(ASCE)0733-9453(2004)130_2(56)

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Geomatics Techniques for Structural Surveying
Jon Mills1 and David Barber2
Abstract: Structural engineers may utilize geomatics techniques for precise and accurate measurements of discrete poin
façade recording, and the production of engineering drawings and plans. Techniques commonly used include direct measurem
tape or gauge or, more recently, observations made from a reflectorless total station. Photogrammetric methods are also
structural surveying. Terrestrial laser scanners have recently taken large steps in development and have the potential to beco
survey tool. An overview of current recording techniques along with an introduction to laser scanning is given, followed by de
test involving terrestrial survey, photogrammetry, and laser scanning at a site in the United Kingdom. Analysis of the results sh
measurement to targeted points using the laser scanner was comparable to measurement using traditional stereo photogram
ods, although care needs to be taken to reduce the impact of mixed pixels and multipath occurring within the scanned scene
DOI: 10.1061/~ASCE!0733-9453~2004!130:2~56!
CE Database subject headings: Photogrammetry; United Kingdom; Surveys; Structures; Measurement.
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Introduction
Geomatics incorporates many disparate methods and techno
that offer surveying engineering and architects flexibility in
design and implementation of structural surveys and reco
schemes. The safety of a structure and that of its occupant
users can be assessed using data collected by these meth
gies; they also provide the basis for decisions concerning
maintenance and care of the building fabric, which may inc
repairs, renovation, or redevelopment. Historic structures re
detailed records to be maintained, which allows academic s
to improve the understanding of a structure’s purpose and v
Additionally, survey data is vital in the event of fire or oth
destructive event~Dallas et al. 1995!.
Techniques commonly used for structural measuremen
clude tape measurements combined with hand recording, an
tical methods, such as theodolite intersection~Banister et al
1998!. In Europe especially, image-based methods such as
fied photography and stereo-photogrammetry are well util
High precision applications may use converging multistation
togrammetric networks that allow precise point measuremen
rapid three-dimensional modeling on a standard desktop or l
PC. More recently active methods, including reflectorless
tronic distance measurements~EDM! and time-of-flight lase
scanning systems, have been developed. In particular, laser
ning systems have potential to be used for structural recor
because they rapidly produce large amounts of three-dimen
1PhD, Senior Lecturer, School of Civil Engineering and Geoscien
Univ. of Newcastle upon Tyne, Newcastle upon Tyne, UK.
2PhD, Post Doctoral Research Associate, School of Civil Engine
and Geosciences, Univ. of Newcastle upon Tyne, Newcastle upon
UK.
Note. Discussion open until October 1, 2004. Separate discus
must be submitted for individual papers. To extend the closing da
one month, a written request must be filed with the ASCE Mana
Editor. The manuscript for this paper was submitted for review and
sible publication on March 25, 2002; approved on August 30, 2002.
paper is part of theJournal of Surveying Engineering, Vol. 130, No. 2
May 1, 2004. ©ASCE, ISSN 0733-9453/2004/2-56–64/$18.00.
56 / JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004
s
-
-
data without~as is required in photogrammetry! the need for in
termediate processing.
This paper outlines the use of photogrammetric and laser
ning techniques in a real-world application: the survey of a
toric building façade. It gives both quantitative and qualitat
assessment of the instrumentation, methodologies, and col
data. An introduction to the techniques will be given in the
lowing sections with particular attention to the relatively n
technique of laser scanning. Results from the survey will the
presented and discussed.
Overview of Terrestrial Photogrammetry
Three-dimensional measurements using photogrammetry ar
sible in two principle network configurations, stereo and mult
tion ~Fig. 1!. Stereo photogrammetry has been traditionally
plied to topographic mapping using imagery from airbo
platforms~Wolf and DeWitt 2000!; however, it may also be us
in terrestrial applications when the viewing of stereopair
three-dimensional models may be useful for the interpretatio
data in addition to three-dimensional measurement. Commo
film-based metric survey camera is used that incorporates a
focus low-distortion lens, film flattening, and other feature
improve the quality of measurement. Nowadays, measure
can be performed using analytical plotting instruments or di
photogrammetric workstations, the latter requiring the scan
of the film negative, or the capture of imagery directly usin
digital camera. Digital systems allow semiautomated mea
ment and the production of orthorectified photography, previo
difficult using analytical methods alone~Bryan et al. 1999!.
