Field Studies - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 176

Fig. 16.3 The second author,
with a precision GPS (antenna
sticking vertically out of
backpack). Some ripples are
visible at left. Note the gloves—it
is in fact quite cold. Great Sand
Dunes National Park and
Preserve. Photo by Cheryl
Zimbelman

when photography required glass plates). Much of Bag-
nold's field work relied on photography as a means of data
acquisition—not only of dunes, but also of the paths of
saltating grains, and the instantaneous measurement of wind
pressures by taking photography of manometer columns.
We have recently seen a new revolution in imaging—the
digital one. Storage of data in image form is, of course,
made vastly easier by modern digital cameras, which are
not only inexpensive but can store thousands of images or
hours of video on small memory cards. The marginal cost of
acquiring an image, both in time (point-and-shoot cameras
take only a couple of seconds to pull out of a pocket) and in
media costs is so close to zero that the field researcher can
essentially take an unlimited number of pictures. Modern
image processing tools allow images to be compared, or
mosaiced together, or features measured precisely.
That said, taking some care can be rewarded in the field.
Near-overhead illumination tends to wash out topographic
shading but may emphasize any compositional sorting (like
patches of red spherules in the Rub' Al Khali, or black
magnetite-rich patches in Death Valley) whereas sensuous
shadows develop near dawn and dusk, emphasizing the
rounded curves of a dune.
Dunefields often lack an intrinsic scale, so inclusion of
reference objects for scale is useful—a backpack, a col-
league or a vehicle. For studying ripple topography, a handy
trick is to mount a horizontal edge like a ruler just above the
ripple pattern, and measure the shape of the shadow of the
edge cast onto the ripple surface.

It is often useful to structure the illumination of a scene,
either in space or time or both, in order to study saltation,
although this is more usually done in the laboratory than in the
field. A long exposure of a closeup of a saltating dune may just
show a blur where the sand is in motion, yet a flash of suitable
length will show the streaks of individual sand grains.
A new avenue for photography in aeolian studies is the use
of photogrammetry to construct 3-dimensional models of a
natural surface; software packages are now available via the
internet (sometimes for no charge) where a series of individual
digital photographs can generate surprisingly robust 3D
models, as long as the surface photographed includes suffi-
cient textural information to allow the software to identify
abundant tie points. As an example, a series of nine photo-
graphs taken while walking around a megaripple field near
Grand Falls in central Arizona was able to result in a good 3D
model of the ripple (Fig. 16.4 ) when submitted to the 123D
Catch web site (Autodesk 2012); no geometric information
was required to be logged in the field (other than including
something for scale in the photographs), since the software
automatically reconstructs all camera positions while gener-
ating the surface model. Utility of this new photogrammetric
technique, however, may be restricted to sites where the
photographed surface has either sufficient textural or albedo
diversity to provide abundant tie point opportunities, which is
not always the case on many natural dune surfaces.
Another new capability is afforded by inexpensive digital
timelapse cameras that can take thousands of images over a
period from a few hours to many months. These may be