600
September 2017
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
only developed for paper/hard surface maps. Decades passed
before we transitioned to soft copy, digital modeling and rep-
resentation of land topography. Accordingly, contours should
have no place in today’s digital mapping environment.
B
reaklines
Put simply, a breakline is a three-dimensional line in a digital
mapping environment used to represent a sudden or abrupt
change or break in the terrain’s smoothness. The need for
breaklines emerged to help the labor-intensive map-making
process using photogrammetric stereo-compilation. It is not
economically feasible to trace the ground and generate height
information for every single square foot of the land through the
3D digitization process. The industry devised an approach that
helped the map-making process and made it more affordable
for users. During the early stages of digital map making, ter-
rain topographies were modeled by pulling contours in which
the operator fixes the floating mark at certain contour eleva-
tions and manually digitizes the terrain where the elevation
is equal to the set contour elevation. This process is similar to
what the industry used for decades prior to the digital era of
map making. However, instead of scribing the contours on the
scribe sheet, as described earlier, the contour lines are formed
digitally within a digital graphic or CAD file. Later, experts
largely replaced this process of modeling the topography by
direct drawing of contours with the new practice of generating
what some call a Digital Terrain Model (DTM). DTMs usual-
ly contain sparse 3D points, sometimes called “mass points,”
to represent terrain elevation in flat and rolling terrain, and
breaklines to represent abrupt changes in the terrain. The
mass points and vertices of the breaklines are then later con-
verted to a Triangular Irregular Network (TIN) from which
contours and other terrain models are derived in a digital envi-
ronment. The density and frequency of mass points were in no
way close to the density of today’s lidar points cloud. The densi-
ty of compiled mass points depends on map scale and the relief
in the terrain.
Collecting mass
points with post
spacing of 50
feet to 150 feet
was and still is
a common prac-
tice when mod-
eling the ter-
rain using stereo photogrammetry. Modeling the terrain with
a denser mass point becomes costly, which makes the case for
introducing the concept of breaklines. The breakline concept
was introduced to avoid modeling the entire project with dense
mass points, as it was a manual, labor-intensive process. With
coarse post spacing of mass points and with the absence of
breaklines, terrain modeling is inaccurate, as it fails to repre-
sent the true shape of the terrain.
L
idar
D
ataset
With the familiarity of today’s lidar technologies, I will forgo
the lidar introduction. When we started using lidar in the mid-
1990s, lidar technology was in its infancy, and pulse repetition
rate, which contributed to the points cloud density, was very
low. Collecting a lidar-based points cloud with a post spacing of
5 meters was a normal practice at the time. A few years later, as
lidar technology matured, we transitioned to 2-meter post spac-
ing. This evolved to where we are today—collecting USGS QL2
lidar data with a density of 2 points per square meter and a post
spacing of around 0.71 meter (or 2.3 feet), which is the normal
practice. Considering the latest advances in lidar technologies,
soon Quality Level 1 (QL1) lidar data with a density of 8 points
per square meter and a post spacing of around 0.35 meters (or
1.15 feet) will be the norm within the industry. I would like the
reader to ponder the fact that a lidar dataset with a post spac-
ing of 71 cen-
t i m e t e r s
or even 35
centimeters
is replacing
old
photo-
grammetric
methods of
modeling the
terrain with
post spacing of 15.2 meters (50 feet) or coarser. This means,
with lidar we are able to survey a terrain elevation every 71 cen-
timeters or even 35 centimeters. That is something cost prohib-
itive even when we perform land-surveying techniques to model
the terrain. Field survey for road construction is performed with
profiles or cross sections every 30 feet or so, and stockpile volu-
metric computations are done by surveying minimal spot eleva-
tion along the toes and ridges of the pile.
Now that I’ve provided some history and background, let us
discuss Kent’s questions. I will address the concepts proposed
in the same order as Kent’s submission:
Key takeaway about breaklines:
Breaklines were introduced to
more accurately capture changes
in terrain in the presence of
coarse mass points, or what we
refer to today as points cloud.
Key takeaway about lidar dataset:
Lidar points cloud is the most
comprehensive method of modeling
the terrain. It provides an elevation
post down to every foot of the
terrain.
“lidar points cloud is the most
comprehensive method of modeling
the terrain. It provides an elevation
post down to every foot of the terrain”
“The breakline concept was
introduced to avoid modeling the
entire project with dense mass points,
as it was a manual, labor-intensive
process”
