=Paper=
{{Paper
|id=Vol-2320/paper4
|storemode=property
|title=Architectural Heritage: 3D Documentation and Structural Monitoring Using UAV
|pdfUrl=https://ceur-ws.org/Vol-2320/paper4.pdf
|volume=Vol-2320
|authors=Danila Germanese,Maria Antonietta Pascali,Andrea Berton,Giuseppe Riccardo Leone,Davide Moroni,Bushra Jalil,Marco Tampucci,Antonio Benassi
|dblpUrl=https://dblp.org/rec/conf/ircdl/GermanesePBLMJT19
}}
==Architectural Heritage: 3D Documentation and Structural Monitoring Using UAV==
https://ceur-ws.org/Vol-2320/paper4.pdf
Architectural Heritage: 3D Documentation and
Structural Monitoring Using UAV?
Danila Germanese1 , Maria Antonietta Pascali1 , Andrea Berton2 , Giuseppe
Riccardo Leone1 , Davide Moroni1 , Bushra Jalil1 , Marco Tampucci1 , and
Antonio Benassi1
1
Institute of Information Science and Technologies - CNR, via G. Moruzzi 1, Pisa
56127, Italy,
danila.germanese@isti.cnr.it
2
Institute of Clinical Physiology - CNR, via G. Moruzzi 1, Pisa 56127, Italy
Abstract. Architectural heritage preservation and dissemination is a
very important topic in Cultural Heritage. Since ancient structures may
present areas which are dangerous or difficult to access, Unmanned Aerial
Vehicles may be a smart solution for the safe and fast data acquisition. In
this paper we propose a method for the long term monitoring of cracking
patterns, based on image processing and marker-based technique. Also
the paper includes the description of a pipeline for the reconstruction of
interactive 3D scene of the historic structure to disseminate the acquired
data, to provide the general public with info regarding the structural
health of the structure, and possibly to support the drone pilot during
the survey. The Introduction provides a state of the art about the crack
monitoring from visible images; it follows a description of the proposed
method, and the results of the experimentation carried out in a real
case study (the Ancient Fortress in Livorno, Italy). A specific section is
devoted to the description of the front-end of augmented reality designed
for heritage dissemination and to support the drone usage. Details about
the future works conclude the paper.
Keywords: Crack quantification · Crack monitoring · Photogrammetry
· UAV · 3D rendering.
1 Introduction
The structural deterioration of architectural heritage is an old problem. Among
the large set of structural features of an ancient building or structure, we de-
voted this paper to the problem of monitoring and measuring missing or de-
formed structural elements, cracks and fissures. Nowadays, visual inspection is
the most used technique to detect damage or to evaluate their variation over
time. Nonetheless, such technique may be time consuming and expensive, and
even not possible if the access to critical locations is forbidden for safety reasons.
?
Partially supported by the Tuscany Region, FAS program.
2 D. Germanese et al.
On the other hand, the constant growth of digital technologies, led to novel
and efficient low-cost hardware and applications, supporting the monitoring of
specific regions and the assessment of the mechanical stability, thus preventing
critical events.
In this paper we describe a method to track over time the variations of
the cracking patterns in buildings. Also we show its applicability in a challeng-
ing case study, namely the Fortezza Vecchia – an ancient fortress in Livorno
(Italy). The study was carried out in the framework of MOSCARDO project
[9], devoted to the development of a monitoring system able to collect and pro-
cess data about the structural health of ancient buildings, and to provide alert
notifications in case of anomalies; of course, the implementation of such a func-
tionality requires the acquisition of a reference data set over at least one year, in
order to set reliable threshold values, e.g. with respect to the seasonal variations
(vegetation, environmental light, temperature, humidity). Among the techniques
aiming at monitoring the structural integrity of buildings, we used the marker-
based method, as it allows for a non-destructive analysis of cracking patterns.
We also demonstrated, in a previous pre-application study [8], the feasibility of
this approach.
2 State of the art
The literature provides a number of studies proposing alternative methods to
visual inspection. Such methods, in general, divide into two di↵erent groups:
invasive and non-invasive approaches. Here, we briefly list a comment some of
the non-invasive methods, in particular those which are more suitable to be
applied in cultural heritage.
Close-range digital photogrammetry includes a large family of methods, as
reported by Remondino and co-workers [17]. In general, it is based on several
acquisitions of a set of images used to produce a 3D point cloud of the scene.
