ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXX
International CIPA Symposium 2023, June 25 -30, 2023, Florence Italy
Evolution of Recording Methods: The Aachen Cathedral World Heritage
Site Documentation Project
a
b
b
b
b
b
c
D. Pritchard* , M. Griffo , M. Attenni , R. Barni , C. Bianchini , C. Inglese , Y. Ley
a
Robert Gordon University, Aberdeen, Scotland – d.pritchard1@rgu.ac.uk
b
Dipartimento di Storia, Disegno e Restauro dell'Architettura, SAPIENZA Università di Roma, Piazza Borghese 9, 00186 Roma Italy
(marika.griffo, martina.attenni, roberto.barni carlo.bianchini, carlo.inglese) @uniroma1.it
c
RWTH Aachen University, Lehrstuhl für Architekturgeschichte, Chair of Architectural History, Schinkelstraße 1 D-52062 Aachen
ley@ages.rwth-aachen.de
KEYWORDS: 3D digitisation, recording methodology, dimensional quality, terrestrial laser scanning, digital photogrammetry.
ABSTRACT:
Modern terrestrial laser scanners and photogrammetric imaging systems can provide highly accurate and objective as-built records of
existing architectural, engineering, and industrial sites. This comprehensive digital recording benefits culturally significant places like
heritage buildings, monuments, and other vital structures. The collected data can be instrumental in various ways, including aiding in
conservation, management, monitoring and repair efforts and serving as an educational resource for scholars and the general public. These
technical capabilities are especially well-suited for architecturally complex, ornate buildings like the Aachen Cathedral UNESCO World
Heritage site. This paper describes the recent recording efforts at the Aachen Cathedral and is a comparative study of the previous
documentation work done at the Cologne Cathedral.
The 3D documentation of the Aachen Cathedral UNESCO World Heritage Site is an ongoing collaborative project between the Sapienza
Università di Roma, Rome, Italy, Robert Gordon University, Aberdeen, Scotland, and in partnership with RWTH Aachen University, and
the Dombauhütte Aachen.
1.
INTRODUCTION
Ongoing technological advancements have significantly
improved the performance and capabilities of remote sensing
technologies. These systems make it possible to comprehensively
record cultural heritage sites faster, more efficiently, and less
costly than in previous years. To gauge the impact of
technological evolution in this field, studying and comparing the
latest systems and methods at a World Heritage site with previous
similar projects is essential. Additionally, looking at past and
recent events, important heritage sites and collections must be
appropriately and precisely documented.
The Aachen Cathedral and Cologne Cathedral UNESCO World
Heritage sites in Germany are excellent comparative
documentation case studies. The two structures are the most
famous cathedrals in Germany and are excellent examples of
Gothic architecture. The Cologne Cathedral represents High
Gothic architecture, while the Aachen Cathedral combines
Romanesque and Gothic architectural styles. Cologne was
documented in 2017 (Pritchard et al., 2017), and Aachen is an
ongoing project.
Digitally documenting these sites regularly is necessary to keep
pace with the latest technological advancements and ensure we
have the most up-to-date and accurate information. Given the
rich cultural history of the two cathedrals and their importance as
World Heritage sites, studying and comparing the latest digital
documentation methods and systems at this location can provide
valuable insights into the effectiveness and potential of these
technologies in protecting and preserving our cultural heritage
for future generations.
Article 29 of the World Heritage Convention requires all
UNESCO World Heritage properties to provide regular reports
on the condition at their site. These reports are used to assess the
sites and, if necessary, to decide on adopting specific measures to
resolve recurrent problems. In the Third Cycle of the Periodic
Reporting (2018-2024), the Aachen Cathedral described the 3D
laser scanning and imaging project, demonstrating the value of
3D documentation as a digital preservation and conservation tool
(UNESCO).
2.
HISTORY OF THE CATHEDRAL
The UNESCO World Heritage Site of Aachen Cathedral has a
long and varied architectural history. Around 1200 years ago,
Charlemagne established his court as a Palatine complex at the
*
Corresponding author.
