308
May 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
breakdown summarizes the main evolutionary stages in the
mapping industry:
1.
The Analogue Photogrammetric Era:
This stage
is considered the cradle of the mapping process
and technology, since it set the foundation for later
advances in photogrammetry and mapping. As the
name implies, this stage employed very primitive
(analogue) technology and procedures that were
highlighted by the use of the analogue stereo plotters
(such as Kelsh and WILD A series), PUG machines,
and scribed or mylar maps.
2.
The Analytical Photogrammetric Era:
Advances
in computer and computing technologies enabled
the replacement of most of the mechanical parts in
an analogue stereo plotter with a computer-driven
photogrammetric math model. Because of this
computer-modeled photogrammetric geometry, the
new stereo plotter, the “analytical stereo plotter”, is
reduced in size and simpler to operate. The analytical
plotter uses a pair of dispositive or film negatives to
form the stereo pair.
3.
The Digital Photogrammetric Era:
This era
is a byproduct of the advances made in the fields
of film scanning and digital camera technologies.
A computational photogrammetric model within
a computer completely replaced all parts of the
analytical plotter. A digital representation of the
image, through a film scanning process or digital
camera imaging, completely replaced the film. The
outcome of this era is a softcopy stereo plotter where
the user views the digital stereo model on a screen
using special stereo viewing glasses.
4.
The Lidar Era:
This era introduced us to a mapping
method that did not follow photogrammetric protocol
or restrictions. It revolutionized the digital terrain
modeling process—mapping with the speed of light.
Like never before, we are able to collect detailed
contours of the terrain with the speed and the
efficiency offered to us by lidar mapping. The digital
terrain modeling during the previous eras was
achieved by collecting sparse mass points (three-
dimensional map features) and break lines. I use
the word “sparse” because a photogrammetry-based
dense collection was economically prohibited, since an
operator, using a stereo plotter, manually collected
those mass points.
When first introduced to the market during the
early 1990s, lidar (linear mode lidar) faced tremendous
operational challenges, as well as, productivity and quality
limitations. These challenges resulted in a decade or
more of mistrust and slow acceptance of the technology
by the mapping community. Tremendous innovations by
manufacturers over the last two decades have resulted in a
reliable, accurate, and very efficient 3D laser-based mapping
technology that is well trusted and used today.
New Developments: Geiger Mode
and Single Photon Lidar
Innovations never seem to stop or slow down when it comes
to lidar systems. In the last few years, we started hearing
about new lidar technologies that are slightly different from
linear mode, including Geiger mode, single photon, and
FLASH. These lidar systems share a common principle:
splitting a single laser pulse into multiple sub-pulses to
increase the density of information reflected by the ground
object. Segmented detectors are used to receive the back
scattered sub-pulses, resulting in a denser point cloud. The
concept of segmented detectors is similar to the focal plane
array design of digital cameras. As with digital cameras,
lidar systems based on these detectors are scalable. It gets
larger, i.e. more pixels, as the manufacturing process of the
semiconductor wafers advances with time. FLASH lidar will
not be discussed in this article as it is not, at least for the
time being, marketed for wide area data acquisition.
Although they are somewhat different, Geiger mode and
single photon lidar share a common characteristic. They
both use single photon sensitive detectors. Unlike linear
mode lidar that requires a flux of 500 to 1,000 photons to
detect the returned signal, Geiger mode and single photon
lidar need only a few photons to detect the returned signal.
This makes it possible to design lidar systems based on a
lower-power laser that can be flown from higher altitudes.
“The new Geiger mode and single
photon lidar offer a better use of
the photons generated by the laser
source, resulting in a denser point
cloud from the same or a less efficient
laser source.”
