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Close range underwater photogrammetry for high resolution survey of a coral reef: A comparison between reconstructed 3-D point cloud models from still image and video data

Authors:

Abstract

Coral threat levels from climate change have increased around the globe. Coral reefs are nature's best coastal protection device [MS48]. They dissipate portions of the wave energy through a system of multi-scalar tunnels to gradually reduce the power of large swells. As complex and permeable underwater structures, reefs refract waves instead of reflecting them which results in sand deposition instead of erosion [GP17]. Currently, reefs are threatened around the globe because of rising sea temperatures due to global warming, elevated levels of CO2 from pollution acidifying the oceans and radical practices such as dynamite fishing. Architects study their geometry to develop artificial coral reef systems to regrow premorse parts of corals and coastal protection devices [Vo18]. To understand the reef geometry detailed surface configurations and textures of a natural coral reef, a workflow was developed for close-range underwater coral reef monitoring that outputs high precision 3-D point cloud models. Utilizing the case study site of Gili Trawangan, Indonesia, underwater data from high-resolution still image and video data were collected of a natural coral reef and 3-D reconstructed precise point cloud models from both datasets. In this paper, both reconstructed point cloud models are presented and results from underwater photo-and videogrammetry are compared followed by discussing the potential of both methods for a close-range underwater survey. The accuracy and reliability of both techniques by measuring objects of known size is demonstrated.
Close-range underwater photogrammetry for
high-resolution survey of a coral reef: A comparison
between reconstructed 3-D point cloud models from
still image and video data
Verena Vogler
InfAR at Faculty of Architecture
Bauhaus University
Belvederer Allee 1, 99421 Weimar, Germany
verena.vogler@uni-weimar.de
Abstract: Coral threat levels from climate change have increased around the globe.
Coral reefs are nature's best coastal protection device [MS48]. They dissipate
portions of the wave energy through a system of multi-scalar tunnels to
gradually reduce the power of large swells. As complex and permeable
underwater structures, reefs refract waves instead of reflecting them which
results in sand deposition instead of erosion [GP17]. Currently, reefs are
threatened around the globe because of rising sea temperatures due to global
warming, elevated levels of CO2from pollution acidifying the oceans and
radical practices such as dynamite fishing. Architects study their geometry to
develop artificial coral reef systems to regrow premorse parts of corals and
coastal protection devices [Vo18]. To understand the reef geometry detailed
surface configurations and textures of a natural coral reef, a workflow was
developed for close-range underwater coral reef monitoring that outputs high
precision 3-D point cloud models. Utilizing the case study site of Gili
Trawangan, Indonesia, underwater data from high-resolution still image and
video data were collected of a natural coral reef and 3-D reconstructed
precise point cloud models from both datasets. In this paper both
reconstructed point cloud models are presented and results from underwater
photo- and videogrammetry are compared followed by a discussion of the
potential of both methods for close-range underwater survey. The accuracy
and reliability of both techniques by measuring objects of known size are
demonstrated.
Keywords: Close-range underwater photogrammetry, underwater
videogrammetry, coral reef monitoring
1
1 Introduction
1.1 Why do architects underwater survey coral reefs?
Coral reefs form excellent study objects for the exploration of high-resolution
3-D scanning and modelling methods. They are geometrically and structurally
complex and present many challenges regarding 3-D scanning and modelling of
their intricate surface configurations [VSW19]. In this section, I introduce two
3-D surveying methods to capturing 3-D models of a natural reef at close-range
used during my field research in Gili Trawangan in Indonesia: underwater
photogrammetry (UW photogrammetry) and underwater videogrammetry (UW
videogrammetry).
Photogrammetry multiview 3-D reconstruction, or
Structure-From-Motion (SfM), is a technique for constructing
three-dimensional structures from two-dimensional imagery from
images.
Videogrammetry is a measurement technique based on the principles of
photogrammetry [Gr97]. Instead of still images it uses extracted image
frames from video footage.
I used both methods to retrieve information of high resolution underwater images
and videos to recover the exact three-dimensional position and colour of surface
points of a natural coral reef. The principle of underwater photogrammetry does
not differ from that of terrestrial or aerial photogrammetry but it is necessary to
take into account certain elements that may cause disturbance, in particular the
refraction of the diopter water-glass and the presence of the housing [BLL02].
