, 2007; Furukawa et al., 2010), all of which may be run independently if desired....
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"‘Structure-from-Motion’ photogramme..." refers methods in this paper
...…in the 1990s, this technique has its origins in the computer vision community (e.g. Spetsakis and Aloimonos, 1991; Boufama et al., 1993; Szeliski and Kang, 1994) and the development of automatic feature-matching algorithms in the previous decade (e.g. Förstner, 1986; Harris and Stephens, 1988)....
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..., 1993; Szeliski and Kang, 1994) and the development of automatic feature-matching algorithms in the previous decade (e.g. Förstner, 1986; Harris and Stephens, 1988)....
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