To monitor over time the crack opening, the point clouds generated at di↵erent
dates are compared. Such comparison may be performed by exploiting di↵er-
ent techniques, such as (i) conventional analysis, which uses statistical tests to
compare the estimated 3D coordinates of the same points [22]; (ii) shape anal-
ysis, e.g. by matching surfaces [10] or comparing their shape signatures [3], or
comparing a specific shape parameter (the surface area associated to each crack)
complemented with a bootstrap testing to detect only statistical meaningful vari-
ations in cracking pattern [1]. However, among these papers, only the last one
tackles the application of 3D shape methods to the crack analysis, representing
the crack as a surface, hence not allowing for the monitoring of specific critical
points along the crack.
Other methods aim at automatically detecting and measuring structure dam-
ages and cracks using image-based algorithms, which allow for specifically filter-
ing out the cracking patterns, such as in [5]. In this work, two pipelines for the
crack segmentation are described: the former evaluates the color level for each
pixel, and enhance the structural discontinuities by adding more ”white” or
3D Documentation & Structural Monitoring 3
more ”black” to make the structural discontinuities even darker (unfortunately
this approach fails when the structure walls are not clear); in the second one,
the cracking pattern is filtered out by automatically detecting edges (by apply-
ing a Gaussian blur and subtracting the filtered image from the original one).
Even if this study provides image-based techniques for the crack segmentation
no quantitative analysis of the structural damages is proposed.
The method proposed by Jahanshahi et al [13], [12] is based on 3D recon-
struction of the scenario, image segmentation and binarization, and two clas-
sifiers (SVN and NN) to detect, isolate and distinguish the pattern related to
small cross-sectional structural defects (0.4-1.4mm). On one hand, such method
showed robustness when applied to images captured from any distance (20 m
in their experimental tests) and acquired using any resolution and focal length
(600 mm in their experimental tests); on the other hand, it seemed to be suitable
only for detection of small cross-sectional defects over homogeneous background.
In general, the principal drawbacks that image-based methods show are re-
lated to noise removal, edge detection, registration, application of morphological
functions, colour analysis, texture detection, segmentation. Among the major
challenges, the removal of noise due to the edges of doors, windows, and build-
ings. In the work of Jahanshahi [11] the pro and cons of cracks automatic detec-
tion in civil infrastructures performed with image-based methods is discussed.
Ellenberg [4] describes the main issues related to image acquisition performed
by UAV, i.e. the environmental conditions, the setting of camera parameters, the
distance of the camera (the greater the distance of the camera, the lower the
accuracy with which the crack width will be calculated), and angle of orientation.
Other studies aimed at marking the most critical points of a crack: any
displacement identified by the coordinates of the targets is used to calculate the
force field (tensile and shear forces) along the discontinuities of interest. Such
approach aim at enhancing the accuracy and the repeatability of the feature
point extraction task [21].
Nishiyama and colleagues [16] exploited reflective targets (i.e., targets made
by glass droplets, in order to reflect the light as much as possible) to mark the
point of interest of a discontinuity. They aimed at assessing the displacements
of the two surface portions of the crack by acquiring a number of images of the
marked crack, implementing photogrammetric algorithms and calculating the
coordinates of the targets.
In [21], another method, based on Hough transform and homography tech-
niques, is proposed to correct the perspective error, detect targets, and identify
their planar coordinates and geometric centers.
Benning et al. [2] prepared the surface of structural elements of pre-stressed,
reinforced and textile concrete by a grid of circular targets. Images were cap-
tured simultaneously and the measure of the relative distances between adjacent
targets was repeated in time intervals. Therefore, the evolution of the cracks and
discontinuities present on the surface of the concrete specimens was monitored
over time. In addition, a Finite-Element-Module was developed, which simulated
4 D. Germanese et al.
the test: thus, the results of photogrammetric measurements were compared with
the numeric tension calculation and iteratively improved. In these studies, the
method was applied only in the case of planar, small cross-sectional cracks.
The markers can also be home-made, as reported in [19], in combination with
useful suggestions on their dimensions, materials. Such method su↵er from the
position of the camera: the greater the distance of the camera, the lower the
accuracy with which the centroid coordinates will be calculated.