One major influence on the quality of 3-D reconstructed models from photo- and
videogrammetry is visibility, a measure of the distance at which an object can be
distinguished [SB81]. Underwater vision is limited by large numbers of
individual invisible particles dissolved in the water. Image and video data
collected at a low visibility of less than 10 meters shows poor alignment rates in
image processing software [VSW19]. UW photogrammetry equipment is
financially affordable, transportable and can be handled by only one diver.
Underwater photo- and videogrammetry for underwater surveys are currently
under investigation in Archaeology, Geology and Marine and Conservation
Biology. Since 2014, Hydrous, a U.S. based non-profit organisation has created
the campaign, ‘open access oceans’ for engagement with marine environments,
collecting underwater image data for use in close-range underwater
photogrammetry of natural coral reefs around the world. These models are clean
3-D polygonal mesh models and textures of different coral topologies and
exploited as Open Access Models on SketchFab, an online 3-D content library
2
[Sk19]. The resolution in this library is less precise than in this approach
presented. In April 2016 the French section of Reefcheck, a non-profit
organization dedicated to the conservation of tropical coral reefs, used a GoPro
Hero 4 Black to 3-D reconstruct a 305 m2 coral reef near Reunion Island from
1625 video frames extracted from video footage. Their goal was to identify
bleached areas of the reef through a digital textured surface model of the reef
[Pi16]. Their resulting 3-D surface models lack details regarding the exact
geometry of individual corals species. However, rebuilt textures are of low
resolution and show poor resolution in areas where surface geometry becomes
more complex.
1.2 The case study coral reef in Gili Trawangan, Indonesia
The case study object is a natural reef at a depth of approximately 13 meters,
about 100 meters off the shoreline of Gili Trawangan island in Indonesia. The
reef is 100 cm long, 100 cm wide, and at its highest point, 80 cm high. The goal
of the experiments was to achieve high precision rates of 1-5 millimeters for 3-D
models from UW photo- and videogrammetry. In this paper, the unique
underwater workflow at close range for high accuracy 3-D models of corals
using UW photo- and videogrammetry is proposed. Precision values for both
reconstructed 3-D point cloud models from (i) still images and (ii) video footage
with 25 manual UW measurements of the reef are compared. Survey results
demonstrate a high level of detail, completeness of the overall model, reliability
and application in the field for both methods. Based on evaluation the
deployment criteria for each underwater survey method is then proposed.
2 High precision methods for 3-D reconstruction from UW
Photo- and Videogrammetry
After a general introduction of the survey technology used, this section focuses
on the implementation and validation of UW photo- and videogrammetry.
During the underwater field survey in Indonesia, the focus was on a complete
3-D scan of a natural reef with the Canon EOS 5Ds camera system. This camera
system uses one of the best image sensors (50.6 megapixels) and highest output
resolution (8688 × 5792 pixels) on the current market. Together with a Canon 50
mm 2.5 macro lens inside of a SEACAM underwater housing 5DMKIII, two
SEACAM strobes (SF150D) and one video light completed the system. The
camera system has the capacity to optimize sharpness and clarity of
high-resolution images through a low-pass cancellation filter. This unique feature
lowers the risk for digital artefacts in photographs. The macro lens was selected
to prevent image distortion.
3
Figure 1: Getting ready to videoscan a natural coral reef in Gili Trawangan, Indonesia,
following a lawn-mower pattern using Canon EOS 5Ds.
Two complete datasets of a natural reef were collected one from 1260
high-resolution still images (8688 × 5792 pixels) and the other one from 912
extracted frames from video footage (1920 × 1080 pixels) (Table 1). Stills and
videos were taken at a distance of 25- 40 cm between camera and object
following a so-called “lawn-mower” photogrammetry pattern with 60% of side
and 80% of forwarding overlap (Figure 1) [Ag18]. Camera settings such as
aperture value, ISO number and image resolution were kept constant respectively
for each dataset. PhotoScan Pro Version 1.4.4 (Agisoft) image processing
software was used to reconstruct 3-D point cloud models. Both datasets (DS1
and DS2) were collected at a visibility of approximately 30- 35 meters.
Figure 2: As a reference for the original size of the natural reef, we took about 25 manual
measurements to scale both reconstructed 3-D point cloud models and to calculate
deviations between the original and digital reconstructed 3-D model.
