This document summarizes a workflow for creating seamless photomosaic logs of paleoseismic trench exposures using structure-from-motion photogrammetry. The workflow involves 4 main steps: 1) trench excavation and grid preparation, 2) photography of the trench using a DSLR camera on an extendable pole, 3) generating a 3D model from the photos using software, and 4) rectifying and printing the final log. Common challenges like additional excavation and trenches with bends are addressed. Following this detailed and field-tested workflow allows for accurate, high-resolution trench logs to be created efficiently.

1. 7th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 30 May to 3 June, 2016, Crestone, Colorado,
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INQUA Focus Group on Paleoseismology and Active Tectonics
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Seamless Photomosaic Trench Logging Using Trench-Based Photogrammetry Methods:
Workflow and Case-Studies
Gray, Brian (1), Nicholas Graehl (1), Serkan Bozkurt (1), John Baldwin (1), and Kevin Clahan (1)
(1) Lettis Consultants International, Inc., 1981 N. Broadway, Suite 330, Walnut Creek, CA 94596, Email: bgray@lettisci.com
Abstract: Modern photo-logging methods of paleoseismic trench exposures include Structure-from-Motion (SfM) and Image-
Based Modeling (IBM) techniques that reconstruct 3D surfaces to create seamless, high-resolution photomosaics designed for in-
situ trench photo logging. Given the often large expense and time-critical nature of paleoseismic investigations, efficient and
accurate production of seamless photomosaic trench logs is an essential component to modern paleoseismic investigations. We
present our field-tested workflow, based on methods developed by Reitman and Bennet (unpublished USGS report) and Reitman et
al. (2015), that includes photo acquisition techniques, software reconstruction, and post-processing methods. Techniques are
illustrated to demonstrate the workflow on various types of trench excavations in remote field settings using basic equipment. Our
workflow builds on experience using SfM methods in more than 25 paleoseismic trenches in various tectonic settings, trench styles,
and site conditions.
Key words: Structure-from-Motion (SfM), Image-Based-Modeling (IBM), Paleoseismology, Photogrammetry, Photomosaic
Introduction
Over the last few years, the development of Structure-
from-Motion (SfM) and Image-Based-Modeling (IBM)
techniques have revolutionized the production and
improved the quality of photomosaic logs generated for
paleoseismic studies. Rendered obsolete is the need for
perfectly-positioned orthogonal photos taken with the aid
of the “Trench-O-Matic” and gone are the sleepless nights
consumed by painstaking Photoshop mosaicking
sessions. Paleoseismic trenches can now be
photographed in as little as a few tens of minutes and
seamless trench mosaics can be generated in the time it
takes to (casually) consume a couple of end-of-the-day
beverages. However, efficient SfM model generation
requires practical knowledge of photography, image
processing, and the software-rectification development or
the entire process can become cumbersome. With
experience from more than 25 paleoseismic trenches, we
present a field-tested workflow that builds upon the
workflows outlined by Reitman and Bennet (unpublished
USGS report) and Reitman et al. (2015). We specifically aim
to address typical challenges associated with the use of
this relatively new paleoseismic tool.
Because of the added time, expense, and logistical
challenges of purchasing and bringing survey equipment
to remote field sites, the majority of our recent trenches
utilizing SfM-based logs have been set up without
absolute survey control. The methods presented herein
are geared toward log production in the absence of Real-
Time Kinematic (RTK), total station, or Light Detection and
Ranging (LiDAR)-based survey control. Instead we use
local grids generated for the trench exposure, averaged
handheld GPS points for endpoints and corners, and
trench length and orientation to place the trench in an
absolute coordinate system (e.g., UTM Zone 10, WGS 84,).
General Workflow
The goal of any photomosaic trench logging exercise is to
develop an accurate, archive-quality, orthorectified image
of the exposure. Efficient generation of a photomosaic log
requires four basic steps: (1) trench excavation and grid
preparation, (2) photography, (3) model generation, and
(4) final log rectification and printing. In addition, having
the proper equipment has proven critical in a number of
our excavations. The following is a rundown of equipment
we typically have on hand for log production. For SfM
processing, we recommend using a computer with at least
16+ GB of RAM, an Intel i7 processor, and a reasonable
graphics processing unit (GPU) as it may be utilized for
processing power. A powerful desktop computer with a
solid-state hard drive and optimized GPU is ideal (we use
settings outlined in the Agisoft PhotoScan user manual),
but high-powered laptops, especially those with solid-
state hard-drives can be a great alternative if portability is
necessary. For photography, a moderate-to-high-
resolution camera (minimum 12 MP) with semi-manual
settings to manipulate ISO (light sensitivity), aperture,

2. 7th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 30 May to 3 June, 2016, Crestone, Colorado,
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exposure compensation, and white balance is ideal. A
relatively wide, albeit undistorted, viewing angle, wide
aperture (to allow low-light photography), and variable-
angle LCD screen are important features to have in shored
trenches as both space (wall to wall) and light are usually
limited. Mirrored digital cameras (DSLRs) are not
recommended for several reasons. First, DSLR lenses often
have greater out-of-focus portions of the image at wider
apertures (for low light photography) and greater edge
distortion than mirrorless cameras. This will cause major
headaches when the SfM software attempts to pair
images. This is especially true for benched exposures
where multiple planes may be represented in a single
image. Second, they are typicallymore expensive, heavier,
and less expendable than most mirrorless cameras. A
high-quality point and shoot that takes a nearly
undistorted photo with a wide viewing angle is preferred.