During the Middle Ages, pilgrimages to Aachen to visit the tomb
of Charlemagne and the precious relics increased steadily. Then,
since the year 1349, sanctuary journeys have taken place every
seven years, during which the four relics - the diapers of Jesus,
the loincloth of Jesus, the garment of Mary and the beheading
cloth of John the Baptist - are presented to the pilgrims for ten
days (Gormans A., Markschies A., 2012).
The large flows of visitors to Aachen Cathedral required a series
of expansion measures: First, in 1350, two Gothic chapels were
added on top of the Carolingian stair towers of the west building
for the storage and instruction of the relics, and in the mid-14th
century an ornate and roofed breach was built between the west
building and the Octagon (Faymonville, 1916, p. 90). Between
1355 and 1414: The construction of the Gothic Choir Hall. To
connect the new building with the octagon, the eastern end of the
original Carolingian building was demolished, and the interior
was expanded extensively. From now on, the Charlemagne shrine
with the bones of Charlemagne and the Mary shrine with the four
relics found their place there.
Figure 1: Carolingian ensemble of structures in comparison to the
current building ensemble of the Cathedral. Judith Ley / drawing:
Frédéric Schnee.
end of the 8th century (Figure 1). The focus was not on the
creation of a settlement but on the construction of impressive
stone buildings, which were visible at a distance on a hill in the
middle of the valley basin, distinguished from the other houses
made of wood and clay in half-timbered construction and thus
symbolised the new centre of the empire as well as the exaltation
of the kingship. In addition, the complex provided sufficient
space for the administration, economy, arts and sciences, thus
uniting central functions in one place. Aligned in an exact northsouth axis, the secular palace building Aula Regia was in the
north, and the original building of Aachen Cathedral, the Palatine
Chapel, was in the south. The site consisted of an octagonal
central building with a sixteen-cornered gallery and a west
building as an entrance portal dedicated to Mary (Pieper /
Schindler 2017, p. 31, 32).
After the death of Charlemagne, Aachen developed into the
coronation site of the German kings from the 10th to the 16th
century. The building underwent extensive construction and
expansion measures, of which only the most important can be
mentioned below.
For the time of the Romanesque building epoch, the construction
of a cloister, the Quadrum, as well as the emergence of three
chapels can be proven in the 11th century, which were rebuilt,
expanded or demolished in the following centuries and are
therefore preserved today only in a few fragments. (Buchkremer
1955, p. 58).
Aachen Cathedral was continuously expanded in the following
centuries by chapel extensions. In the southwest, the Matthias
Chapel, the oldest chapel extension, was built at the same time as
the Gothic Choir Hall. The first floor of this Gothic chapel,
preserved to this day, was used from the beginning as a sacristy
and the upper floor as an archive. The Anna Chapel was added to
the west side of the Matthias Chapel in the middle of the 15th
century. Initially, the expansion housed arcades with entrances to
the cathedral on the first floor, while the chapel room was on the
upper floor. At the end of the 18th century, the first floor was
closed, connected to the Matthias Chapel and has since been used
as an extension of the sacristy (Maintz, 2007). To the southwest
is the extension of the Hungarian Chapel, built in 1357 as a
Gothic chapel on a rectangular ground plan with a hipped roof
and donated by King Ludwig of Ungan. Due to a lack of
maintenance and the city fire of Aachen in 1656, the chapel was
so severely damaged that it was replaced by a new Baroque
building in 1746. (Siebigs, 2000).
To the northwest of the cathedral, the Gothic Chapel of St.
Nicholas and St. Michael was built between 1473 and 1485. It is
located on the site of a Romanesque predecessor building, which
is still remembered by three Romanesque portals in the current
building stock (Konnegen, 2015). The large building, in addition
to its liturgical function, with six altars, whose whereabouts are
unclear, provided space until the 18th century for the burial
places of the clergy of the canonry (Faymonville, 1916, p. 65),
whose memorial plaques are still visible there today (Hugot,
1988, p. 48).