Multistation convergent networks~Fig. 1! use more than tw
images~typically many more!, allowing the use of superior ne
work design with a larger observation redundancy, thus imp
ing accuracy, precision, and reliability~Fraser 1996!. The increas
in observation redundancy often permits this method to use
metric cameras, because self-calibration can be performed w
the adjustment procedure~Fryer 1992!. Commercial softwar
such as Photomodeler, 3D Builder, and Kodak Dimension~Mills
et al. 2000! are just three low cost software packages that may be
ent of
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apture
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sing
e
for a
is
eam
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int,
rows
used to perform this method of measurement. The developm
the internet now means that photogrammetric measuremen
even be made online~Drap and Grussenmeyer 2000!. Advances
in high-resolution digital cameras, which eliminate the need
film processing and provide inherent image stability, complem
the use of multistation convergent networks by allowing an
tirely digital measurement workflow. Digital cameras also al
greater flexibility in network planning, because there are fe
restrictions on the number of images that can be captured as
pared to their film-based counterparts~metric film cameras have
reputation for being difficult to use, while digital cameras ten
be based on simpler, more familiar, 35 mm SLR camera des!.
Such developments have improved the efficiency of photog
metric measurement and made photogrammetry accessib
nonspecialists.
Overview of Laser Scanning
Although laser scanning in a survey environment may be as
ated more with airborne applications such as terrain modelin
flood risk assessments~Wehr and Lohr 1999!, terrestrial lase
scanners are also available. Terrestrial laser scanners oper
one of three principles: triangulation, time-of-flight, or ph
comparison. Triangulation scanners record an object’s s
using trigonometry. They generally use a Class 1 laser~IEC 2001!
to emit a point or stripe of laser light, and a charged cou
device~CCD!, mounted at an offset to the laser source, to de
the returning laser energy. In order to perform a measurem
triangle between the laser source, the point on the object w
the laser strikes the surface~the object point!, and the CCD de
Fig. 1. Principle photogrammetric network configurations
Fig. 2. Triangulationlaser scanning
n
tector is formed as illustrated in Fig. 2. A known baseline betw
the laser source and detector and measurement of the in
angles of the triangle allows theXYZ coordinate of the obje
point to be calculated. Using a mirror, the laser can be defl
over the object and multipleXYZ coordinates can be obtaine
Scanners of this type afford precise high-resolution measure
however, as the distance from the scanner to the object incr
the angles become smaller and harder to measure and,
quently, the measurement becomes less precise. In additi
range increases the visibility of the laser source decreases, f
limiting measurement precision. For these reasons, triangu
systems are generally limited to short ranges, commonly less
2 m, and are therefore inefficient for use on the large object
structures typically found in structural applications such as fa¸ade
measurement. It is, however, interesting to note that triangul
scanners can provide measurement, in optimum configuratio
better than 10 microns.
Scanners more relevant to large scale applications are tim
flight systems that measure range to an object point using
pulse or phase comparison methods. Timed pulse systems
a pulsed diode laser, enabling them to operate at longer r
than triangulation systems. By measuring the time betwee
emission of a pulse of laser energy and the detection of th
flected signal, the sensor to object distance can be calculated
technique of range measurement is similar to the method
ployed by the Distomat DI-3000 EDM instrument. Timed pu
scanners can typically measure upwards of 1,000 points pe
ond with an accuracy from 6 to 100 mm, depending on the r
and system in question. There is an increasing number of m
available, with systems developed by Cyra Technogies Inc.~Gor-
don et al. 2001!, Callidus Precsion Systems GmbH~Niebuhr
2001!, and Riegl Laser Measurement Systems GmbH~Ullrich
et al. 2001! operating on this principle.
Through the use of continuous wave~CW! lasers, scanne
based on phase comparison allow increased rates of data c
as compared to pulsed systems. For example, Zoller and Fr
ch’s LARA 25200 produces up to 625,000 points per second~Zol-
ler and Fro¨hlich 2002!. The principle uses the phase shift betw
the transmitted and received wave to calculate the range t
object point ~Heinz et al. 2001!. This concept is analogous
contemporary EDM equipment that also uses phase comp
to determine range.
As laser energy is emitted from the laser it diverges, cau
the instantaneous field of view~IFOV! of the laser to grow in siz
in proportion to the range traveled. The angle of divergence
particular laser allows the size of beam to be calculated~from
edge to edge! for a particular range. The edge of the beam
normally taken to be the level where the intensity of the b
drops below 37%~Fig. 3!. Divergence is normally quoted in m
liradians, allowing a simple estimation of the IFOV, or footpr
of a particular beam; a beam with a divergence of 1 mrad g
Fig. 3. Beam divergence~O’Shea et al. 1977!
in diameter by 100 mm for every 100 m traveled.
JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004 / 57
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In both timed pulse and CW-based systems, the las
scanned over the subject using either a rotating mirror, mec
cal movement of the laser source, or through a combination o
two. The resolution, or horizontal and vertical spacing betw
data points, is determined by the amount of movement o
mirrors or the scanner assembly and the range at which the
ner is operating. It is normal to use the same resolution in bot
horizontal and vertical axis in order to prevent bias in the des
tion of horizontal or vertical features. For example, if a brick w
is scanned with a higher resolution in the vertical axis, the h
zontal mortar joints are more likely to be recorded than the
tical joints; without knowledge of the differing horizontal a
vertical resolutions, a user may believe this is a true repres
tion of the wall. However, in practice, especially with subje
with large depth variations, it is difficult to get a homogene
resolution over an entire scan scene. This must be noted
using and presenting scanner data.
Cartesian coordinates are calculated using the angle of d
tion ~horizontal and vertical! and range to a single point observ
to by the scanner~Wunderlich 2001!. These coordinates are bas
on an arbitrary reference frame with its origin at the sca
location. One scan results in many points being measured
point or data ‘‘cloud’’ is the outcome. Scanning from differ
positions may be required in order to overcome occlusions w
a scene; however, as each scan is referenced to an arbitrary
dinate system, it is necessary to transform scan clouds i
common reference frame before use. This process is know
registration.
The registration of scan clouds involves determining the
rameters of a three-dimensional transformation to rotate
translate the scans to a single reference frame. A common m
used in the processing of data from triangulation scanners
equally applicable to data from time-of-flight systems, is
matching of shapes using techniques such as the iterative c
point algorithm~Besl and Mckay 1992; Pulli 1999! where two o
more sets of scan data with overlapping coverage are mathe
cally compared to obtain the transformation parameters. H
ever, in order to transform the data to a known reference sy
for example, a site coordinate system, a traditional transform
is required involving the identification of control points and ite
tive least squares estimation of the transformation param
This is the method of registration often used by time-of-fl
systems, rather than shape matching, and normally involve
use of targets to identify conjugate points. As error in the m
surement of a targeted point will affect the accuracy of the tr
formation parameters, it also affects the absolute accuracy
transformed scan cloud; therefore, the accuracy of individua
geted points are important even when only surface measure
rather than discrete point measurement, is of interest.
The ideal choice of target varies from scanner to scanne
methods of measurement are based on either intensity or s
Where systems allow the intensity of the reflected pulse t
measured, highly reflective targets may be used. When co
points are scanned at a sufficiently high resolution, returning
nals with high intensity can be identified as returns from the
get. However, as reflective targets work poorly at acute an
such targets may restrict the geometry of the scanning net
Where more flexibility is required, or when using systems tha
unable to record intensity, three-dimensional shape targets m
used. These targets can be reduced to a single common poin
a scan of the target from any direction. For example, the
cloud of a spherically shaped target can be used to recove
parameters of the sphere~i.e., location and size!. The location of
58 / JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004
-
-
t
,
.
the sphere can then be used as a targeted point despite no
observation to that point. As the same point will be derived f
scanning any side of the spherical target, the scanning netw
more versatile as a result. In practice, a combination of inte
and shape targets are often used.
Once registered, the preparation of measurements, draw
and models from the scan cloud can begin. Such inform
extraction may involve algorithms to fit computer-aided de
~CAD! primitives, such as planes and cylinders, to the point
It may alsoinvolve using a meshed surface to produce con
sections, or rendered models. Such processing is generally c
out using a manufacturer’s proprietary software and may the
transferred to a commercial CAD package. Direct use of p
data in standard CAD packages or within standard survey
ware is generally impractical due to the large amount of da
question, although some software and CAD plugins do allow
manipulation of scan clouds within CAD software.
Case Study: Survey of Hastings Tower, Ashby
Castle
The advantages of laser scanning as compared to existing
niques stems from the high rate-of-capture and density of t
dimensional data~Boehler et al. 2001!. Structural application
previously unsuited to traditional taped measurements or p
grammetry, such as the recording of large statues and monum
may now be possible with laser scanning. Therefore, the com
son of scanning, especially with image-based survey is of
interest to engineers who require structural survey informatio
who perform structural monitoring. Quantitative assessme
accuracy and precision is required to provide confidence in
ner data, while qualitative judgments are useful for practitio
wishing to make effective use of this new technique. Prev
tests to provide benchmark information performed by Lichti e
~2000! used reflective targets on a first-order EDM baseline a
a high-precision dam-monitoring network. The project descr
in this paper selected a complex architectural subject and
lected photogrammetric and laser scanner data in order to a
comparison of the two techniques.
The south fac¸ade of Hastings Tower, a partially ruined 1
century structure, part of Ashby Castle located to the northea
Birmingham, U.K., was selected as a suitable site~Fig. 4!. This
provided a suitably stable and complex subject with various
face textures~including areas of smooth masonry and area
deteriorated stonework! and allowed testing of the scanner
ranges up to 80 m.