4
Table 1: Technical data for 3-D reconstruction experiments from still image and video
data.
Canon EOS 5DS R
Still image data (DS1)
Video data (DS2)
File format
JPEG, RAW
MPEG
Resolution
8688 ×5792 pixels
1920 ×1080 pixels
Light source
Two SEACAM strobes (SF150D)
One video light
Underwater battery
lifetime (camera
and light source)
Camera 70 min,
SF150D strobes at 25% approx. 800
still images
Camera 50 min,
video light at 100%
approx. 35 min
2.1 UW Photogrammetry 3-D point cloud model
The still image data from DS1 was processed in PhotoScan Pro Version 1.4.4 and
the resulting point cloud model was cleaned in Cloud Compare V2.10.1, an
open-source 3-D point cloud and mesh processing software [Cl17]. 25 manual
measurements from around the model were then compared with measurements
taken from the scaled digital point cloud model and calculated a precision for the
final 3-D point cloud model of a range between 2 to 9 mm. The final point cloud
model is complete and displays high detail of the geometry and texture of corals
(Figure 3-8).
Figure 3- 4: Overall point cloud model reconstructed from 1260 still images (DS1) with
camera positions. The model has 621,912,135 colored points.
5
Figure 5- 8: Resulting details of reconstructed UW photogrammetry point cloud model
(DS1) from still images.
2.2 UW Videogrammetry 3-D point cloud model
The second model 912 video frames (1920 × 1080 pixels) were extracted at a
frame extraction rate of 15 frames per second (fps) from DS2. Following the
lawn-mower pattern method, the top, left, right, back and front faces of the
natural reef were captured in five video files. Extracted frames had the correct
image overlap between 60% and 80% to be aligned and processed in PhotoScan
Pro Version 1.4.4. The results were cleaned and scaled to the resulting point
cloud model in Cloud Compare V2.10.1 and calculated deviations of a range
between 7 to 25 mm. The overall 3-D model is complete from all sides but has
several holes (Figure 9).
6
Figure 9- 10: Overall point cloud model reconstructed from 912 video frames (1920 x
1080 pixels) (DS2) with camera positions. The model has 74,505,524 points.
Figure 11-14: Resulting details of reconstructed UW videogrammetry point cloud model
(DS2) from extracted video frames.
3 Results and discussion
3.1 Comparison of results from UW Photo- and Videogrammetry
The comparison criteria of both methods is precision, level of detail, model
completeness in the resulting point cloud model and overall time to generate a
3-D model. Both datasets, from UW photo- and videogrammetry reconstructed
7
3-D point cloud models describe the overall surface of the scanned coral reef.
Our reconstructed 3-D model from UW photogrammetry is cleaner and describes
in high detail resolution and colour the scanned geometry of individual corals
(Figure 5-8). This can be attributed to the high-resolution input data from still
images of 8688 × 5792 pixels, as well as, razor-sharp and perfectly illuminated
images. Therefore, UW photogrammetry is a more reliable method to reconstruct
highly accurate and complex 3-D models from UW data at close range than UW
videogrammetry (Figure 15-18). UW data collection and image processing took
much longer for our UW photogrammetry models than for UW videogrammetry
models. Data collection in the field for DS1 took twice as long as for DS2.
Model processing time in PhotoScan Pro Version 1.4.4. took 8 times longer for
DS2 than DS1. Even though the point cloud model from video frames is less
precise, UW videogrammetry is still a convincing method to quickly capture the
overall geometry and texture of the test coral reef with an average precision of
+/- 7 mm (Figure 11-14). An average precision of +/- 3.5 mm for the 3-D model
was calculated from DS1 (Figure 15-16). Both UW photo- and videogrammetry
have the potential to monitor complex underwater landscape models at high
precision and are applicable. In commercial environments greater financial
resources are available.
Figure 15-16: UW photograph (left), screenshot of 3-D point cloud model from DS1
(right). This enlarged view of the 3-D model represents one of the more complex areas of
the scanned reef.
8
Figure 17-18: Extracted frame from UW video (left), 3-D model from DS2 (right). This
view shows the same region of the scanned reef as in Figure 15-16.
Table 2: Technical details for 3-D reconstruction of point cloud models from image and
video data.
