Kuchitsu et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0260-7(2019) 6:12

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Simple evaluation of the degradation stateof cultural heritage based on multi-view
stereoNobuaki Kuchitsu1*, Masayuki Morii1, Shuji Sakai2 and Hiroki Unten2

Abstract
In this study, degradation of cultural heritage is discussed based on the comparison between the present shape of theWareishi Rock Cliff Sculpture and its replica made 30 years ago through simple three-dimensional measurement using a
multi-view stereo. The Wareishi Rock Cliff Sculpture, an important cultural property of Hiroshima Prefecture, is located neara shoreline. Local people often highlight that the sculpture seems to have recently been suffering from rapid degradation
because of the direct impact of tidal ebb and flow. The present three-dimensional shape of this sculpture wasinvestigated through multi-view stereo technique to accurately estimate the current degradation state. The sameinvestigation was performed on the replica of the sculpture made in 1986 using a mold and exhibited in the Hiroshima
Prefectural History Museum. The difference between the present shape of the sculpture and that of the replica revealed
through the above investigation can be regarded as degradation of the sculpture that has occurred over the previous 30
years. The result is as follows. The difference was detected mainly on the middle part of the sculpture, which is a beltshaped portion parallel to the sea surface that is ~ 20 cm high. The total area where more than 3 mm of displacement
was detected constitutes ~ 0.56% of the whole measured area. Accordingly, the degradation of the Wareishi Rock Cliff
Sculpture does not seem to be recently accelerated but seems to be progressing at a regular pace since it was carved,
especially in relation to the sea level rise. If the degradation extends up to the face of the sculpture in the future, somecountermeasures may be required. The method used in this study will contribute to the conservation of cultural heritage
through evaluating the degradation state of the heritage precisely and to the study of weathering by detecting longterm weathering.
Keywords: Wareishi Rock Cliff Sculpture, Sea level change, Replica, Hiroshima Prefectural History Museum, Sagi Island

Introduction
Research background

Heritage is our legacy from the past, what we live withtoday, and what we pass on to future generations(UNESCO World Heritage Centre 2005). In addition, as
the Japan Consortium for International Cooperation in
Cultural Heritage (2013) underlines, cultural heritage is atestament that has been created over humankind’s long
history. The protection and management of cultural heritage are means of ensuring their maximum possible vitality, values, and functions to the benefit of current andfuture generations (Szmelter 2013). Conservation of* Correspondence: kuchitsu@tobunken.go.jp
1
Tokyo National Research Institute for Cultural Properties, 13-43 Ueno Park,
Taito-ku, Tokyo 110-8713, Japan
Full list of author information is available at the end of the article

cultural heritage is often compared with human healthcare(Smith and Přikryl 2007). In such an analogy, degraded
cultural heritage is compared with a sick person. To prevent cultural heritage from further degradation, diagnosisof the present heritage condition is given priority to perform any conservation treatment or restoration work. It issimilar to the case that no medical treatments are provided to a patient before a precise medical check. The
causes of the degradation should further be revealed basedon the present condition like identification of the disease.
Finally, countermeasures can be discussed by considering
how to remove the revealed causes in a manner similar to
the extraction of the cause of a disease. As basic medical
studies are indispensable for human healthcare, studies on
precise description and weathering are needed for conservation of cultural heritage.

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Kuchitsu et al. Progress in Earth and Planetary Science(2019) 6:12

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Fig. 2 Locality map of the Sagi Island, Hiroshima, and Itozaki. Sagi Island
is located in the Hiroshima Prefecture floating on the Seto Island Sea

of cultural heritage (Ikeuchi et al. 2007; Jordá et al.
2011). Finding a difference between the results of twotime measurements of the same target with a certain
period leads to detecting the weathering during that
period (Gomez-Heras et al. 2008). On the contrary,

Fig. 1 The Wareishi Rock Cliff Sculpture. a During low tide (2016) and b
during high tide of a spring tide (2012). The sculpture was carved on a
granitic core stone and appears above the sea level during low tide. The
lower mass of the core stone below the chest of the sculpture is covered
with dark-colored materials. The whole sculpture sinks below the sea level
during high tide of a spring tide

Stone weathering has been investigated on several stonemonuments from such a viewpoint (Doehne and Price2010). Among them, studies on the on-going weathering
rate are especially effective to evaluate the present condition of stone monuments (Winkler 1986), for example, toascertain whether the target monument requires an immediate measure to stop weathering or continuous prudent
monitoring for a while. To understand the present weathering condition of target monuments, the first step would be
to measure the precise shape of the stone surfaces.
Contact and non-contact methods have been practiced
to measure the surface change rate of stones (Moses etal. 2014). Although contact methods, such as with micro-erosion meter, have been practiced on quite a few
objects (Stephenson and Finlayson 2009), a non-contact
and non-destructive method is generally preferred foravoiding value reduction of the cultural heritage. The
non-destructive laser scanning measurement technique
is often applied to describe the present surface condition

