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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
Moreover, it is greatly influenced by environment. Scanning in
rain and snowfall cannot be performed in principle. It is not
suitable to use VIVID900 when the sunlight is on an object.
Fig.1 VIVID900
Fig.2 Cyrax2500
2.2 The Measurement Method
In order to create 3D digital data utilizing laser-scanning
technology, we start by obtaining a set of 3D position data
points called "point clouds". First of all, a laser scanner is set up
and fixed in the most desirable place. Measurable area is
scanned for several seconds to several 10 minutes (dependent
on a type of scanner). Most of the time, it is required to perform
laser measurement of the whole object, and that is done by
installing a scanner in various positions around the target. The
measurements from various directions will result in highly
precise 3D data.
In order to obtain data of the whole surface of the object, it is
usually necessary to conduct several to dozens, sometimes
hundreds of scans from all around the object.
Therefore, the determination of the positioning of a scanner
becomes important at measurement work. Not only does it
determine the amount of data obtained, but also it has
considerable possibility of influencing subsequent work. The
shape and size of an object, and other conditions must be
considered to judge the most appropriate way for measurement.
It reality, it is very difficult to obtain point clouds for all
detailed areas of the object, and some occlusion tend to occur.
In order to reduce occlusion to the minimum, it is required to
check occlusion by aligning all the point cloud data obtained, as
soon as the measurement is over, within the duration of the
measurement period, since the result determines whether
additional measurement is needed or not.
When an object is large in scale, it might become necessary to
construct scaffolds for the scanning activity. Moreover, in order
to ease the data processing work that follows, usage of marker
targets on an object might become useful.
In laser scanning, it is important to create detailed plans
beforehand, with precise knowledge of the location and
characteristics of an object well in advance, and also to perform
sufficient examination on the method of data processing.
2.3 Data Processing
The quantity of the point cloud data acquired by laser scanning
becomes huge, when scanning is performed many times.
Although the point cloud data can be processed by using the
Software attached to a scanner, it does not have sufficient
function to create the general-purpose 3D data which can be
used for CAD, CG, and VR purposes. Therefore, in order to
process data smoothly, it is necessary to use the software
designed solely for processing point cloud data. Usually, data
processing is performed using exclusive software with special
functions, for example, to interpolate, reduce, edit, and
polygonize point cloud data.
In using marker targets for measurement, point cloud data is
aligned using the coordinate values of targets. However it takes
a long time to set and measure targets in this way. On the other
hand, alignment of the point cloud data is usually performed by
finding the duplicated part of adjoining data. This method can
save time and effort, and will lead to shortened alignment
processing time. But there is possibility of error accumulation,
in this case. :
Therefore, in order to keep to minimum the accumulation of
errors, making all point cloud data applicable to calculation for
the alignment becomes necessary, at the same time.
Both methods are often combined, so that main parts of the
objects are aligned by using targets, and the rest of the parts are
aligned by finding duplication parts, resulting in efficient and
more precise work (Fig. 3).
Following this process, 3D polygon data is obtained through
merging process. It might become necessary to edit the
occulusion parts not acquired by scanning, as necessary.
aligned by duplication parts
aligned by using targets
Fig.3 Combined Methods
3. PRACTICAL USE OF 3D DATA
3. General
Since the 3D data created can be visualized real time on a
computer, the application possibilities expand, from just using it
for protection of cultural properties, to utilizing it in a wide
variety of fields, including education, research, and multimedia
activities. The following items are the practical examples
regarding application of laser scanning technology to different
areas. Authors would like to evaluate subjects, problems, and so
on.
3.2 3D Digital Recording (Uraga Dock)
3.2.1 Purpose: Since Uraga dock (former Uraga Shipyard of
Sumitomo Heavy Industries, Ltd.) was established in 1897,
many ships and ferries have been built and repaired.
The Uraga dock is one of the important industrial heritages of
modernization, symbolizing the town of Uraga for a long period
of time.
Even after its close in March 2003, the dock has drawn great
attention from both citizens and the local government. The
future alternative uses of the property have been discussed,
including its reuse as museum, preserving existing facilities.
This attention led to the 3D recording of the present Uraga dock
using laser-scanning technology.