Operating the scanner over longer distances has obviou
vantages in terms of time~and therefore cost!. The test provided
real-world application of the survey techniques, a fact that
emphasized by the variety of weather conditions encounter
the time of the survey~heavy rain and winds, accompanied
periods of bright sunshine!. This was not a laboratory-controll
experiment, a deliberate decision so as to provide a good de
tion of the problems encountered in the normal survey env
ment.
Eighty retroreflective targets~Beyer 1992! were attached to th
main façade of the tower. These were distributed mainly on
lower portion of the fac¸ade, although some targets were place
higher levels where possible. The targets were designed
compatible with all the methodologies in the test~this included
theodolite intersection and polar observations using a tota
tion!, allowing the comparative analysis of techniques. The tar-
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gets were attached a day prior to measurement to allow the
sive to become firm, ensuring stability for the duration of
survey.
A local site system was established for the test site with
X-axis running from left to right along the fac¸ade, theY-axis
perpendicular to the main face of the fac¸ade, and theZ-axis ver-
tical. Control observations were made using a total station
two photogrammetric networks, one stereo and one multista
were observed. These two networks used a film-based m
camera and a nonmetric digital camera, respectively. Has
Tower was then scanned from four different scanner posi
~from three different ranges!, with multiple scans made at ea
location.
Terrestrial Survey
Horizontal and vertical angles were observed from two su
stations that were positioned to allow favorable intersec
angles, while simultaneously allowing line of sight to the majo
of the targets. A Leica TCRA1003 total station was used fo
observations; this instrument has 3 seconds of arc angular
sion with a range measurement precision of62 mm12 ppm to
reflective tape~Leica 1998!. Three rounds of angles were o
served from each station, and distance observations were
made to each point allowing the target coordinates to be c
lated using four methods, intersection from both stations, p
observation from station 1, polar observation from station 2,
Fig. 4. South fac¸ade of Hastings Tower, Ashby Castle
Table 1. Average Standard Deviation~1 Sigma! Taken from
STAR*NET Adjustment of Terrestrial Survey
Method
Standard deviation~mm!
X Y Z
Intersection 0.5 2.0 1.0
Polar from station 1 1.1 3.0 1.1
Polar from station 2 1.1 2.0 2.0
Combination 0.2 1.0 1.0
a combination of the intersection and range observations. All
cessing was performed using the STAR*NET least squares adju
ment package.
Network preanalysis using STAR*NET estimated a precisio
of 61 mm at the 1 sigma level for intersection observations,
62 mm for polar methods. Table 1 shows the average precis
the targets for the four methods calculated using the STAR*NET
adjustment package. The average standard deviation of the
dinates achieved using the intersection observations is bette
1 mm in X and equals 1 mm inZ. In Y, however, it is twice th
value estimated by the network preanalysis. As predicted b
preanalysis, the polar measurements are of lower precision
the intersection measurements, but are mostly below 2 mm
combination approach produces the best result, with all
achieving a standard deviation of 1 mm or better. Although
precision of measurement did not quite meet the estimated
sion due to errors in observation, these values indicate tha
data are suitable for use as control data.
Multistation Convergent Photogrammetry
A Kodak DCS660 camera was used to observe a 10-image
tistation convergent photographic network. The DCS660 is pa
the Kodak DCS series of digital cameras~Fraser and Short
1995; Shortis et al. 1998!. It is a professional ‘‘off-the-shelf
camera incorporating a 3,040 by 2,008 CCD sensor, produci
image containing over 6 million pixels. The retro-reflective
gets were illuminated using a flashgun to produce highly sig
ized points in the imagery and thereby permitting automated
surement using centroiding algorithms~Clarke et al. 1993!. The
network was processed using a self-calibrating bundle adjus
with internal constraints. 1,113 observations were used in th
justment with a redundancy of over 800 observations; scale
connection to the local site system was provided using mea
ments from the terrestrial survey. All image measurement
adjustment computations were performed using the Vision
trology System~VMS! ~Robson and Shortis 1998!. The averag
standard deviation of the target coordinates estimated from
covariance matrix of the least squares adjustment was 0.2
and 0.2 mm in theX-, Y-, andZ-axes, respectively. In this case,
with the terrestrial survey, the network was restricted to ca
stations in front of the fac¸ade. The precision of the measurem
in the Y-axis is therefore of slightly lower precision than theX-
andZ-axes; however, the relatively high precision of this met
~as compared with the terrestrial survey! allows it be used wit
confidence as a comparative dataset for the assessment
stereo photogrammetry and laser scanning methodologies.