Reconstructed point cloud
model from still images:
Dataset 1 (DS1)
Reconstructed point cloud
model from extracted video
frames: Dataset 2 (DS2)
Total number of images/
videos
1260 still images
912 extracted video frames
(15fps)
Time to collect data/
Number of dives
2 dives/ in total 75 min
1 dive/ 20 min
Number of partial
models
2 partial models (2 chunks )
1
1 partial model (1 chunk)
Alignment rates
Chunk 1: 603 of 618 (97,6 %)
Chunk 2: 619 of 642 (96,4 %)
Only 1 chunk: 912 of 912
(100%)
Processing time of
dense cloud
Chunk 1: 3 days and 21hours
Chunk 2: 3 days and 13 hours
Total: 4 days and 9 hours
23 hours and 32 minutes
Number of points
overall model
621,912,135 points
58,498,527 points
Precision of 3-D model2
+/- 2- 5 millimeter
+/- 7- 15 millimeter
Detail and completeness
of model
High detail of overall model
and individual corals
Overall geometry was
reconstructed, low detail of
corals, model has holes
Alignment rates were similar for both models: 603 of 618 (97,6 %) and 619 of
642 (96,4 %) still images and 912 of the 912 (100%) extracted video frame
images were aligned (Table 2).
2We compared 25 manual UW measurements of the coral reef with 25 digital measurements of the
scaled point cloud model and calculated deviations at each measuring point. Both the minimum and
the maximum value represents, respectively, the smallest and largest deviation over all 25 measuring
points.
1In PhotoScan Pro, chunks allow to include similar image and video data in one dataset. Several
chunks can be included in the same file and datasets can be combined.
9
3.2 Sources of error in UW photogrammetry from stills and video
data
Following the exact protocol as in VSW19, the same error sources affect both
3-D scan methods. In short, the potential source of error in our underwater
photo- and videogrammetry experiments is using not calibrated lenses for
underwater refraction. It is challenging to determine the refractive index for
seawater as water temperature, salinity and wavelength were changing
parameters during our experimental [JKK16]. In the experiments, this effect was
ignored. Therefore, deviations in both compared point cloud models from UW
photogrammetry include refraction errors and errors from uncalibrated lenses.
Calibration and refraction errors multiply as the area covered grows. In the
experiments, datasets of the natural reef captured at different distances to test
how this effect evolves were not included. In future proposals, the experiments
would benefit from testing methods in a wider area. The underwater camera
system used was confined in an underwater housing with one viewing the scene
through a macro port, a flat piece of glass. Light rays entering the camera
housing are refracted due to different medium densities of water, glass and air.
This causes linear rays of light to bend and the commonly used pinhole camera
model to become invalid. When using the pinhole camera model without
explicitly modeling refraction in SfM methods, a systematic model error occurs
[JK13]. Photogrammetry models are susceptible to alignment errors causing
scaling errors and ghosting, a phenomenon when two image data sets are
combined and reconstructed more than once at a different location. Even in
well-aligned high precision models, an error of 3 cm is possible depending on
how accurate the scaling has been performed in photogrammetry software. A
high amount of moving objects such as fish, soft corals, shadows from sun or
light sources projected onto the sand and backscatter, strobe reflections from
moving particles in water, all of them cause image noise to appear in the images
and can cause misalignments of collected data. Visibility is the key parameter for
good alignments in both, photo- and videogrammetry. Both datasets, DS1 and
DS2, were taken with a visibility larger than 30 meters with more than 96% of
the collected images able to be aligned.
4 Conclusions
In times of climate change and ecological crisis, this workflow offers a unique
approach to UW survey coral reefs at close range using UW photo- and
videogrammetry to reconstruct high precision 3-D point cloud models of corals
and coral reefs. Aiding perception and understanding of the living underwater
10
environment as 3-D reconstructed models visualized in high detail and creating
accurate 3-D surface and textures configurations of the scanned natural reef.