Fig. 3 The present surface condition of the sculpture (2016). The lower
part of the sculpture is generally degraded compared with the upper part

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Fig. 4 Replica in the Hiroshima Prefectural History Museum (made in1986). The replica has continuously been located inside theMuseum. The lower mass below the stomach of the sculptureis dark-colored

three-dimensional measurement based on photogrammetry has also been applied to measure cultural heritage
because the technique is simple and quick comparedwith the laser scanning method (Westoby et al. 2012;
McCarthy 2014; Fujii et al. 2018). Therefore, it is expected that the degradation speed of cultural heritage
may be easily estimated without any special equipment,
by measuring the same target twice with an interval
using such a method and comparing the data.
However, it is necessary to understand long-term weathering for long-term conservation of cultural heritage. To
detect a long-term degradation, an adequate length of interval is required. To solve this problem, it is expected to beeffective to measure an old replica of a target made in thepast together with the original. If the replica is close enoughto the original, the difference can basically be regarded as
the progress in the degradation of a target since the replica
was made. In this study, the degradation of the Wareishi
Rock Cliff Sculpture in the previous 30 years is discussed bycomparing it with its replica made 30 years ago.
Wareishi Rock Cliff Sculpture

The target of this study is the Wareishi Rock Cliff
Sculpture (Hiroshima Prefecture 2008) (Fig. 1a), designated as an important cultural property of the Hiroshima
Prefecture, in the Sagi Island, Mihara City, Japan (Fig. 2).
An inscription besides the sculpture shows that it wascarved in A.D. 1300, directly on a core stone (Holmes
1978) of Cretaceous Hiroshima Granitic Rocks, locatednear a shoreline. No salt efflorescence has ever been reported on the core stone. The sculpture is a bas-relief ofan approximately 1 m tall sitting Buddha. The ~ 4 m
globular granitic core stone appears above the sea level
during low tides (Fig. 1a), whereas a part or all of it sinks
below the sea level during high tides (Fig. 1b). The averageFig. 5 Photographing the Wareishi Rock Cliff Sculpture. It took approx. 6
min using a compact digital camera basically by one person during low
tide. The person is 170 cm tallsea level during the high tide of 2016 was estimated to bearound the chin or neck of the sculpture based on the tidetable in Itozaki (Japan Meteorological Agency 2017), thenearest point of the Meteorological Agency (Fig. 2). The
lower mass of the core stone below the chest of the
sculpture is covered with dark-colored materials that seemto be algae (Fig. 1a).
About the degradation state of the sculpture, the original surfaces are supposed to be still retained on theupper body, including the face, because of the sharpnessof the edges (Fig. 3). On the contrary, the edges of thelower body, including the garment, are not sharp, indicating that the lower part of the sculpture is degradedmore than the upper part (Fig. 3). Local people emotionally insist that the degradation of the sculpture is progressing rapidly because of the impact of sea waves;however, this notion does not have any scientific data.
Because an investigation must be conducted only during
low tide, quantitative data about the sculpture’s degradation state have not been obtained to date.
The replica

There is a replica of the Wareishi Rock Cliff Sculpture
made of fiber-reinforced plastics (FRP) in 1986 (Fig. 4),
which is located in the Hiroshima Prefectural History(2019) 6:12

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Museum (Hiroshima Prefectural History Museum 1998). It
is said that the replica was made not by imitating with human eyes but by directly copying the sculpture using amold; first, the mold was formed by using silicon on theoriginal sculpture through thin vinyl sheets, and then, thereplica was made with FRP styling in the mold. Because thereplica has been conserved inside the Museum since then,the difference between the shapes of the present Wareishi
Rock Cliff Sculpture and of the replica can be basicallyregarded as the degradation during the previous 30 years.
The lower part of the replica is also dark-colored, but theupper boundary of the darkened part is around the stomach of the sculpture (Fig. 4), i.e., lower than that of thepresent Wareishi Rock Cliff Sculpture, around the chest(Fig. 1a).

the planar distribution of displacement. Fifty-seven photographs (~ 70 cm wide each) were taken of the original
sculpture, and 43 photographs (~ 120 cm wide) of the replica were taken using a digital camera “PowerShot G7X”
from Canon Company. The measurement, conducted inJune 2016 by a single person, took ~ 6 min for the
sculpture and ~ 4 min for the replica.
There are several artifacts around the bottom of the original sculpture built in later eras (Fig. 1a), which hamper
the observation of the surface of the original sculpture.
Therefore, the measured area of the original sculpture wassmaller than that of the replica.

