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Today, tools available to the field geolo- The app, currently available for iOS only, The EF tool provides rapid measurement
gist are much the same as they were a cen- comprises two principal tools: edge fabric of preferred shape orientation of grains
tury ago: devices for measuring angles, (EF), for measuring and quantifying pre- and can pick up fabrics too subtle to detect
bearings, and distances, and a few categori- ferred orientation, and color index, for by eye alone, such as in Figure 1B. The EF
cal measurement aids such as an acid bottle determining the percentage of dark miner- tool works by examining the orientations
and a magnet. Although mobile phones and als. The color index (CI) tool can be used to of grayscale gradients, which are typically
laser rangefinders are replacing the com- estimate the abundance of any component particle edges, in the image. To illustrate,
passes and tape measures of yore, the that can be separated from others based on Figure 2A shows a 50-by-50 pixel image of
domain of properties that can be measured grayscale, such as light-colored pheno- two gray stripes and a gray dot. At each
is largely the same. Field studies are typi- crysts in a volcanic rock. pixel, there is a direction of maximum
cally the prelude to a comprehensive set of grayscale brightness increase, and its
laboratory measurements of chemical com- THE EDGE FABRIC TOOL length corresponds to the sharpness of the
position, porosity, mineral age, mineral or gradient. These vectors are shown in
clast preferred orientation, remanent mag- Edge Fabric Figure 2B; blank areas are where the
netism, and other useful things. Such labo- Measuring fabrics such as bedding, folia- brightness does not vary, and the vectors
ratory studies could be significantly en- tion, and lineation is a large part of geologic have length zero. Most of the long bright-
hanced if some of these properties could be fieldwork in structural geology, petrology, ness vectors point away from the centers of
measured in the field. If such measurements and sedimentology. The resulting data the stripes, and a smaller number point at
could become routine and ubiquitous, then (strike and dip or trend and plunge) are various other directions. In Figure 2C, all
field studies would produce richer results. quantitative and easily digitized. However, of the vectors from B have been translated
It is possible to bring devices into the a quantitative assessment of the strength of to the origin and rescaled. There is a clear
field to measure chemical composition, the fabric is difficult with traditional field concentration perpendicular to the trends
magnetic susceptibility, gamma-ray emis- tools, and weak fabrics are difficult to of the stripes, which defines the orienta-
sions, rock hardness, and other rock prop- reproducibly measure and quantify if they tion of the long axis of the edge fabric
erties. These tools are valuable for mapping can even be seen at all. ellipse (EFE; Fig. 2D).
subtle variations that may be unmappable
by eye (e.g., Parkinson, 1996; Aydin et al.,
2007; Dühnforth et al., 2010; Coleman et
al., 2012), but they are expensive and not
widely employed. As a result, aside from
orientation measurements, fieldwork is still
done in a mostly qualitative or semiquanti-
tative manner, using phrases such as
“strong fabric,” “coarse-grained,” “dark,”
or “poorly sorted,” rather than quantitative
measures. For structural analysis, several
algorithms have been developed for semi-
automated fabric determination from images
(e.g., Launeau et al., 1990; Ailleres and
Champenois, 1994; Vinta and Srivastava,
2012), but these require processing in the lab.
In this paper, we introduce a mobile app,
StraboTools, which allows rapid field mea-
surement and quantification of three quan-
tities: fabric orientation, fabric strength,
and the percentage of dark or light minerals
in the field of view.

STRABOTOOLS
The StraboTools app provides quantitative
data at the outcrop that are otherwise difficult
or impossible to estimate in the field or that
might be subject to large uncertainty and user-
to-user variation. The app was developed for
work in plutonic rocks such as granite, but it
can be used for fieldwork in any type of rock
and for study of thin sections and aerial photo- Figure 2. (A) An image of gray stripes 50 pixels on a side. (B) Vectors showing magnitude and direction
graphs as well. The analysis uses a photo- of the brightness gradient at each pixel. (C) Vectors in B translated to the origin, showing strong group-
ing perpendicular to dominant edges, along with scaled eigenvectors and ellipse defined by them.
graph taken within the app or imported into it. (D) Edge fabric ellipse (EFE) derived by rotating ellipse in C 90°.
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