The Chapel of Charlemagne and St. Hubert was built in the
northeast in 1455. In this place, a vigil was kept by the future
kings before their coronation (Hugot, 1988, p. 48). The Gothic
building on a heptagonal plan houses the chapel of St. Hubert on
the first floor with an entrance to the cathedral, and on the upper
floor, there is a chapel dedicated to Charlemagne (Grimme, 2000,
p. 105).
The Romanesque cloister northwest of the building ensemble
was replaced in the 15th century by a new Gothic building
(Faymonville, 1916, p. 58). The square it enclosed, the so-called
Quadrum, passed through many functions, from a cemetery to a
garden and the school building site to the present-day schoolyard
of the cathedral school. The Gothic building had richly decorated
vaults, partially replaced by simple vaulted constructions after
withstand the detonations of many bombs. The foundation of a
cathedral fire brigade, which extinguished smaller fires from
1941 onwards, deserves special mention (Schein /Wentzler,
2008).
2.1 UNESCO World Heritage
At the second session of the UNESCO World Heritage
Committee in July 1978, Aachen Cathedral was listed as one of
the first twelve World Heritage Sites and the first German one.
The outstanding universal value of the building is achieved by its
great historical value as a unifying symbol of Charlemagne's
empire and its unique structural characteristics.
Figure 2: Image of the Catherdral bell tower.
The following four criteria were established for inscription as a
World Heritage Site: Criterion (i) identifies the Palatine Chapel
as an exceptional artistic creation and the first vaulted structure
north of the Alps. Criterion (ii) describes the significance of the
building as having made it a prototype of religious architecture
that has inspired copies and imitations. Criterion (iv) emphasises
that the structure is an excellent example of the family of Aulien
chapels based on a central plan with tribunes. Criterion (vi)
describes the historical importance of the burial place of
Charlemagne and the coronation place of German emperors until
1531, as well as the invaluable significance of the cathedral
treasure.
On the one hand, the World Heritage Committee concluded the
integrity of the building since all the features and structures
convey its significance as the Palatine Chapel of Charlemagne is
present. On the other, the authenticity of the building is noted
since its form and design, material and substance, use and
function as a church, and the most important place of pilgrimage
north of the Alps have remained unchanged.
2.2 Historic Threats to the Cathedral
Figure 3: Ceiling of the Palatine Chapel.
widespread destruction by the town fire in 1656 (Faymonville,
1916, p. 104). With few exceptions, the surviving Gothic vaults
were destroyed during the Second World War (Buchkremer,
1955, p. 57).
During the Baroque era, the cathedral's interior underwent a
contemporary revision, emphasising a decoration with the
symbolism of the veneration of Charles and pilgrimage.
However, today there are no reliable sources about the Baroque
decoration in detail (Grimme, 2000, p. 113). The most
recognisable constructional measures to date are the new
construction of a one-story portal porch in front of the west
building of the cathedral in 1788 (Clemen, 1916, p. 107) and the
new construction of the Hungarian chapel to the south of it
starting in 1756 (Siebigs, 2000).
In the following period, the cathedral was repeatedly the scene of
political events. During the Secularisation at the end of the 18th
century, the ancient columns of the Octagon were removed by the
French and transported to Paris to be later installed in the royal
palace (Konnegen, 2016, p. 39). With the Second Peace Treaty of
Paris in 1815, the Kingdom of Prussia decided to return the
columns to Aachen, but they could not be reinstalled in the
Octagon until 1840 (Faymonville, 1916, p. 69).
The Second World War caused extensive damage to Aachen
Cathedral, but the building and the surrounding ensemble could
During the 1200-year history of Aachen Cathedral, the building
has been confronted with various threats. To give an insight into
these, three categories of threats will be briefly presented: The
threats posed by political developments and disputes, as well as
those of a structural-technical nature and those posed by the
pilgrims coming to Aachen since the Middle Ages.
The threats of political developments and disputes have already
been illustrated using the example of the removal of the ancient
columns in the Octagon as part of Secularisation and the later
reinstallation by the Kingdom of Prussia and the damage caused
by the Second World War. The city of Aachen and Aachen
Cathedral has always been at the centre of European disputes
since the city and the building have a tremendous symbolic
character for European identity and, due to their location in the
border region between France, Belgium, the Netherlands and
Germany, they were also geographically at the centre.