Stereo Photogrammetry
Stereo photography was captured using a Wild P32 metric
era, with a camera to object distance of approximately 2
providing an average photo scale of 1:350. Sixty-six targets
visible in the stereopair, of which seven were used to orienta
Table 2. Average Standard Deviation~1 Sigma! and Root-Mean
Square~RMS! Error of Analytical Photogrammetry Measuremen
Standard deviation~mm! RMS error~mm!
X Y Z X Y Z
2.4 11.9 2.7 2.3 9.9 2.2
stereomodel using the coordinate values obtained from the bundle
JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004 / 59
Zeiss
f the
s forof the
-
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rdi-
tically
ent;
e
Eq.
for
s
sure-
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rgets
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ower
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ion 1
adjustment. A trained photogrammetric operator then used a
P3 first-order analytical plotter to measure the coordinates o
remaining 59 targets. Measurement was repeated 10 time
each point; Table 2 shows the average standard deviation
measurements and the root-mean-square~RMS! error as com
pared with the coordinate values from the bundle adjustmen~not
including the seven control points!.
As with both the total station and bundle adjustment coo
nates, theY-axis has the lowest precision. The parallax Eq.~1! can
be used to evaluate the precision of measurement theore
possible in theY-axis;
dh5dpS dbD s f (1)
where dh5standard deviation of height measurem
dp5precision of parallax measurement;d/b5base to distanc
ratio; ands f5scale factor of the photography. Rearranging
~1! results in
dp5
dh
S dbD s f
(2)
In this case, using the standard deviation of the depth~Y-axis!
measurement, the parallax value can be calculated as 8mm. The
P3 should be capable of eliminating parallax up to 1mm ~Zeiss
1987!, which would result in a theoretical precision of 1.4 mm
measurements in theY-axis. Use of Eq.~2!, therefore, indicate
that optimum precision has not been achieved. Although mea
ment precision, in part, relies on the stereoscopic acuity o
operator, it also depends upon target visibility and size. Ta
that are too large make it difficult to return to the same point
time. Although the target design was considered a good com
mise for this test, the targets were required to be used with
different methodologies and therefore the targets may have
too large for optimal stereoscopic measurement. The RMS
of all three axes is within the standard deviation of measure
and shows that no systematic error was present in the me
Fig. 5. Riegl LMS Z210 scanner on site at Ashby Castle
ments. The mean error in all these axes was less than 1 mm.
60 / JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004
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Laser Scanning
Laser scanning of Hastings Tower was performed using a
LMS Z210 scanner manufactured by Riegl Laser Measure
Systems GmbH, Austria~Riegl 2001!. The system uses a tim
pulse method of range measurement, rotates mechanically
horizontal plane, and uses a rotating polygonal mirror to de
the beam in the vertical~giving a system field of view of 333° b
80°!. The quoted precision of angular measurement is60.018°
for the rotating mirror and60.036° for the mechanical mov
ment. Range precision is quoted as625 mm120 ppm. With a
mean divergence of 3 mrad, the footprint of the laser ener
150 mm at 50 m. A standard 12-volt car battery is used to p
the system and a laptop computer acts as a control and dat
age unit with the scanner itself normally mounted on a su
tripod ~Fig. 5!.
Laser scanning was performed at 30 m~Positions 1 and 4!, 50
m ~Position 2!, and 80 m~Position 3! from the main face of th
facade.~The two scans at 30 m were used to investigate h
slight change in the aspect of the scanner affects the result! At
each position, multiple scans were collected to allow an as
ment of precision. The resolution of the scan varied dependin
range, with a resolution of approximately 50 mm at 30 m,
mm at 50 m, and 150 mm at 80 m. Although the targets were
50 mm in diameter, the divergence of the laser beam mean
returns from the targets were received at all three ranges. T
were extracted from the scan data using the manufactures
ware, LPM-Scan, which uses a centroiding technique, simil
target measurement in VMS, to extract features of high inte
~Pfeifer and Rottensteiner 2001!. Fig. 6 shows an image form
from the intensity of returns received from the tower at Positi
Fig. 6. Scan of Hastings Tower, shaded by intensity
Fig. 7. Standard deviation of range for targets extracted at Posit
wer.
for all
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~note the targets of high intensity on the lower area of the to!
Fig. 7 shows the standard deviation of range measurement
targets at Position 1.
