2. IMPLEMENTACIÓN DEL PROTOTIPO
2.3. PROTOTIPO FINAL
2.3.3. MOVIMIENTO ELEVACIÓN Y DESCENSO DEL MARCADOR
Future endeavors using the SR should involve modifications of the SR and the experimental setup to allow the use of the altimeter included in the instrument, which should allow
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measurement of the vertical component of velocity. Also useful will be robust video analysis of experimental rockfalls to determine translational velocities and trajectory bounce heights, which will allow full calibration of rockfall models using translational velocity, rotational velocity, and bounce height.
With high-resolution surface data available in the form of photogrammetry models or the sub-meter resolution DEMs expected for much of NH in the next five years (Carswell, 2015), 3D rockfall modeling may become possible for rock cuts. The SR could be used to ensure accurate models for dangerous slopes. Further investigation of slope roughness parameters and coefficients of restitution could bring even lower-resolution data into agreement with measured SR data.
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APPENDIX A. CREATING A POINT CLOUD USING STRUCTURE FROM MOTION FREEWARE
The following document detailing a methodology for creating a three dimensional point cloud using structure-from-motion techniques was written for the New Hampshire Department of Transportation, Bureau of Materials and Research, during an internship in January, 2018. It is used here with permission.
Acknowledgements
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Original Photo
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CREATING A POINT CLOUD USING STRUCTURE FROM MOTION FREEWARE Introduction
The goal of this document is to describe methods for creating and georeferencing a point cloud of an object or area of interest using freely available software or software available to the New Hampshire Department of Transportation (NHDOT). This is written assuming the application is digitizing a rock cut exposure for structural analysis and change detection. However, the
methods described may be applied to any three dimensional object for which pictures can be obtained.
Structure from Motion (SfM) is a photogrammetry technique that uses overlapping 2D pictures to model 3D objects. There are many references discussing SfM, including the documentation of the software used below, related references, online blogs, and published literature. Johnson and Salisbury (2015) gives a good introduction to SfM. References used to create this document are presented at the end.
Software
Listed below are the software packages discussed in this document, their sources, and notes on installation and use. Wikipedia lists many more photogrammetry packages, both licensed and free, here: https://en.wikipedia.org/wiki/Comparison_of_photogrammetry_software
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Software Website Installation Notes Free?
VisualSFM http://ccwu.me/vs
fm
Download from website. Standalone executable needs no installation. Requires PMVS/CMVS
file cmvs.exe, pmvs2.exe,
and genOption.exe placed in the same folder as VisualSFM.exe for dense reconstruction. Very easy to use. Matches photos and generates a point cloud. Yes PMVS/CMVS http://www.di.ens. fr/cmvs/
Download from website. Three executable files must be placed in the same folder as the accompanying SfM software. Files
needed: cmvs.exe,
pmvs2.exe, genOption.exe.
Used for dense point cloud reconstruction. DOT website filters may restrict access to the download website. Yes SfM_Georef http://www.lancas ter.ac.uk/staff/jam esm/software/sfm _georef.htm
Download from website, along with required Matlab runtime library.
Sfm_georef.exe requires no install; Matlab runtime library requires admin privileges for installation. See website for required runtime version. Used for georeferencing SfM data. Yes CloudCompare http://cloudcompa re.org/ Help/Documentati on: http://www.cloud compare.org/doc/ wiki/index.php?tit le=Main_Page
Download from website; .exe file requires admin privileges for installation.
Used for georeferencing , analyzing, and comparing point clouds. Yes SplitFX https://www.splite ng.com/products/s plit-fx-software/
Download requires admin privileges for installation. License code or dongle required to run the software.
Used for analyzing point clouds.
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Software References
The creators of the freeware software packages request that the following citations be referenced in publications when they are used.
VisualSFM
Changchang Wu, 2013. "Towards Linear-time Incremental Structure From Motion", 3DV http://ccwu.me/vsfm/
Changchang Wu, 2011."VisualSFM: A Visual Structure from Motion System", http://ccwu.me/vsfm/
Changchang Wu, 2011. Sameer Agarwal, Brian Curless, and Steven M. Seitz, "Multicore Bundle Adjustment", CVPR
Changchang Wu, 2007. "SiftGPU: A GPU implementation of Scale Invariant Feature Transform (SIFT)", http://cs.unc.edu/~ccwu/siftgpu
Furukawa, Y., Curless, B., Seitz, S. M., & Szeliski, R. (2010). Towards internet-scale multi-view
stereo. In Proceedings of the IEEE Computer Society Conference on Computer Vision and
Pattern Recognition. (pp. 1434-1441). [5539802] DOI: 10.1109/CVPR.2010.5539802
Furukawa, Y., Ponce, J. (2010). Accurate, dense, and robust multiview stereopsis. IEEE
Transactions on Pattern Analysis and Machine Intelligence, 32(8), 1434-1441. DOI: 10.1109/CVPR.2010.5539802
CloudCompare
CloudCompare (version 2.X) [GPL software]. (YYYY). Retrieved from http://www.cloudcompare.org/
(replace X and YYYY with the appropriate version).
