AIRBORNE LASER MEASUREMENT TECHNOLOGY IN JAPAN
Takayuki OKUBO, Japan
Key words: GPS, Laser Profiling, Garbomirror
scanner, CG, 3d-modeling.
Abstract
Kinematics GPS and inertia movement instrumentation technology has
developed rapidly, which achieved to provide a high and stable
accuracy of spatial positioning, attitude in three dimensions for
high-speed movement platform such as aircraft.
The airborne laser measurement system which is based on the laser
scanning technology, has been integrated to the GPS inertia movement
instrument on the aircraft platform.
The advantage of airborne laser data is the possibility of
displaying, operating and generating of 3D spatial model in the
computer. Further development is being done to extend to wide variety
of applications in scientific manner.
The applications based on these airborne laser data are used in the
field of mapping, analysis of land deformations, city modeling, flood
control simulation, and CG or virtual realities.
Also, the research and development on spatial operation function
technology for three-dimension spatial model acquired by airborne
laser measurement system, is now applied for the expansion uses of
latest 3D-GIS.
The analysis result of the airborne laser measurement, by using the
electronic ground control points as reference points, which is
established by Geographical Survey Institute, extending over 1000
points in Japan, and the research result concerning the preparation of
the manual for public survey, indicates the vision of the prospect of
the laser measurement in Japan.
1. INTRODUCTION
After the Cold War, military affairs technologies were opened to
private enterprises, which the technologies of laser scanner devices
and the positioning system in high speed movement platform were able
to be developed.
Aero Asahi Corporation (AAC) has owned a fixed wing (Cessna207) and
the helicopter (AS350), employed the airborne laser measurement
system, ALTM 1025 that had been developed by Optech Inc. under our
specifications, which generates a high dense DEM data in 3D.
As a result, AAC was succeeded to expand the range of 3D data
acquisition up to 25cm-30cm, comparing to 1m range in the year 1997.
In the year 2000, the airborne laser measurement system (ALTM1225)
has been integrated to mobilize with the digital CCD color camera,
pixel size 2000x3000, which acquires the image simultaneously. The
result from laser profile system shall be used as an exterior
calibration parameter, to generate orthogonal image that synchronizes
with the DEM data.
Also, the reflection intensity data of each laser spot has
potential uses for distinguishing the difference of materials.
This report describes the outline of the airborne laser profile
system and case studies about applications such as actual land surface
analysis and others.
2. AIRBORNE LASER PROFILING
(1) Airborne subsystem
ALTM1025 had been developed in connection with survey work on
obstacle tree felling ranges to power lines to detect the wire more
than 10mm from a position above 150m and coniferous sharp tree tops
without being left. The system is composed of an airborne measuring
subsystem and a ground post-processing subsystem.
Fig.1 shows the airborne subsystem, which is comprised of sensor
head, computer and recorder racks, GPS receiver and power supply. Some
devices are housed in sensor head such as laser transmitter and
garbomirror scanner, position and orientation system (POS) sensor and
CCD video camera. GPS antenna is laid on the roof. RTK navigation
subsystem is loaded unconnected with ALTM subsystem, which is
comprised of antennas and receivers of GPS and beacon, PC and display.
And also the ground GPS are included for kinematic deferential
processing by carrier phase.
Fig.1 Airborne system
GPS provides a long-term accuracy but sometimes interrupted by
outage, multipath and nose in the short term.
On the other, POS has a good accuracy in the short term but suffers
from a long-term drift. This integrated positioning system provides a
very good accuracy by compensating each other for their weakness.
The garbomirror scanner oscillates in the two directions, flight
path direction and its perpendicular, and generates a parallel
scanning lines by harmonizing with flight speed.
The coordinates and elevation of laser beam reflecting points are
determined by GPS position, POS orientation, scanning angles and
ranges of laser, so that we have to measure the relative position of
each sensors before flight.
Table 1 shows principal specifications of the ALTM 1025.
Table 1. specification
The ground post-processing subsystem comprises such devices as a
data processing PC with 10 GBytes memories; data analyzing PC, data
processing of the ground post-processing is shown in Fig.2.
Fig.2. Logical sequence of post-processing
The ground post-processing subsystem has an excellent software that
provides a DEM without trees, buildings and other constructions. The
prescribed laser points software is based on J. Lindenburger (1993),
which is to eliminate the data of man made structures and elevated
points except ground surface within a specified limit.
(2) Accuracy of data
The measured laser point includes a complex error derived from each
devices of airborne system.
The errors that generated by each device of airborne system may
accumulate to form a complex error for the measured point. (Angle data
are converted into length in case of flight height 400m). Table 2
shows the accuracy of data.
Table 2. Accuracy of data
Fig.3 shows the relative errors between actual survey and airborne
laser scanning.
This result was obtained by targeting a ground control point, which
is elevated about 1m from the ground.
Fig.3 Relative errors of airborne laser survey (m)
The distribution of error ranges from about +0.5m to -0.5m; E-W
+0.33 to -0.17m, N-S +0.48 to -0.11m and elevation +0.32 to -0.54m;
average -0.01m in E-W, 0.22m in N-S and 0.02m in elevation.
Another survey data, statistic become as follows (Table3).
Table 3. Statistic of error (n =21)
(3) Data processing
Each data is converted into UTM coordinate system and then to
Bessel and WGS84 by the ground post-processing subsystem.
MICROSTATION software that was designed by Bentley Systems Inc. is
used to convert the measured data into DEM.
Fig.4 shows bird's-eye and side views of a topographic surface
sample before and after processing. These DEM are created in 50m-grid
spacing because the space of each shot is about 25cm and that of each
scanning line is about 40cm. The DEM before smoothing has many spikes,
whose height range 15-20cm in average, derived from the obstacles on
the land surface and the little disorder of scanning. To remove these
noises, we have processed the data by the matrix smoothing method;
number of columns and raw of center in ten.