Kuchitsu et al. Progress in Earth and Planetary Science

Methods/Experimental
Photography

The measurement is required to be carried out easily andquickly because it should be done only during low tide for
efficiency. For this reason, multi-view stereo techniquebased on photogrammetry was used in this study. This
technique measures the unevenness of targets not only on
particular spots but also on the whole area by taking photographs from various viewpoints (Fig. 5) and matchingthem. In this study, this technique was applied to detect

Matching

Three-dimensional coordinates were constructed by thephotographs using the multi-view stereo technique by Sakai
et al. (2015). First, the positions of the camera center andthe optical axis of each photograph were estimated using
OpenMVG. Second, by combining more than two photographs of the same spot taken from various viewpoints, partial three-dimensional coordinates were calculated based on
the principle of binocular stereopsis. Then, the total
three-dimensional coordinates of the Wareishi Rock Cliff
Sculpture and of the replica were integrated by compilingthe partial three-dimensional data. The three-dimensional

Fig. 6 Orthographic projection of the Wareishi Rock Cliff Sculpture. The x-y line shows the position of the cross section (Fig. 10)

(2019) 6:12

Page 5 of 9model of the original sculpture is shown in Fig. 6 and thatof the replica in Fig. 7.

detected within a partial three-dimensional form
model (Fig. 8) is always less than 3 mm; thus, the influence of swelling or shrinkage of FRP is negligible.

Kuchitsu et al. Progress in Earth and Planetary Science

Detecting the displacement

After adjusting each three-dimensional form model,
the smallest deviation between the Wareishi Rock
Cliff Sculpture (Fig. 6) and the replica (Fig. 7) was
calculated by aligning the three-dimensional models
through the method of iterative closest point (ICP),
minimizing the total deviation of the models (Besland McKay 1992). The software of Geomagic XOS of
the SMARTTECH Company was used in this step.
The calculation was carried out on the 31 partial
three-dimensional form models (Fig. 8), to avoid impact from broad distortion caused by swelling or
shrinkage of FRP. Each partial model area (Fig. 8)
was selected to keep the broad distortion smaller thanlocal displacements. Measurement error of thismethod is theoretically estimated to be less than 0.7
mm based on former research about multi-view stereo (Sakai 2015). Places where more than 3 mm ofdisplacement was detected were extracted as the degraded area in the previous 30 years by this study torecognize the difference by naked eyes. The distortion

Fig. 7 Orthographic projection of the replica

Results
The total measured comparable area between the original sculpture and the replica was 2.62 m2. The maximum displacement between the original sculpture
and the replica was detected as 9.45 mm and themean displacement as 0.878 mm. The areas wheremore than 3 mm of difference was detected between
the original sculpture and the replica are indicated on
the three-dimensional form model of the replica
which can be regarded to retain the shape closer tothe original shape of the sculpture carved in A.D.
1300 (Fig. 9). The total area where more than 3 mmof difference is observed is 0.0148 m2, which corresponds to ~ 0.56% of the whole measured area.
The difference between the shapes of the original
sculpture and the replica was detected mainly on the
middle part of the core stone constituting a horizontal belt ~ 20 cm parallel to the sea surface (Fig. 9).
The upper surface of the belt where displacement
concentrates concurs with the upper surface of theKuchitsu et al. Progress in Earth and Planetary Science(2019) 6:12

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Fig. 8 Positions of the partial three-dimensional form models where the difference between the original sculpture and the replica was calculated.
Each partial model area was selected to keep the broad distortion smaller than local displacements, free from swelling or shrinkage of FRP

dark-colored area of the original sculpture (Figs. 1aand 9), and the lower limit of the belt concurs withthe upper surface of the dark-colored area of the replica (Figs. 4 and 9).

Discussion
The present shape of the core stone is nearly globular
without extreme encroachments (up to several centimeters in depth: Fig. 1a). If weathering of the corestone was severe, some geomorphological features
should be detected. The area where the displacement
was detected between the original sculpture and thereplica does not cover a wide range (only ~ 0.56%),
indicating that the current detectable degradation of
the Wareishi Rock Cliff Sculpture has occurred only
in limited areas.
The detected displacement areas are scatteredmainly within the belt-shaped area where dark colorization has progressed in the previous 30 years. The
dark-colored area, the upper line of which concurs
with the average seawater surface during high tide, isconsidered to relate to the supply of the seawater
during high tide (Fig. 10). Then, it is possible that therise in sea level in the previous 30 years has resulted

in the degradation during that period. In fact, theaverage sea level in Hiroshima, the nearest spot where
the continuous sea level data of these 30 years areavailable (Fig. 2), has risen up to 20 cm (Geospatial
Information Authority of Japan 2018), and there areno significant active faults between Sagi Island andHiroshima. In other words, the average sea level during high tide may be related to the degradation of the
sculpture physically and/or chemically. Because thelower part of the sculpture has not retained the original surface (Fig. 3), it can be concluded that thedegradation of the Wareishi Rock Cliff Sculpture has
not been proceeding only recently but continuously
since being carved.
If the degradation speed is fixed, the total degradation can be calculated based on the displacement of
the previous 30 years, ~ 0.56% in the whole measurement area (0.56%/30 years × 716 years = 13.4%); a total
degradation of 13.4% occurred after being carved. The
past degradation is assumed to have progressed
mainly in the lower part of the sculpture, because theaverage sea level during A.D. 1300 has been estimatedto be much lower than the present level based on thestudies of local climatological change in Japan (Tange