Respective practical, technical, environmental criteria and parameters for
high-resolution 3-D reconstruction and comparisons from resulting 3-D models
from photo- and videogrammetry of the same coral reef are discussed. Both
methods result in 3-D point cloud models of different precision and detail and
require different amounts of time and resources. Therefore, UW photogrammetry
could be implemented for detailed studies of high precision surveys of individual
corals whereas UW videogrammetry could be used for faster scans of larger
survey areas. Both methods can be applied to study details of growth processes
of corals or e.g. to monitor different stages of coral bleaching. Point cloud
models represent the physical form of underwater objects and can be used as a
tool for spatial analysis, Virtual Reality models or WebGL models in online 3-D
content libraries such as SketchFab [Sk19]. Underwater point cloud models can
be converted into digital surface models for structural analysis, hydrodynamic
modelling, or digital fabrication such as 3-D printing to represent scanned reef
areas as physical model. Furthermore, at high visibility both methods can be
exploited in other areas of marine and environmental modelling for static objects
such as underwater inspection and monitoring of oil and gas pipelines.
5 Acknowledgements
I thank my colleagues and collaborators Jan Willman (Faculty of Theory and
History of Design, Bauhaus University Weimar), Sven Schneider (Faculty of
Architecture, Bauhaus University Weimar), Thomas Gebhardt (Computer Vision
and three-dimensional Geodesy, Bauhaus University Weimar) and especially
Bert van der Togt (Baars CIPRO) who provided insight and expertise that greatly
assisted the research. I thank Georg Nies (Unterwasserfotografie Deutschland,
GeNieS GmbH) for assistance with underwater photography. For operational
support during my field research, I thank Delphine Robbe (Gili EcoTrust,
Indonesia), Steve Willard (Dive Central Gili), Matteo (Dreamdivers Gili T.) and
for assistance in the field Philipp Semenchuk (Department of Conservation
Biology, Vegetation and Landscape Ecology, University of Vienna). This
research is financially supported by a doctoral fellowship of the German
Academic Scholarship Foundation (or Studienstiftung des deutschen Volkes) and
Andrea von Braun Foundation, which provided funding for material and travel
expenses for our experiments.
11
References
[BLL02] Butler J.B.; London E-G., Lane S.L: Through-water close range digital
photogrammetry in flume and field environments. in Photogrammetric
Record,Vol.:17, pp.419, 2002.
[GP17] Goreau, T.; & Prong, P.: Biorock Electric Reefs Grow Back Severely Eroded
Beaches in Months. Journal of Marine Science and Engineering, 5(4), 48, 2017.
[Gr97] Gruen, A.: Fundamentals of videogrammetry - A review. Human Movement
Science, 16(2–3), 155–187, 1997.
[JK13] Jordt-Sedlazeck, A.; & Koch, R.: Refractive structure-from-motion on
underwater images. In Proceedings of the IEEE International Conference on
Computer Vision, pp. 57–64, 2013.
[JKK16] Jordt, A.; Köser, K.; & Koch, R.: Refractive 3D reconstruction on underwater
images. Methods in Oceanography, 15–16, 90–113, 2016.
[MS48] Munk, W. H.; & Sargent, M. C.: Adjustment of Bikini Atoll to ocean waves.
Eos, Transactions American Geophysical Union, 29(6), p. 855–860, 1948.
[Pi16] PIX4D: Surrounded by marine creatures, French association Reef Check used
Pix4Dmapper software and two GoPro cameras to map underwater. Retrieved
from
https://www.pix4d.com/blog/underwater-mapping-3d-coral-reefs-enhance-envir
onmental-reefs-surveys, 2016, October 26.
[SB81] Smith, R. C.; & Baker, K. S.: Optical properties of the clearest natural waters
(200-800 nm). Optical Society of America, 20(2), 177–184, 1981.
[Sk19] Sketchfab: Open Access Coral Models- Available for Download. Retrieved
from
https://sketchfab.com/thehydro.us/collections/open-access-coral-models-availab
le-for-download, 2019, January 25.
[VSW19]Vogler, V.; Schneider, S.; Willmann, J.: High-resolution underwater 3-D
monitoring methods to reconstruct artificial coral reefs in the Bali Sea: A case
study of an artificial reef prototype in Gili Trawangan. JoDLA Journal of Digital
Landscape Architecture, Issue 4-2019, 275- 289. Berlin/Offenbach: Wichmann
Verlag im VDE VERLAG. e-ISSN 2511-624X, 2019.
[Vo18] Vogler, V.; Andrea von Braun Foundation: Sonderheft Korallenarchitektur.
Briefe zur Interdisziplinarität. Munich, Germany: Oekum Verlag. ISSN
1865-8032, 2018.
Software:
[Ag18] Agisoft LLC: Agisoft Photoscan Memory Requirements. Retrieved from
https://www.agisoft.com/pdf/tips_and_tricks/PhotoScan_Memory_Requirement
s.pdf, 2018.
