29-08-2014, 01:45 PM
LiDAR
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Abstract
LiDAR Technology is being used to develop various products such as DEM’s and
Contours. The objective of this paper is to compare LiDAR to Photogrammetry using a
digital camera. This comparison revealed that the LiDAR bare earth points were within a
reasonable accuracy from the stereo models developed using an aerial triangulation
solution, but the surfaces and contours generated using the LiDAR dataset were not
depicting accurately the terrain mainly because of the lack of breaklines. The
combination of the photogrammetric breaklines and LiDAR helped improve the final
contours and surfaces although it introduced some noise because of the differences
between the LiDAR points and the stereomodels.
- Introduction:
The use of the Light Detection and Ranging (LiDAR) is increasing worldwide day after
day. Since its introduction to the market over a decade ago, LiDAR has been widely used
by a variety of users belonging to different industries. The LiDAR technology offers the
development of various products such as Digital Elevation Models (DEM’s), Digital
Surface Models (DSM’s), and Limited Planimetric Features. The advantages of the
LiDAR technology can be summarized in the speed at which DEM’s are derived, the fact
that LiDAR can be acquired at night time and finally the fact that LiDAR can penetrate
foliage to reach the ground. This paper aims at comparing surfaces derived by
Photogrammetry and LiDAR in an effort to better understand how they can both be
combined.
2- LiDAR: Description of Technology
A LiDAR sensor is an active laser remote sensing sensor which utilizes a laser beam to
measure the distance from the sensor to an object. When loaded onboard an aircraft (In
this case it may be also be referred to as ALTM), the laser beam departs the aircraft and
intersects with objects on and/or above the ground. The timer determines the time that it
takes every pulse to leave the sensor, intersect with the object and then return back to the
sensor. LiDAR systems allow for multiple returns, which means that the same pulse can
be reflected more than once (five returns usually). The ABGPS and IMU allow the
computation of the location and orientation of the aircraft while acquiring the data.
Because the pulses travel at the speed of light, the precise coordinates of the various
returning objects are determined. Additionally, the intensity data can also be retrieved
from the LiDAR dataset. The intensity is simply the difference in energy between the
initial pulse and the reflected one. The intensity values once displayed provide an image
similar to photography. The LiDAR product is a cloud of points which require further
processing (Often referred to as filtering) to separate between various classes of points
such as Vegetation, Bare Earth DEM’s, Buildings…etc.
Comparing Surfaces
The next step of our study was to compare surfaces derived from bare earth LiDAR and
photogrammetrically compiled DTM consisting of mass points and breaklines. The
surfaces were generated by developing TIN’s and then creating a post spacing grid of 1m
(For both the DTM and the Bare Earth Surface)
Conclusions
A
To summarize the results of this test, we can say that:
• LiDAR is accurate compared to Photogrammetry.
• The terrain may not be accurately represented by LiDAR because of the lack of
breaklines. The contours generated by LiDAR are not accurate around areas
where breaklines are required such as ditches for example. Certain applications
may not require the breaklines (Such as Tree Height Measurement, 3D City
Modeling, Building Height Measurement…etc), therefore LiDAR may be the best
solution because of its advantages presented earlier in this paper.
• Once supplemented with photogrammetrically captured Breaklines, the quality of
the contours does improve but a noise is introduced which needs to be removed.
This combination can generate contours that are comparable to the ones generated
using Photogrammetry techniques. In general, it is not financially attractive to
acquire aerial photography and LiDAR in order to generate contours except in
situations where LiDAR data is required or already exists.