PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
September 2017
599
Question: Although today’s lidar data is more accurate
and denser than previously achieved, and therefore
more accurate and reliable in representing the
topography of the land, we have some clients who
are generating or request we generate contours from
dense lidar datasets for different reasons and uses.
We always strive to provide our clients with sound
approaches that help them make best use of their
budgets. Considering the current state of technology,
what is your opinion on the following concepts?
1. The idea of generating contours in the presence
of a dense and accurate lidar dataset;
2. The need for breaklines to augment the lidar
dataset before generating the contours;
3. Lidar-derived contours are more accurate than the
contours generated from traditional photogram-
metric practices;
4. With U.S. Geological Survey (USGS) Quality Level 2
(QL2) lidar data, is it safe to say, “The contour lines
are a visual representation of the lidar data” when
we deliver the contours to clients?
Kent Park, Woolpert Inc., Bluffton, IN
Dr. Abdullah:
Thank you, Kent, for bringing this very important
topic to my attention. The discussion of generating contours
from lidar data and whether this process requires breaklines
is haunting the industry, and there has not been clear action
to settle it. In the hope to inform users and data providers on
this important topic and to remove myths surrounding it, I
will try to shed some light on the concept of contour gener-
ation from lidar and the role of breaklines. The best way to
Breaklines for Lidar Data, Do We Really Need Them?
start discussing this topic is to provide the history and evo-
lutionary stages that terrain modeling has experienced over
the years. Without understanding the history, it is difficult to
devise a way forward.
C
ontours
A contour line is a line connecting places of equal elevation.
Using contour lines is the most common way of numerically
representing land elevation. In the 18th and 19th centuries,
land-surveying practices did not allow for the recording of re-
lief across the land, as it was not practical. Only a limited
number of spot elevations at survey points were surveyed.
That was the practice until U.S. Geological Survey (USGS)
topographical survey maps included contour representation
of terrain relief. After reflecting on the period that contours
were introduced to represent the topography, experts con-
cluded paper maps and other hard surfaces were the only me-
dia to document and represent land relief. Even contours gen-
erated from photogrammetric methods were scribed on scribe
sheet before being printed or reproduced on paper. A scribe
sheet is made of a stable plastic base material, coated with
a material de-
signed for easy
removal
us-
ing a scribing
tool to produce
a cleanly cut
line. Therefore,
the
concept
of contours to
represent
to-
pography was
Key takeaway about contours:
(1) Contours were the only
method available to model terrain
elevation numerically on paper
media or paper maps. (2) Contours
are unnecessary and it should not
be used to define a topographic
surface in digital form.
“contours should have no place in
today’s digital mapping environment”
Photogrammetric Engineering & Remote Sensing
Vol. 83, No. 9, September 2017, pp. 599–602.
0099-1112/17/599–602
© 2017 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.83.9.599