Fig. 4. Before and after tree and noise removing
images
3. APPLICATION EXAMPLES
3.1 Actual land surface analysis
Fig.5 - fig.10 is an analysis comparison figure of the same spot in
scale of 1/2,500. The 2m-interval contour line indicates the feature
of the river terrace and an alluvial fan, which is able to distinguish
more clearly the difference when airborne laser results (fig.6) are
compared with aerial photogrammetry results ( fig.5).
Furthermore, fig .7 is a color elevation map, generated from the
DEM, which can visualize the equivalency of the river bottom, and
recognize the temporal change of riverbank of both side.
Fig.8 is a slant incline division map, which indicates the slope
degrees by color. The results can be referred as an interpretation of
topographic.
The traditional method of topographic interpretation is to identify
the division such as in fig.8 by using the topographic maps prepared
from aerial photogrammetry (fig.5). The indistinct area shall be
surveyed by additional field survey, which will increase the total
cost volume and time. Airborne laser measurement can eliminate those
works by measuring the actual land surface automatically in high
accuracy by using the last pulse mode.
Fig.9 and fig.10 are the samples, which the results from first and
last pulse data have been visualized by the preparation of the
artificial shaded relief image.
The characteristics of these images are the indicated features in
shadows, which result from optical devices such as aerial photographs,
are not able to indicate the objects inside the shadows of trees and
buildings. This means that the results from airborne laser measurement
have no connection with optical ray, which can also be surveyed in
night time.
3.2 An astonishing land deformation by abrupt volcanisms
Mt. Usu reopened the eruption activity on March 31, 2000.
The topography is changed by tectonic movement as a result of
eruption activity, and a large quantity of volcanic ashes, which
spouted out and accumulated at the crater outskirts.
The second measurement was done in April 26th which was
approximately 1 month after the eruption, which still the smoke rose
from several places of the crater.
As for the laser pulse, more than 15% of laser ray were not able to
penetrate the smoke and clouds, which make extra days to capture the
complete landform feature, by taking the chance of the change of wind
direction.
The two temporal results which indicated the topography change,
expressed by shaded relief map (fig.11), is able to distinguish the
particular movement area, which was the peripheral area of crater in
the south.
The results have indicated that the recent eruption area was not
the largest deformed area, whereas the western portion changed
greatly, shown in the contour map in fig.12.
The upheaval change map (Right side of fig.12) indicates the
quantity of height difference of the first and last measurements,
where the maximum deformation was 65m in western part, and 25m in
Mt.Kompira area.
Accordingly, the quantity of topography change during March 31st to
April 26th was 39,533,199m3.
The final evaluation from this survey can be stated as follows:
The magma reservoir rises in the peripheral of southern crater
group, and this was evaluated as a possibility of existence of
potential dome, which has not still spouted out with essence magma on
the surface of the landform.
3.3 City modeling
The experiment of city modeling by using airborne laser measurement
results have been started from July 2000.
The random elevation points that were acquired under 1m interval
expresses the feature of the urban area. Fig.12 indicates the
possibility of visualization of the elevation and the shape with high
accuracy by using the high density laser points within 50cm range.
However, in the case of preparing 3d-urban database in vector data,
the outline of each structure has to be modeled, by generating the
surface from the random elevation points.
The difficulties of generating surface from random elevation points
such as in fig.14, the feature outline is still hardly difficult to be
determined automatically, because the algorithm of software shall
include a self-evaluation technique to identify the outline of the
structure.
Fig.15 indicates the color evaluation map, which is prepared for
city consulting, mobile-phone construction, and city visualization.
3.4 CG and Virtual Realities
The keyword for 21st century must be Virtual Realities, which the
visualization techniques for three dimension models have been
developed rapidly during this decades. CG technologies exist as the
fundamental, which achieves the growth of the visualizing market and
merge with other technologies such as the WEB. The visualized spatial
data of topographic and urban area was prepared from the last pulse
50cm interval random point data, which can be downloaded from the
Internet by using Web3D technologies.
The latest Web3D technologies have the function to compress 3D data
to minimize the volume. Fig.16, is one of the example using XVL
(Extended VRML) technology from Lattice technology Inc., which can
compress the 3D data to approximately 1/50 comparing with the VRML
2.0.
4. SUMMARY
The article indicates that there is a large potential capability to
apply airborne laser measurement technology to a new market.
Virtual Reality is one possibility, because preparation of 3D
database in high accuracy, and preparation of orthogonal images by
using digital CCD color camera, mobilized with the ALTM 1225 system,
is getting inexpensive.
Thus, in the near future, the opportunity to use urban or
topographic data in 3D shall be more popular, and we believe that
airborne laser measurement technology shall be one of the largest
solutions of this 3D world.
Airborne laser measurement Product name: ALMAPS(Asahi Laser
Mapping System)
Fig.5 Map by aerial photogrammetry
Fig.6 Map by airborne laser measurement
Fig.7 Color elevation map
Fig.8 Slant incline division map
Fig.9 Shaded relief image (first pulse)
Fig.10 Shaded relief image (last pulse)
Fig.11 Shaded relief map (temporal images)
Fig. 12 Contour map and Upheaval change map
Fig. 13 Bird's eye view image
Fig.14 City modeling
Fig.15 Color elevation map
Fig.16 Web3D technology
REFERENCES
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CONTACT
Takayuki Okubo
AERO ASAHI CORPORATION
Sunshine 60, 43rd Fl.
No.1-1, 3Chome
Higashi-Ikebukuro
Toshima-ku
Tokyo 170-6070
JAPAN
E-mail: takayuki-ookubo@aeroasahi.co.jp
30 April 2001
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