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Fig. 9 Displacement map between the Wareishi Rock Cliff Sculpture and the replica. Ortho-image facing to the replica. Red represents more than 3 mm ofthe detected displacement, and green represents less than 3 mm of the detected displacement. a (pink line): The upper line of the present alga lushness
area. b (blue line): The upper line of the alga lushness area in 1986 (Lines “a” and “b” were drawn by the same person based on the naked eye observation.)2013). The original surface seems to be mainly lostby more than 13.4% on the lower part of the present
sculpture, indicating that the previous degradation
was not considerably smaller than the recent degradation. Consequently, it is not considered that the
Wareishi Rock Cliff Sculpture has recently been suffering from increasing amount of rapid degradation
during the previous 30 years. The present condition ismore likely to be caused by the historically continuous degradation.
However, if the sea level rises further, it is possiblethat the upper essential part of the sculpture will be
subjected to degradation, i.e., the face. In such a scenario, some countermeasures may be required. Conservation measures must be discussed by considering
the process to remove the degradation causes rather
than strengthening the object. Because the degradation area basically concurs with the average sea level
during high tide, floating materials such as driftwood
are considered to be one of the causes of degradation.
In addition, some barriers in front of the sculpture
may become an effective measure against further degradation by such causes than consolidation of the
sculpture with synthetic resins. Thus, not subjective

but prudent and objective consideration is required
for the conservation of cultural heritage.

Conclusions
The degradation state of the Wareishi Rock Cliff
Sculpture, an important cultural property of Hiroshima Prefecture, was discussed based on thethree-dimensional measurement using multi-view stereo of the present sculpture and the replica made in1986. The area where more than 3 mm of displacement was observed constituted ~ 0.56% of the wholemeasured area, and the area was distributed in a 20
cm high belt-shaped portion parallel to the sea surface, indicating a possibility that the rise in sea level
during the previous 30 years may have influenced thispartial degradation. Therefore, some countermeasures,such as building barriers in front of the sculpture,
may be required if the degradation extends to theface of the sculpture in the future.
In this study, 30-year degradation of the target
monument has been revealed; further, the conservation policy has been proposed based on the measurement of the original and the replica at the same time.
Because the method is relatively simple and quick, itKuchitsu et al. Progress in Earth and Planetary Science(2019) 6:12

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Availability of data and materials
The data of this study are available from the corresponding author on
reasonable request.
Authors’ contributions
NK proposed the topic and conceived and designed the study. MM
promoted the cooperation with coauthors and owners of the targets. SS andHU clicked photographs and calculated the three-dimensional coordinates.
All the authors have read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Tokyo National Research Institute for Cultural Properties, 13-43 Ueno Park,
Taito-ku, Tokyo 110-8713, Japan. 2Toppan Printing Co., Ltd, 1-3-3 Suido,
Bunkyo-ku, Tokyo 112-8531, Japan.
Received: 30 March 2018 Accepted: 15 January 2019

Fig. 10 Cross section of the Wareishi Rock Cliff Sculpture. The
positions of “x” and “y” are shown in Fig. 6. a (pink line): The upper
line of the present alga lushness area probably represents thepresent average sea level during high tide. b (blue line): The upper
line of the alga lushness area in 1986 probably represents theaverage sea level during high tide in 1986

will contribute to the conservation of other cultural
heritages. Moreover, replicas are sometimes made forgeological characteristic outcrops, such as faults,
unconformities, folds, and fossil containing outcrops.
In such a case, this method will contribute to the discussion of the weathering rate of various rocks basedon the displacement during a long period.
Acknowledgements
The measurement of the Wareishi Rock Cliff Sculpture was performed
through the support of Mr. Hideyuki Matsuda from the Mihara City; he also
provided permission for the publication of this result. The measurement of
the replica was executed in the presence of Mr. Tomohiro Yamamoto of
the Hiroshima Prefectural History Museum who also obtained thepermission for the publication of the result. The authors express theirsincere gratitude to them.

Funding
This work was supported by the “Research on the degradation factors of
outdoor monuments and their conservation measures” funded by the Tokyo
National Research Institute for Cultural Properties.

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