This unit introduces students to Structure from Motion (SfM). SfM is a photogrammetric technique that uses overlapping images to construct a 3D model of the scene and has widespread research applications in geodesy, geomorphology, structural geology, and other subfields of geology. SfM can be collected from a hand-held camera or an airborne platform such as an aircraft, tethered balloon, kite, or UAS (unmanned aerial system). After an introduction to the basics of SfM, students will design and conduct their own survey of a geologic feature, followed by an optional (but highly encouraged) introductory exploration of SfM data after returning from the field.
This unit begins in a classroom setting, with a lecture presentation followed by the distribution of the SfM manuals and assignment packets. The lecture includes background on why geodetic surveying techniques are used in the earth sciences, and how SfM works. Similar material is provided in the manual. This segment should be brief (thirty minutes to one hour) to allow time for SfM surveying in the field. If possible, have another instructor photograph a small portion of the classroom while giving the first lecture so students can view the data. Roughly fifteen photos is ideal, as this number allows for quick data processing to generate a model of part of the room. These photos may be collected and processed prior to the lecture, to ensure the model is done processing. Take a break between the two presentations to show the students the equipment and have them practice setting up targets to break up the lecture time. Inventory the equipment to ensure that all necessary components are present; while this is happening, students may fill out the blank equipment list in their assignment packets. Students should bring all typical field equipment with them (writing utensils, straight edge/ruler, field notebook, AND calculator*).
Solution Manual Atmospheric Science An Introductory Survey 1
The field data collection portion of an SfM research project is a small component of the complete workflow. This step can be completed by individuals, pairs, or groups of students, as well as completed by an instructor with no student presence. As soon as field data collection has finished, use SfM software (the student exercise shows how to use Agisoft, but many other open-source options are available; see SfM Guide) to generate the 3D model of the field site. This could take eight to twelve hours, depending on the number and resolution of photos used as input. Make sure to georeference the model no matter the chosen workflow; this is necessary in order to measure features within the model. It is possible to load camera GPS data into Agisoft to speed up the model-generation process, specifically the first step of aligning photos. However, this data should be supplemented with ground control points from the GPS setup prior to surveying to achieve TLS-level accuracy.
In many cases it is very beneficial to use high-precision GPS to measure specific locations of ground control points. This definitely helps to generate a more accurate 3D model, comparable to the results produced by TLS. However, if GPS is not available, the GPS locations associated with each photo will work well enough to produce a less-accurate model (possibly with more internal distortion). In addition, you may use scale bars to integrate scale into your model. The value in investing effort in GPS alongside SfM depends on the geoscience application for which you are doing the survey and especially whether it depends on being tied to a global reference frame. Another module in the GETSI Field Collection covers field GPS - High Precision Positioning with Static and Kinematic GPS/GNSS.
If teaching this unit/module in a classroom setting or on campus, try to choose an object/feature to survey that is similar to a geologic outcrop or feature. It is quite difficult to accurately survey statues, trees, or any other feature that requires 360 of data, and these should not be used as a feature in the introductory unit.
A worksheet has been provided for students to calculate the area of their photos to determine the number of photos needed for a survey. The worksheet requires some knowledge about the camera used: the focal length, sensor dimensions (width and height), the aspect ratio of the photographs, and the effective megapixels of the camera. These variables may not be listed in the manual for the camera, but can be easily found using an Internet search.
Much of the formative assessment can be done through observations of and discussions with students individually, in pairs, or periodically in the whole group. Students can also hand in their work from the field, including field notes (atmospheric conditions, metadata) and sketch of survey setup. The work for formative assessment could be graded based on completion. 2ff7e9595c
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