After data collection, outlier detection using the t-test was
formed in order to assess the quality of the data. However, de
this process, it is clear from Fig. 7 that some targets show a
precision of range measurement~for example, target 174 has
range standard deviation of over 0.5 m!. In order to make a
effective assessment of the scanner, all targets with a sta
deviation outside the 99% probable error limit~based on th
manufacturer’s standard deviation! were considered to be in err
and rejected. Five targets were removed from Position 1,
three, four, and two targets removed from Positions 2, 3, a
respectively. This procedure was applied to all four positions;
8 shows the standard deviation of range measurement fo
accepted targets at Position 1.
For all four positions, targets close to ground level and
stonework edges with large relief changes were most likely t
gross errors. It is noticeable that Position 4, the second at a
of 30 m, had fewer gross errors than Position 1, also at 30 m
9 details the location of targets in error at Positions 1 and 4
Fig. 10 is an image formed from the repeated scans perfo
at Position 1. The standard deviation of range measurement a
the scanned area is shown by the intensity of the pixel at
location. Predictably, the trees located to the sides of the im
have a high standard deviation~as the wind would move the tre
during scanning and they are at long range and relatively t!;
however, other areas of low precision occur at the edges o
tower. The high standard deviation only occurs at edges w
large depth displacements occur, for example, between the
and the sky.
Fig. 8. Standard deviation of range for accepted targets extrac
Position 1
Fig. 9. Location of removed points at Positions 1 and 4
s
Having rejected the outliers, the average precision of r
measurement was 7.2, 10.0, 10.5, and 8.7 mm for Positions
respectively. At each position, the target coordinates used t
entate the stereo-photogrammetry were used to transform t
maining targets onto the local site coordinate system. The
dard deviation and RMS error~as compared to th
photogrammetric bundle adjustment! for each target is given
Table 3.
At Position 1, the standard deviation of theX-axis is 9.4 mm
and at Position 3 it is 14.4 mm. At Position 2, however, i
approximately 60% larger than Position 3, despite the lower r
from 80 to 50 m. TheZ-axis also has a higher standard devia
at Position 2 than at Position 3. TheY-axis is the axis of lowe
precision for all four locations and becomes less precise as
increases. Both Positions 1 and 4, both at 30 m, show a s
precision of approximately 6.0, 16.0, and 7.5 mm inX, Y, andZ,
respectively. The RMS error of Positions 1 and 4 in theX andY
axes is within the precision of the measurements; howeve
both positions the RMS error value for theZ-axis exceeds th
standard deviation. The RMS error exceeds the precision of
surement in theZ-axis at all four positions. All RMS error valu
for the Y-axis are within the standard deviation of measurem
Discussion
Terrestrial Survey
Traditionally, discrete point measurement has involved the
nique of intersection, particularly for provision of photogramm
ric control and monitoring applications. Polar positioning to
reflective target has been possible for a number of years
recently reflectorless total stations have allowed measurem
nonsignalized points. The results of the terrestrial measurem
demonstrate the precision of both intersection and polar mea
Fig. 10. Standard deviation of range for different areas of scan
Position 1
Table 3. Average Standard Deviation~1 Sigma! and Root-Mean
Square~RMS! Error ofLaser Scanning
Position
Range
~m!
Standard deviation~mm! RMS error~mm!
X Y Z X Y Z
1 30 9.4 15.6 6.9 6.1 11.7 7.
2 50 22.6 28.3 19.4 16.9 13.2 21
3 80 14.4 36.0 17.5 30.8 23.0 21
4 30 8.4 15.6 8.3 5.5 7.7 14.
JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004 / 61
roach
ough
n the
is tes
van-
thods
dun-
ution
aving
strate
ject
tion
hich
com-
sure-
ry in
ing a
ces
is of
tion 1
of
o the
oint; a
from
ion o
olu-
tan-
on-
o the
the
even
01
less
ulted
g a
rgets
ision.
-
due to
-
m a
large
e
ith
sment
nal
ting
m to
ved.
duced
. 10
tion 1,
ntar-
w the
ixed-
elec-
to
o be
in-
ing of
ution
and
oto-
sing
lity
eo-
ricted
cess,
gital
pleted
gram-
from
nd
duc-
rly in
raw-
og-
ilding
lysis.
onal
cted
ments; the best result was achieved by a combined app
using angles and distances from both survey stations. Alth
the choice of intersection or polar measurement depends o
required measurement precision, the results achieved in th
argue for the use of polar measurement with its obvious ad
tage of speed and comparable precision to intersection me
However, it should be stressed that the lack of observation re
dancy in this method requires care in the planning and exec
of measurement to avoid data integrity problems, such as h
insufficient observations to position a particular point.
Photogrammetry
The bundle adjustment results obtained in this test demon
the ability of photogrammetry to determine high-precision ob
coordinates of discrete points. The high precision of the solu
has allowed this method to be adopted as a standard with w
the laser scanning and stereo-photogrammetry could be
pared. The more conventional method of stereoscopic mea
ment highlighted the lower precision of stereo-photogrammet
the depth axis; this must be properly considered when plann
photogrammetric survey that is to meet project tolerances.