Properly designed for cultural heritage, our solution addresses the major
open problems related to the cracking pattern of the ancient structures, where
the fissures may be wide, large, and often non-planar. The markers allow for an
accurate quantitative measure and for a long-term monitoring. In addition, the
use of the UAV allows for exploring all the most critical points of the structure,
even the non-accessible ones.
3 Methodology
Our main case study is the Fortezza Vecchia, an ancient fortress in Livorno
(Italy): its walls are difficult to be monitored, because the fortress is partially
surrounded by the sea (see Fig. 1). A very important feature of most of the cracks
along the walls of the fortress is that their sides are quite far and definitely they
don’t lie on the same plane, as visible in Fig. 1, Bastione della Capitana. It makes
very difficult to obtain an absolute and accurate measurement of the separation
of the sides with standard methods. For this kind of structures, irregular, out-
door, subject to environmental agents and to seasonal changes (for vegetation
or even some weeds on it), photogrammetric reconstruction quality may be not
enough, especially when the aim is to compare over time measurements of the
cracks along the structure. Hence, we decided to use markers to provide a com-
plete and stable 3D information about specific fiducial points along the crack, to
be tracked over time. Such data include: (i) the set of the 3D coordinates of each
marker’s corners, (ii) the set of the distances between the barycenters of each
pair of markers, and (iii) the angle variations between the reference frame as-
sociated to each marker. Other advantages of such minimally invasive technique
are that it enables:
– reaching high accuracy when performing a quantitative analysis of the crack;
– using Unmanned Aerial Vehicles (UAVs) to acquire and, possibly, process
on board the images:
Also, the markers represent optimal reference points with respect to the use of
UAV for data acquisition. It is worth noting that UAV-based technologies allow
for a fast and highly repeatable data acquisition, even in area difficult to access,
thus reducing costs and risks.
We chose to use the ArUco markers because they are black and white square
planar coded markers [6] easy to use, and they can be reliably detected under a
wide range of environmental conditions; in addition, such markers allow for an
3D Documentation & Structural Monitoring 5
Fig. 1. Evident structural defects in the walls of the ancient fortress, in the area named
Bastione della Capitana (Fortezza Vecchia, Livorno, Tuscany, Italy).
6 D. Germanese et al.
accurate and robust camera localization. Our method is based on the Simulta-
neous Localization and Mapping, described in [15], which is optimized for the
creation of 3D map of the markers visible in the images acquired. A sequence of
frames of the same scene is acquired and at each frame the graph-pose is esti-
mated minimizing the re-projection error in the detection of the marker corners.
The output of the algorithm are the 3D coordinates of the corners with marker
ids. Then, the Euclidean distances between the markers’ barycenters and poses
are computed.
In the previous pre-application work [8], we showed the feasibility of such a
marker-based approach for the assessment of the crack opening, simulated in a
laboratory. We estimated a measurement accuracy of less than 1 mm, using a 18
Mega-pixels mirror-less (Canon EOS M), with a focal length set at 24 mm. The
acquired images have a resolution of 5184 x 3456 pixels. The same hardware
has been used for the experimentation described in the Section 4. As regards
the visual impact of the markers placed on an ancient structure, such as the
wall of the Old Fortress in Livorno, we printed markers with two di↵erent side
lengths: 0.1 m and 0.2 m. We found that, as all of them were detected correctly
, the smaller ones may be used for the next experimentation, without loosing in
accuracy.
4 Experimental setting
Six pairs of markers, four small pairs (0.1 m side length) and two big ones (0.2
m side length), have been glued along to the sides of a complete vertical cut
in the walls of Fortezza Vecchia in Livorno, located in the area called Bastione
della Capitana: the highest pair of markers is glued to the wall about 3.3 m from
the ground. The camera used for 3D reconstruction and photogrammetry is the
Canon EOS M, a 18 Mega-pixel mirror-less with a sensor APS-C of 22.3 x 15
mm (aspect ratio 3 : 2). The maximum video resolution is of 1920 x 1080 pixel
at 30 fps. It weighs 298 g and has dimensions 108 x 66 x 32 mm. The focal length
varies in the range 18-55 mm. E.g., setting the focal length at 24mm, and the
target at 1.5 mt, the field of view will be of 1.39 m (width) and 0.93 m (height),
and the pixel resolution (computed from the camera fact-sheet) will be of 0.27
mm. The camera has been calibrated using a ChArUco board as in [8], and at
each acquisition a sequence of at least 6 images were acquired 3 m, 6.5 m and
9 m far from the wall. The focal length of the camera has been set at 24 mm;
this implies that, for example, when the camera is 6 m far from the target, the
field of view is of 5.575 m (width) and 3.725 m (height), and the pixel resolution
(computed from the camera fact-sheet) is of 1 mm.