Photographing a sheet of graph paper provides a simple
test of lens distortion at various focal lengths. Our Canon
Powershot SX50-HS (shooting RAW) has produced the
most consistent results so far. Additionally, cameras that
perform extensive automated in-camera post processing
(i.e., enhancing color, contrast, and sharpness) may also
produce images that are difficult for the SfM software to
reconstruct. If necessary, post processing should be
completed by the user to avoid altering the images to a
point where the software can no longer match images.
For slot trenches, we have found that using a sturdy
extendible paint pole with an affixed tripod adapter to
attach the camera, coupled with a radio-controlled shutter
switch on the camera, significantly reduces the time
required to photograph the trench (Figure 1). For example,
a recent trench on the Rodgers Creek fault measuring 23
m long by 3.7 m deep took approximately 50 minutes to
photograph one wall with approximately half the trench
requiring a small amount of additional time to manage
shading. Avoiding the hassle of climbing up and down
shores, moving boards, and awkward photography
positions reduced photography time in half compared to
what we anticipated for handheld photography. Logs are
printed in the field on 11x17 photo paper using a Canon
Pixma iX6820 inkjet printer which prints well and is
relatively inexpensive to purchase.
For field processing of SfM logs, a portable power pack or
power inverter connected to a car battery can be helpful
as Agisoft PhotoScan is a power-hungry software that
quickly consumes laptop batteries. Field processing is
especially helpful when time is critical or the exposure
location is remote. A portable power source also facilitates
printing of logs directly in the field.
While often site and fault conditions limit how the trench
is excavated, it is important to keep in the mind that
straight trenches with vertical walls, or at the very least
planar walls, are the quickest and least-problematic
trenches to generate SfM-based photomosaics for.
Trenches that curve and/or have irregular walls will be
considerably more challenging to rectify. This is because
the SfM IBM are exported as orthorectified images
projected onto a single x/y plane. As the wall becomes
progressively more oblique to the projection plane, the
rectified image becomes progressively more distorted.
Large variations in trench azimuth are best managed with
multiple orthophoto exports and subsequent stitching to
complete a log that appears as a single plane. If bends in
the trench are required, for the sake of photomosaic
development, it is best to excavate in linear sections rather
than a continuous curve as each section can be exported
with a single orthophoto defined by a single plane. Like
other steps of the process, our photography method
emphasizes efficiency. The ultimate goal is to obtain the
best log with the least number of acceptable photos as
model processing times increase substantially with large
photo sets or if photos are difficult to align. As noted
above, for slot trenches we use a paint pole and remote to
systematically photograph the trench in top-to-
bottom/bottom-to-top rows, maintaining a roughly 50%
vertical and side-to-side overlap (utilizing the camera’s tilt
Figure 1: Pole photography using a sturdy paint pole with
camera attachment, radio remote, and camera with a
variable-position (upturned) screen.

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screen to track movement). Benched trenches are usually
easier to photograph in rows.
The standard rule is to achieve adequate coverage without
overdoing it, as dealing with too many photographs
consumes valuable time. Conversely, taking too few
photos will result in gaps in the mosaic that may require a
significant effort to fill/repair. To maintain color and
exposure consistency, we lock the ISO sensitivity, white
balance, and aperture before starting the photography
session. As in Reitman et al. (2015), we typically take an
additional row of photos at the top and bottom of the
trench for completeness. Fault zones are often
photographed a second time as a way to provide
additional photos in case the first set does not have
adequate coverage. This additional photo set generally
does not increase processing time because it only used if
there are gaps in the first set. Lighting should be carefully
controlled to avoid bright spots or sunlight on the camera
lens as this may cause problems during SfM processing.
Photographing the exposure when shaded or cloudy is
best and avoids high contrast exposures. We typically try
to avoid taking overly oblique photos or photos that of
anything outside the trench where possible as extraneous
portions of photos will need to be cropped during
processing. Additionally, it is important to keep loose
objects stationary and to avoid change from one image to
the other. Notably, flagging with important sample or
kinematic labels should be nailed to the wall if wind is
present.
Model processing and export closely follow the methods
outlined by Reitman and Bennett (2015) with a few
exceptions noted here. We are meticulous about masking
out extraneous portions of images such as excavation
spoils, trees behind the trench, etc. In PhotoScan, these
masked-out sections are excluded using the “constrain
features by mask” toggle in the “Align Photos” window.