Laser Scanning: Target Precision
By applying the special law of the propagation of variances~Wolf
and Gillihani 1997! to the laser scanning instrument varian
quoted by the manufacturer, a value of 18 mm in each ax
measurement can be calculated for scans performed at Posi
The observed results for theX-axis show a standard deviation
half that value, approximately 9 mm in each axis. TheY-axis also
has a precision of less than 18 mm. This may be attributed t
method used to reduce the scan data to a single targeted p
weighted average used to extract the high intensity targets
the scan data may have improved the measurement precis
an individual point. More surprisingly, given the angular res
tion of the vertical mirror as compared to the horizontal, the s
dard deviation for theZ-axis is much better than expected. C
sultation with the manufacturer suggested this effect is due t
variation of energy over the IFOV of the laser; in general,
variation of energy in a beam of laser energy is not an
distribution ~Hecht 1998; Riegl, personal communication, 20!.
In the case of the Riegl LMS Z210, the variation of energy is
significant in the vertical axis than the horizontal; this has res
in a higher precision in theZ-axis despite the instrument havin
lower angular resolution in this direction.
Laser Scanning: Target Errors
Two effects may explain the cause and location of the ta
removed from the assessment due to their low range prec
The first is a multipath effect~Runne et al. 2001!, more com
Fig. 11. Multipath effect in laser scanning
monly associated with global positioning system applications,
62 / JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004
t
.
.
f
where the apparent range to the target has been increased
reflections on the surrounding stonework~Fig. 11!. A second ef
fect is that of mixed pixels, where the reflected energy fro
single pulse is received from two surfaces separated by a
distance~Fig. 12!. Mixed pixels ~Hancock et al. 1998! are more
likely to occur in scanners with a large IFOV~lasers with a larg
angle of divergence! and with points located close to edges w
large depth separations. The points removed from the asses
could be attributed to either multipath or mixed pixels; additio
work would be required to investigate this further. It is interes
to note that the second 30 m position, which was located 4
the left of the first 30 m position, had only two targets remo
This suggests that errors in target measurement can be re
via proper positioning of both the targets and scanner. Fig
shows the standard, for the whole scanned scene, at Posi
and highlights the repeatability of range measurements to no
geted points. Edges with a large depth displacement sho
highest standard deviations; possibly indicating that the m
pixel effect is the cause.
Methodology for Recording
Although measurement accuracy is an important part in the s
tion and adoption of a particular survey method, its ability
supply an appropriate result quickly and efficiently must als
considered.
Image capture for photogrammetry can be difficult for the
experienced, as use of metric cameras requires careful plann
camera stations and targeting networks. Although high resol
digital sensors are currently available, they are expensive
generally nonmetric in design; therefore, the majority of ph
grammetric survey in architectural recording is performed u
film. Suitable lighting conditions are required for good qua
photography, and not all locations will easily allow ster
coverage to be achieved; for example, its use in small rest
areas may not be suitable. In addition to a development pro
scanning of the film may be required to allow the use of di
photogrammetric workstations. These stages must be com
before measurement can begin and further expand the photo
metric work flow. Furthermore, the use of stereo models
which to plot detail is a specialist skill requiring training a
experience to ensure high quality products are delivered.
Despite limitations, the use of photogrammetry for the pro
tion of vector drawings is an established technique, particula
Europe, and is capable of producing three-dimensional line d
ings with a very high level of detail. Good quality metric phot
raphy alone is a valuable product for the assessment of bu
condition, or as the basis for historical research and ana
Photogrammetry also allows products in addition to traditi
line drawings. The collection of the surface models, colle
Fig. 12. Mixed-pixels effect in laser scanning
through automated or semiautomated photogrammetric methods,
y of
ollec-
gra-
rmed
tho-
el is
hows
tings
met-
is for
on-
an
ring
sed.
il to
od of
e as
ation
emi-
pro-
hive
this
shed
lected
s are
rther
ject.
cess,
cult
l,
rent
uce
ork
ow-
much
ction
laser
great
tech-
the
have
pho-
ing.
pre-
-
ista-
d 0.2
the
ision
Riegl
, and
for
ompa-
ow-
com-
ability
nner
lace-
d by
not
ith a
Care-
prob-
hed
cen-
cord
hby
a
allows the investigation of important details in the topograph
an object such as tool marks or areas of weathering. The c
tion of a surface model allows the production of orthophoto
phy. Orthophotographs are images that have been transfo
from the perspective projection of a photograph to an or
graphic projection used on a map or plan. The surface mod
used to remove distortions due to changes in relief. Fig. 13 s
an orthophotograph of part of the wall end section at Has
Tower, produced using LH systems SOCET SET photogram
ric workstation. This scaled image could be used as the bas
additional survey work with observations on key points of c
struction, or with important details of deterioration added by
appropriate specialist.