For the “Facets” plugin in CloudCompare (automatic modeling of planar surfaces): Dewez, T. J. B., Girardeau-Montaut, D., Allanic, C., and Rohmer, J.: FACETS : A CLOUDCOMPARE PLUGIN TO EXTRACT GEOLOGICAL PLANES FROM
UNSTRUCTURED 3D POINT CLOUDS, Int. Arch. Photogramm. Remote Sens. Spatial Inf. Sci., XLI-B5, 799-804, doi:10.5194/isprs-archives-XLI-B5-799-2016, 2016.
For the “Compass” tool in CloudCompare (manual picking of planes and traces for structural data):
Thiele, S. T., Grose, L., Samsu, A., Micklethwaite, S., Vollgger, S. A., and Cruden, A. R.: Rapid, semi-automatic fracture and contact mapping for point clouds, images and
geophysical data, Solid Earth Discuss., https://doi.org/10.5194/se-2017-83, in review, 2017
SfM_Georef
James, M. R. and Robson, S. (2012) Automated image-based 3D surface reconstruction for the
geosciences: Accuracy and application, J. Geophys. Res., 117, F03017,
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Photography
All photogrammetry relies on overlapping photos to generate a 3D model. Falkingham (2013), Hesse (2013), and Schoenberger (2017) are three references that discuss requirements and tips
for taking pictures. See References.
Before Taking Photos
Ensure space is available on the camera memory card for a large number of photos: dozens to
hundreds depending on the size of the area/object of interest.
Avoid:
o Times of day that will produce deep shadows on the area of interest
o Large or multiple reflective surfaces (ice, wet surfaces, glass windows)
Large contrasts in light and shadow or large reflective surfaces may create
problems during image matching and model construction.
o Smooth areas and objects. Texture on objects is required to match points between
images; surfaces without texture will not work.
Set up a minimum of 3 targets with known locations that will be easily identified in the
images. These will be your control points. See Tips on choosing control points
o Targets should be spaced out around the area of interest- don’t cluster them.
o Vary X, Y, and Z between targets as much as possible. A linear arrangement of
targets will not provide reference points for all areas of the model, so the model orientation (tilt, yaw, or roll) may be incorrect, even if it is georeferenced to a correct general location.
o Using a target shape with an easily-identified center may make georeferencing easier.
Include several objects of known size or scale when taking pictures for quality control and
scaling.
Measurements to take:
o GPS coordinates, including elevation, of all target locations/control points
Check the measurements before you leave! Do they make sense?
If possible, manually measure the height of the targets above the ground for
later reference. Experience indicates GPS elevations may be inaccurate and their usefulness may be influenced by the units used.
Do not rely on cell phone GPS for locations. Testing shows that cell phone location quality varies widely.
o Dimensions of objects of known size, for scale
o For quality control and model checks:
Orientation of rock cut relative to north and to the road
Height, length, and slope of rock cut
Slope of ground in front of rock cut or dimensions of ditch
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Distance to road
Dimensions of ditch
Ground cover (vegetation and density, gravel, dirt, etc)
Observations of slope material and roughness
Taking Photos
Note the type of camera.
o Maintain the same focal length throughout (ie: use the same camera).
o Wide angles cover more but also add distortion to the images. This may interfere with
model creation.
o Reasonable models with acceptable error have been developed using smart phones.
Ensure GPS location tagging of photos is turned on.
o On an iphone: Settings->Privacy->Location Services (ON)-> Camera.
Use similar lighting for all photos.
Overlap all photos; up to 80 or 90% if possible.
All features must appear in a minimum of 3 images, preferably more.
All control points/targets must be easily visible in at least 3 images, preferably more.
Take a picture or panorama for reference that shows the area as a whole, including the
control points. Annotate this afterwards if necessary to clearly label the locations of each control point.
Take a step or two between photos; don’t remain in the same spot. Take pictures from
multiple angles and multiple distances, instead of a single straight line of photos. The different viewpoints are what allows SfM software to create a 3D model.
Keep the camera as still as possible for each image to minimize blur.
Take pictures that include the ditch/ground in front of the base of the cut. Make sure they
overlap enough with the rock cut to be matched to the rest of the model.
Too few pictures or not enough overlap between images will create gaps in the model. More
pictures are better than too few, but processing time will increase with the number of pictures.
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Ground control points, or feature with known locations, are required in order to put a point cloud in a real-world frame of reference. Any combination of targets/control points for which the