[Ag18] Agisoft LLC: Agisoft PhotoScan User Manual. Professional EditionVersion 1.4.
Retrieved from https://www.agisoft.com/pdf/photoscan-pro_1_4_en.pdf, 2018.
[Cl17] Cloud Compare: CloudCompare Version 2.6.1 User Manual. Retrieved from
https://www.danielgm.net/cc/doc/qCC/CloudCompare%20v2.6.1%20%20User
%20manual.pdf, 2017.
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... Although UW photo-and videogrammetry are signi icantly more constrained than in aerial or terrestrial uses, they are still robust and e icient measuring techniques or underwater environments with limited accessibility [145]. Through recent advances in camera technology and digital image processing so tware, the ability to use photogrammetry or high-precision 3D reconstructions or underwater survey purposes has been greatly improved [146]. There ore, close model. ...
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In underwater environments, cameras need to be confined in an underwater housing, viewing the scene through a piece of glass. In case of flat port underwater housings, light rays entering the camera housing are refracted twice, due to different medium densities of water, glass, and air. This causes the usually linear rays of light to bend and the commonly used pinhole camera model to be invalid. When using the pinhole camera model without explicitly modeling refraction in Structure-from-Motion (SfM) methods, a systematic model error occurs. Therefore, in this paper, we propose a system for computing camera path and 3D points with explicit incorporation of refraction using new methods for pose estimation. Additionally, a new error function is introduced for non-linear optimization, especially bundle adjustment. The proposed method allows to increase reconstruction accuracy and is evaluated in a set of experiments, where the proposed method's performance is compared to SfM with the perspective camera model.
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With the advent of inexpensive and powerful CCD- (‘video-’) cameras and associated image data transfer and processing hardware novel measurement systems, based on these components, are of great potential for human movement recording. This contribution gives an introduction to the fundamentals of ‘videogrammetry’, a measurement technique based on the principles of ‘photogrammetry’. We will discuss the concept of the multi-image (more than two CCD-frames) measurement mode, the bundle adjustment with its variants for point positioning, orientation and calibration, the notions of precision and reliability, system aspects including some details on CCD-cameras and image data transfer, calibration and self-calibration, algorithms for high precision image measurements, and finally show a typical application example.Besides being a highly automated measurement technique, videogrammetry provides for high accuracy (a relative accuracy of 1: 10000 can be reached in trajectory determination with standard components) and truly real-time data processing capabilities. In addition, a great number of particles (> 100) can be measured and tracked simultaneously, and surfaces and their deformation can be determined.Therefore, the videogrammetric technique is of great interest for applications in biomechanics, sport, animation, and virtual reality generation and control.
High-resolution underwater 3-D monitoring methods to reconstruct artificial coral reefs in the Bali Sea: A case study of an artificial reef prototype in Gili Trawangan
  • V Vogler
  • S Schneider
  • J Willmann
Vogler, V.; Schneider, S.; Willmann, J.: High-resolution underwater 3-D monitoring methods to reconstruct artificial coral reefs in the Bali Sea: A case study of an artificial reef prototype in Gili Trawangan. JoDLA Journal of Digital Landscape Architecture, Issue 4-2019, 275-289. Berlin/Offenbach: Wichmann Verlag im VDE VERLAG. e-ISSN 2511-624X, 2019.
Andrea von Braun Foundation: Sonderheft Korallenarchitektur. Briefe zur Interdisziplinarität
  • V Vogler
Vogler, V.; Andrea von Braun Foundation: Sonderheft Korallenarchitektur. Briefe zur Interdisziplinarität. Munich, Germany: Oekum Verlag. ISSN 1865-8032, 2018. Software:
Agisoft Photoscan Memory Requirements
  • Llc Agisoft
Agisoft LLC: Agisoft Photoscan Memory Requirements. Retrieved from https://www.agisoft.com/pdf/tips_and_tricks/PhotoScan_Memory_Requirement s.pdf, 2018.
Agisoft PhotoScan User Manual
  • Llc Agisoft
Agisoft LLC: Agisoft PhotoScan User Manual. Professional EditionVersion 1.4. Retrieved from https://www.agisoft.com/pdf/photoscan-pro_1_4_en.pdf, 2018.