For simple point measurement, for example, in the monito
of stonework, the multistation convergent network could be u
It would also be possible to produce simple vector deta
supplement reports and presentations. Although this meth
photogrammetry requires network planning, it is less restrictiv
compared to stereo photogrammetry. However, as multist
convergent networks are generally used with nonmetric or s
metric cameras, the image quality can be lower than that
duced by a metric camera. The quality of an image for arcpurposes may be an important issue for some projects.
The products that could be produced from scan data in
application rely upon modeling the scan cloud as a me
model. Fig. 14 shows a meshed model of the scan data, col
at 30 m, produced using Cyra’s Cyclone software. Large gap
visible in areas occluded from the scanning location, and fu
scans would obviously be required to fully record this sub
~This may be difficult in some areas due to restrictions of ac
for example at high levels, although this would also be diffi
for other techniques such as photogrammetry!. From this mode
sections may be taken to show the building profile at diffe
levels. The relatively small amount of effort required to prod
this model is in stark comparison to the large amount of w
required in obtaining this type of data by any other means. H
Fig. 13. Orthophoto of wall end section, Hastings Tower, As
Castle~produced with LH Systems SOCET SET!
ever, the use of this data in a traditional sense, for example, in
paper plans and drawings, is difficult, as the scan data loses
of its value when not viewed in three dimensions. The produ
of suitable deliverables is set to become an important part of
scanning for structural survey.
Conclusion
The methodologies used to perform structural survey are of
interest to engineering surveyors where the efficient use of
niques and instrumentation is a main priority, especially in
specification and planning stages. The tests described here
compared the established methods of terrestrial survey and
togrammetry with the relatively new technique of laser scann
The analytical photogrammetric measurement achieved a
cision of 2.4, 11.9, and 2.7 mm in theX-, Y-, andZ-axes, respec
tively, for the measurement of targeted points, while a mult
tion convergent solution achieved a precision of 0.2, 0.4, an
mm in the X-, Y-, and Z-axes, respectively, demonstrating
advantage of this technique in applications where high prec
is required. The laser scan data collected at 30 m by the
LMS Z210 scanner has shown that a precision of 9.4, 15.6
6.9 mm in theX-, Y-, andZ-axes, respectively, is achievable
the measurement of targeted points, and the values are c
rable with the analytical photogrammetric measurement. H
ever, gross errors in the measurement of some targets as
pared with the other techniques and analysis on the repeat
of scans has highlighted the effect of mixed pixels on sca
data with errors occurring at edges with large depth disp
ments. Although in this test targets in error could be identifie
their low range precision, it is possible other situations would
allow this, and the target’s coordinate would be recorded w
suitable precision but also contain a large systematic error.
ful planning of target and scanner positions can reduce this
lem; however, it would be wise not to overlook other establis
techniques. It is recommended that further work should con
trate upon improving the ability of scan data to precisely re
Fig. 14. Meshed model of Hastings Tower~produced using Cyr
Technologies Cyclone 3.0 software!
edges, possibly by augmenting laser scanning with other survey
JOURNAL OF SURVEYING ENGINEERING © ASCE / MAY 2004 / 63
age o
ed to
fu-
dant
t use
ike.
f En-
ing-
gl
s Inc.
f the
with
earch
ital
e-
,
-
ding.’’
the
r the
-
s,
sure-
nts
a-
470.
D.
n,
s and
en
-
ea-
.
ch-
h-
e
5.
,
an-
,
ic
.
8.
ras.’’
ated
c-
ter-
ues
eci-
ea-
,
observations. New procedures and processes taking advant
the strengths of each survey methodology should be develop
efficiently perform structural surveying and recording in the
ture. Benefits including improved precision through redun
observations and reduced costs from time saved by efficien
of methodologies will be seen by practitioners and clients al
Acknowledgments
The writers greatly appreciate the support and cooperation o
glish Heritage and the staff at Ashby de la Zouch Castle, Birm
ham, U.K.; 3D Laser Mapping Ltd. of Nottingham, U.K.; Rie
Laser Measurement Systems, Austria; and Cyra Technologie
Thanks also to Stuart Robson and Mark Shortis for the use o
Vision Measurement System. This research was carried out
the funding of the Engineering and Physical Sciences Res
Council ~EPSRC!, U.K., studentship award 99802980.
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