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September 2017
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
software can handle it more efficiently. In most cases, full
density of the MMS data is used to extract breaklines after
which the data is decimated to a 3-foot grid and delivered with
the breaklines to be used within the modeling/CAD software.
It is worth mentioning here that as computing power and the
ability to store and readily access “massive” amounts of data
advance, the reliance on breaklines in Road Design and Engi-
neering will also diminish to the point of elimination.
2.
Drainage Lines in Vegetated Areas:
Depending on
the density of vegetation, lidar point density in vegetat-
ed areas may be reduced considerably. This results in
inaccurate drainage modeling if lidar points alone are
used to determine the centerlines and boundaries of wa-
ter bodies. Therefore breaklines, whether from imagery
or intensity images, must be compiled to define these
obscured or partially obscured hydro features.
3.
Vertical Walls:
Positioning tall, free-standing walls
such as sound barriers or retaining walls around bridg-
es approaches.
4.
Other:
There may be additional situations that require
the compilation of breaklines.
Finally, if the applications or the end user require breaklines
to augment lidar data and if the accuracy requirement al-
lows, there is a less expensive and more affordable approach
of extracting contours and that is automated contours. For
these type of contours, lidar points cloud and lidar intensity
and/or existing imagery are used through image segmenta-
tion techniques to extract edges that can be used as break-
lines. The process is not perfect yet but we, as an industry,
need to pay more attention to this topic in order to advance
the technique. Woolpert’s latest internal R&D efforts have re-
sulted in a promising quality of breaklines, which also can be
utilized to delineate wetlands, vegetation boundaries, roads
edges and building boundaries for a variety of applications.
Q
uestion
3—I
s
the
lidar
-
derived
contour
more
ac
-
curate
than
the
contours
generated
from
traditional
photogrammetric
practices
?
When contours are generated from elevation posts spaced ev-
ery 35 or 71 centimeters, the maximum length of the triangular
sides of the generated TIN always will be under 100 centime-
ters. This is not the case with the photogrammetric-derived con-
tours modeled from DTM with triangular sides exceeding tens
of feet in length. For that reason, contours generated from lidar
make a better representation of the terrain than the contours
generated from DTM. They are more accurate. For the same
reason, contours generated from a lidar dataset appear jagged
because lidar is more sensitive to the changes in terrain. Some-
times, I describe lidar-based contours as “hypersensitive” due to
the wealth of elevation details they carry.
“with lidar we are able to survey a
terrain elevation every 71 centimeters
or even 35 centimeters” contours
generated from lidar do not look
appealing. However, such contours
are more accurate in representing
the terrain than the photogrammetric
contours”
Q
uestion
4—W
ith
USGS QL2
lidar
data
,
is
it
safe
to
state
, “T
he
contour
lines
are
a
visual
representation
of
the
lidar data
,”
when we deliver the contours to clients
?
If you mean that the contours will have the accuracy of the li-
dar data, then the statement is accurate. Contours generated
from QL2 lidar data are derived from a TIN with triangular
sides under 100 centimeters in length. Therefore, slight in-
terpolation is performed to generate such contours, and these
contours will reflect nearly the actual elevation of the lidar
points cloud at the location where these contours are plotted.
However, contours in general do not represent the lidar data
quality, as the lidar points cloud provides better details about
the relief than the contours alone, unless the contours are
created with a 5- to 10-centimeter contour interval.
I hope I answered your questions. I also hope that this article
benefits many users and data providers who have to follow
widely misunderstood specifications about terrain modeling
when using lidar data. My humble request from users, espe-
cially government agencies who are contracting statewide and
countywide lidar projects, is to investigate end users’ needs for
contours before routinely asking for it. If you have a dire need
for contours, then please do not specify breaklines as a routine
requirement unless there is a good reason for it. Asking for
breaklines, unless generated through automated means, with-
out a good reason does nothing but increase the project bud-
get and delay the final product delivery. Eliminating contours
in general and breaklines in particular from any contract is a
good practice for the fiscally cautious individual or agency. Even
if breaklines are needed to model some features, limiting the
number of features that need to be modeled with breaklines can
help the project budget and expedite product delivery.
**Dr. Abdullah is Senior Geospatial Scientist and Associate at
Woolpert, Inc. He is ASPRS fellow and the 2010 recipient of the
ASPRS Photogrammetric Fairchild Award..
The contents of this column reflect the views of the author,
who is responsible for the facts and accuracy of the data pre-
sented herein. The contents do not necessarily reflect the offi-
cial views or policies of the American Society for Photogram-
metry and Remote Sensing and/or Woolpert, Inc.