Another set of data have been collected recently, but not yet processed, sur-
veying the area with a drone. The drone is a Micro Air Vehicle designed and
assembled at the Institute of Information Science and Technologies of the Na-
tional Research Council of Italy. Having a drone following a predefined path
(made of GPS waypoints), allows to repeat the same acquisition; hence support-
ing the creation of a large dataset of the site of interest over time.
3D Documentation & Structural Monitoring 7
5 Results
As shown in Fig. 2 all markers are correctly detected, and the corners’ coor-
dinates are used to compute the distances between the barycenters, to roughly
check the correlation with the measures performed with a flexible meter (see
Table 1). Being this lesion large and not planar, linear measurements on it are
inherently not accurate.
Table 1. Barycenter distances and approximate ground truth. Last two columns show
the absolute value of the di↵erence between the approximate ground truth and the
barycenter distances computed from the two acquisitions. All values are in m.
Marker pair (ids) ⇠Ground Truth 1st acq. 2nd acq.
id: 27-20 0.33 0.001 0.000
id: 38-23 0.385 0.004 0.003
id: 3-4 0.55 0.007 0.007
id: 31-26 0.33 0.009 0.012
id: 25-18 0.42 0.003 0.011
id: 5-10 0.51 n.a. 0.006
Table 2. Barycenter distances, first and second acquisition, all values are in m.
Marker pair 1st acq. 2nd acq.
ids 3 m 6.5 m 9 m 3 m 6.5 m 9 m
27-20 0.329 0.333 n.a. 0.332 0.333 0.336
38-23 0.386 0.391 n.a. 0.387 0.390 0.394
3-4 0.555 0.557 0.562 0.552 0.558 0.559
31-26 0.334 0.338 0.346 0.334 0.336 0.341
25-18 0.428 0.434 0.444 0.428 0.430 0.436
5-10 0.522 0.523 0.526 0.540 0.526 0.522
In Table 2, the distances between the markers’ barycenters are computed for
the two sets of images manually acquired 3 m, 6.5 m and 9 m far from the crack.
As expected, the approximately ground truth is not always close enough to the
computed distances; nonetheless, the two sets of values from the two acquisitions
seem to agree enough to justify further e↵orts and more experimentation to refine
the method (also regarding the acquisition procedure and the hardware setting)
in order to increase its accuracy in the outdoor setting; for so large cracks in
brick ancient structure, we aim at an accuracy of 1 mm, considered a meaningful
measure. Table 3 shows that in our tests the accuracy of the measurement, even
if showing a bit large standard deviation, does not depend on the marker size.
On the other hand, Table 2 shows that the stability of the measurement should
8 D. Germanese et al.
Fig. 2. ArUco fiducial markers have been placed along the crack in the most critical
points. The markers are correctly detected and identified.
3D Documentation & Structural Monitoring 9
be increased with respect to the camera-target distance. This last point is quite
important with respect to the usage of UAV for the data acquisition.
Table 3. Chart of average values and standard deviation computed with respect to
the two acquisitions, not regarding the distance camera-target (all values are in m).
Marker pair average st dev
id: 27-20 0.332 0.002
id: 38-23 0.39 0.003
id: 3-4 0.557 0.004
id: 31-26 0.338 0.004
id: 25-18 0.433 0.006
id: 5-10 0.527 0.007
This results will be enriched by the analysis of the 3D pose of each marker,
made through the computation of the angle variations between the reference
frame associated to each marker.
6 Augmented reality
In the management of the cultural heritage, 3D rendering, recording and doc-
umentation are a fundamental step [7] towards the enhancement of cultural
heritage. General public may experience an interactive survey of the ancient
structures, possibly including not accessible areas, and explore all the digital
information extracted. As reported in [18], several technologies can be used to
build a 3D digital model. LiDAR (Light Detection And Ranging) is a high qual-
ity remote sensing method that uses light in the form of a pulsed laser to measure
ranges. Despite the proven quality of the LiDAR systems, we preferred to ex-
plore less expensive solutions that achieve comparable results. A simple and
lightweight monocular camera can be used for Structure from motion (SfM), in
order to build the 3D model of the structure from a sequence of di↵erent views
of the object. The number, the quality, and the resolution of the images have
to be taken into account because they may a↵ect very much the time needed to
obtain a full 3D reconstruction.