This can dramatically improve processing times and has
saved a number of our initially failed mosaics. To export
orthophotos, we use the native window and define the
export plane based on markers, using those pre-
determined for the x and y axes. This makes the export
orientation reproducible in case complete updates are
necessary. For benched trenches, we commonly export
one log normal to the vertical risers and a second log
(typically the opposite wall) at a slightly oblique angle that
results in a nice 3-dimensional base that can still be used
as a trench log (Figure 2). The export file type will default
to TIF format which results in images that are often times
hundreds of megabytes in size. We opt for JPEG or PNG
images instead and have had no issues with either.
The final steps to completing the mosaic are rectification
of the log and export to a printable format. We have
worked with Illustrator, Photoshop, and ArcMap to
complete these final tasks and ArcMap is by far the most
accurate and efficient. As in Reitman et al. (2015), we use
the georeferencing tool to add control points based on
grid nail coordinates. However, we use the “spline”
transformation method which allows all points to snap
directly to their respective grid coordinates. This
transformation method is only active once a minimum of
10 points are entered. The ability to rectify the image to
the exact coordinates or markers also allows for easy and
seamless stitching of subsequent images. Once complete,
the desired grid (typically 0.5 or 1.0 m) is overlain on the
rectified mosaic. Final log export is accomplished with an
Figure 2: SfM export options for benched paleoseismic investigations. (a).Standard export orientation using grid markers for a pre-
defined projection plane that is normal to the trench wall. (b). Optional oblique view of the benched exposure, provides additional
context for contacts crossing benches or oblique walls, orthophoto typically exported using an abitrary projection plane.

4. 7th International INQUA Meeting on Paleoseismology, Active Tectonics and Archeoseismology (PATA), 30 May to 3 June, 2016, Crestone, Colorado,
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11x17 map document scaled to the desired output
resolution.
Troubleshooting Common Issues
The following section addresses common photomosaic
log production issues that regularly result in significant
delays and lost field time. These issues include patching in
trench updates (after shore moves, additional excavation,
and re-flagging), log production in trenches with bends,
and lengthy model processing times.
Additional photography and model updates are often
necessary during the course of a paleoseismic
investigation. Reasons may include further excavation,
removal of shores in a fault zone, and significant updates
to interpretations and flagging. Where additional
photography is necessary, care should be taken to
minimize disturbance of the adjacent sections and/or
overlapping tie points. The updated section should be
photographed with significant overlap of the original
undisturbed sections. Without proper overlap, seamlessly
incorporating the new log can be problematic. Care
should also be taken to mimic the original lighting
conditions as best as possible using proper shading and
photographing at the same time of day. Differences due
to cloud cover and soil moisture and largely unavoidable
but can be managed with Photoshop adjustments. Once
the new photo set has been processed and exported, we
incorporate the image directly into the ArcMap
rectification document using the “spline” transformation
method outlined above. Alternatively, the new log
sections can be merged using Photoshop or directly into
the existing SfM model, although these alternative
methods are typically more time consuming and result in
visible seams.
As discussed in the workflow section above, trenches with
bends present specific challenges with respect to log
production. As each orthorectified log is exported from
the SfM model using a given plane normal to the
exposure, trenches with walls of variable azimuth will; (a)
have sections that are exported oblique to that plane
(requiring significant stretching), (b) be logged in
individual sections, or (c) be presented as one log
comprised of multiple stitched orthophotos. Multi-
azimuthal trench walls intended as one exposure will use
the latter method. Each section is exported using a given
set of x/y markers defining the specific export plane.
Planning ahead here is helpful because determining
which grid markers will be used to export each plane is
more easily accomplished in the field rather than back at
the office. The resulting orthophotos are cropped to
remove the unwanted sections and then rectified and
merged in ArcMap using the spline interpolation (Figure
3). If done correctly, the resulting log will be seamless and
distortion from sections oblique to the orthophoto
projection planes will be mitigated.
Conclusions
Paleoseismic exposures can be quickly, accurately, and
seamlessly reconstructed in exceptional detail with
careful planning and anticipation of common log
production issues. For even the most challenging of
paleoseismic excavations, SfM-based photomosaics can
be developed in a fraction of the time of traditional
Photoshop-based methods.
Acknowledgements:
The authors would like to thank additional LCI staff members
Christopher Bloszies, Jereme Chandler, Brian Greer, Alex Remar,
and Patrick Williams who have helped streamline every step of
the process, and Chris Milliner for passing on his knowledge
during LCI’s first big run of PhotoScan logs.
References
Agisoft PhotoScan v. 1.2.4 [computer software]. (2016), accessed
via http://www.agisoft.com/downloads/
Reitman, N. and S. E. K. Bennett, Structure from Motion (SfM)
photo mosaic workflow using Agisoft PhotoScan Professional,
unpublished white paper, USGS Golden, CO, 8 p.
Reitman, N., S. E. K. Bennett, R. D. Gold, R. W. Briggs, and C. B.
DuRoss, (2015), High-resolution trench photomosaics from
image-based modeling: workflow and error analysis, Bull.
Seismo. Soc. Am., Vol. 105, No. 5
Figure 3: Working with bends in the exposure: (a) Map view
of slotted trench with prominent bend. (b) Rectified mosaic
created from two merged sections exported normal to the
exposure, oblique distortion is eliminated and the seam is
largely undetectable.