Walk-around method: in situ and exhibition
specimens
Specimens that are immobile can only be
photographed in situ by walking around them as
far as local conditions allow. This places
restrictions on the lighting conditions, and may
mean that parts of a specimen cannot be
digitized because the camera cannot be brought
into positions required to photograph them.
Figure 8 shows an example of a mounted
dinosaur skeleton in an exhibit hall populated by
obstacles in the form of other exhibit specimens.
Especially problematic are specimens outdoors,
where one is dependent on weather conditions,
and often has no access to electrical power
available. Try to avoid strong sunlight as it
causes high contrasts with dark shadows (also
true in exhibition spaces with natural light, such
as the AMNH dinosaur halls); use reflectors/flash
to brighten them up. A light but uniform cloud
cover is preferable. Avoid shooting during times
of day where the sun shines at a shallow angle
onto the surface of interest (i.e., for subhorizontal
surfaces prefer shooting during mid
day, for strongly inclined surfaces choose times
accordingly).
When a specimen is subject to changing light
conditions, work rapidly to minimize the
differences and ignore people walking through
view (they can be masked out; take more
photographs to ensure that all parts of the
specimen are sufficiently captured).
Mounted skeletons, long trackways and similar
very large and complex objects can be
calculated in chunks and merged later (see
below). If this approach is chosen, the
photography should be adjusted by shooting
series of pictures of parts, with markers for the
later alignment placed beforehand. Each part
should contain its own scale bar, as large as
possible. It is for example possible to measure
railings or other architecturally defined
distances. You may also use a complete
measuring tape or, otherwise, place a yardstick
(ideally at least 2 m) next to each section of the
specimen in a stable position while you
photograph it. To facilitate correct
photogrammetric calculation of repetitive
pattern such as ripple marks on trackways,
place uniquely colored/shaped objects (e.g.,
clothes, tools, etc.) around the specimen.
Typically, specimens with complex shapes in
exhibition settings suffer from uneven lighting
caused by top-down light or spotlights, causing
strong shadows on the undersides of individual
elements. Ground-mounted spotlights
sometimes ameliorate this effect, but are
normally insufficient to allow straightforward
digitizing. If possible, ask for access outside
normal visiting hours, and for the exhibition
lights to be turned off. Many museums have
“cleaning light” that more evenly lights the
specimen. Use reflectors and additional light
sources, and use a flash if nothing else helps. A
second, hand-held and automatically triggered
flash can also be helpful. If nothing else works,
block spotlights or reflections with your hand
(Figure 3C).
The actual process of photography consists of
moving around the specimen and taking
photographs. As simple as this sounds, there
are a number of pitfalls. First of all, it is
important to retain enough overlap between
images, especially if the view of the specimen is
edge-on. Here, it is even more important than
in the turntable method to shoot more images
than one believes to be necessary. Care must
be taken to not shoot panorama series (in
which the camera is pointed in different
directions but remains stationary), as these
image sets have little to no parallax between
individual images. For huge specimens,
especially those too large to fit entirely in each
photograph, e.g. because it is impossible to
gain enough distance or because a higher
resolution of the model is required, it can help
to place visual markers of used positions on the
ground, e.g. small pieces of paper, so that one
can keep an overview of what has already been
photographed.
Exposure must be checked and adapted for
each photograph, as differences can easily
cause color differences which make feature
matching difficult for the program, degrading or
failing alignment. It may even be necessary to
adjust the white balance between shots. RAW
format photographs, in which settings can be
developed later, can be useful under these
circumstances. Especially outdoors, but also in
indoor places with low-mounted light sources, it
is important to avoid casting shadows on the
specimen.
Also, it is often difficult to capture the top
surfaces of huge specimens sufficiently from
ground level. In this case, arrange for a ladder
and a second person to steady it. Long
telescopic masts, the use of kites or UAVs
(Unmanned Aerial Vehicles; e.g. Watts et al.
2012; see http://www.mikrokopter.de) are
useful while photographing large areas.