SfM algorithm, in most of its implementations, consists of the following main
steps: (i) feature point selection in all the images, generally obtained by applying
SIFT (scale-invariant feature transform) or SURF (speeded-up robust features),
(ii) matching of the corresponding features and registration between images (in-
correct matches are usually filtered out with specific algorithms, e.g. RANSAC,
random sample consensus), (iii) detection of the control points, (iv) building of
a dense point-cloud, that is performed by using wide baseline stereo correspon-
dence [20], and finally (v) the surface reconstruction as a polygonal mesh, that
is the final 3D model of the object.
In general, 3D reconstruction algorithms may take hours or days using a nor-
mal pc. The using of multiple parallel GPUs or of cloud computing are strongly
10 D. Germanese et al.
recommended to speed up the whole process. The most popular software of 3D
reconstruction include Agisoft Photoscan3 , which is the first photogrammet-
ric software and allows the user tuning the reconstruction parameters during
the procedure to increase the quality, depending on the input data, on the user
preferences, and on the computing resources; COLMAP4 , an open-source soft-
ware which allows for setting the reconstruction parameters, but does not allow
for interacting with the middle result of the reconstruction phases; Autodesk
Recap Photo5 , which exploits cloud technologies, it is able to processes up to
100 photos at once, but it does not allow for setting any parameter relative to
the reconstruction phases.
Beyond computing dense detailed models [14], we got the best result from
Agisoft Photoscan, by selecting the appropriate key points and processing only
the interesting part of the image. Then, a virtual scene containing the recon-
structed object has been created by using the Unity6 engine. The exploitation
of such type of engine guarantees the easiness of navigation and, at the same
time, the overall representation quality. Inside the scene, users can easily nav-
igate around the reconstructed object and have a quick-look of all the regions
of interest of the structure, e.g. sensors installed to monitor environmental or
structural parameters, as shown in Fig. 3. Cracks may be highlighted and la-
Fig. 3. 3D front end interface. In the main panel, the 3D reconstruction of Torre
Grossa in San Gimignano, Italy, can be explored. Installed sensors are gathered from
MOSCARDO database and displayed directly. When a sensor is selected, last retrieved
data are shown on the right panel.
belled with latest measurements. It is also possible to interact with the cracks
3
www.agisoft.com
4
colmap.github.io
5
www.autodesk.com/products/recap
6
www.unity3d.com
3D Documentation & Structural Monitoring 11
in order to retrieve past calculated values or visualize charts representing the
crack opening evolution over time. The 3D reconstruction of the scene is also
useful to support the drone survey when an alert notifications in case of critical
anomalies is provided.
7 Conclusions and future works
Currently only a few acquisitions have been performed and the plausibility of the
proposed method has been confirmed. Since the preliminary results are promis-
ing, further e↵ort will be devoted to: acquire and process more data acquired
temporally close to each other in order to assess the robustness of the measure-
ment method; acquire and process data on a monthly basis, for at least one
year, to track the evolution of the crack features and possibly develop a model
to detect critical trends; and to test the validity of the UAV application.
Also, our system is minimally invasive, as required in the field of cultural
heritage, the measurements provided are accurate is not enough, it opens the
way to further structural analysis based on the 3D geometric representation of
a structural defect, e.g. looking not only at all the barycenter distances but also
at the pose variation of each marker. Note that our approach works around the
main difficulties of a crack segmentation, in our specific scenario: outdoor, large
and non planar cracks of ancient structures.
On the other hand, the 3D information and technology reveals to be quite
a powerful tool: the reconstruction of the architectural asset could be used to
disseminate the cultural heritage to the general public, and even may support
the expert to analyse the structural data acquired by sensors or the pilot of the
drone to flight over the specific area interested by a detected anomaly.
8 Acknowledgements
This work has been carried out in the framework of the project MOSCARDO
(FAR-FAS 2014) and SCIADRO (FAR-FAS 2014).
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