Challenges of a structural-technical nature have also
accompanied the building over the centuries of its history. In this
context, the heterogeneity of the various components must be
emphasised, which, in addition to stylistic differences, is
accompanied by very complex joints and many materials used.
The anchor system for the static stabilisation of the Gothic Choir
Hall can be cited as an illustrative example. A system of iron
rings and cross ties was used to connect it to the stable central
structure of the Octagon. However, since a static movement of
the Gothic Choir Hall towards the east was discovered at the
beginning of the 20th century, the system had to be analysed and
strengthened again (Siebigs, 1997). This shows that the
extensions of the Aachen Cathedral have been a constant
conditions has become a very present task. For this purpose,
unique materials were developed during construction work on the
cathedral to protect the preserved building fabric and supplement
it in the long term. As climatic conditions change, these materials
must be reviewed and possibly redeveloped. An example of how
changing conditions can be seen at the cathedral is the sculptures
attached to the exterior of the Gothic Choir Hall. Due to
exponentially increasing CO2 emissions over the last centuries, a
dark shell formed around the artworks, initially visible in lightcoloured limestone, as a result of deposits. This is currently being
removed using a specially developed process.
Figure 4: Terrestrial scaning from parapet level.
The additional consequences of mass tourism, in addition to the
challenges of pilgrimage already described, form an essential
task of the present at Aachen Cathedral. On the one hand, action
is being taken on a technical level, for example, by installing a
CO2 extraction system inside to protect the mosaics from
damage. In addition, the humidity inside is constantly regulated
by the heating system. On the other hand, action is taken on an
organisational level. Guides and accessible areas are designed to
protect the structure and equipment to the greatest extent
possible.
3. RESEARCH OBJECTIVES
The 3D documentation of the Aachen Cathedral UNESCO World
Heritage Site is a collaborative project between the Sapienza
Università di Roma, Rome, Italy, Robert Gordon University,
Aberdeen, Scotland, in partnership with RWTH Aachen
University, and the Dombauhütte Aachen. This ambitious and
multi-phase project has two critical objectives, as detailed in the
two associated papers.
Figure 5: South view of Cathedral roofs.
challenge for the building over the centuries up to the present
day.
Another source of threat is the impact on the building resulting
from the enormous streams of pilgrims who have been coming
every seven years since the 14th century to admire the relics and
the royal throne. The throne is a vivid example of the fact that
visitors actively wear down the architectural-historical substance
and that this has a long tradition in Aachen. On the one hand,
people have been sitting on the throne for centuries, as apparent
traces of wear can be seen on the hand rests, on the seat and on
the steps leading up to the throne. On the other hand, the base of
the throne shows that it was also used quite actively for a bowing
gesture: Between the supports of the base, there are clear signs of
wear, which indicate that pilgrims crawled under the throne.
2.3 Contemporary Challenges to the Cathedral
For the long-term protection and conservation of the Aachen
Cathedral World Heritage Site, the continuous development of
new solutions for known or unknown problems and the regular
evaluation of previous measures are necessary. Many of the
described historic threads still constitute challenging tasks, some
under new or changed auspices and require special attention.
As described above, the heterogeneous structure of the Aachen
Cathedral already poses a significant challenge to the
maintenance of the building. In the recent past, protecting the
building from environmental and increasingly changing climatic
This first paper presents the methods employed for dimensional
recording using various advanced technologies. This includes the
development of the most effective approaches, methods, and
tools for capturing detailed geometric data of the cathedral. The
second paper describes the procedures used to represent and
analyse the monument, providing an in-depth understanding of
the structural behaviour through the comprehensive
documentation of its geometry and constituent materials.
To achieve these goals, the project introduces a digitally
integrated methodology that combines Terrestrial Laser Scanning
(TLS), Terrestrial Digital Photogrammetry (TDP), and Aerial
Digital Photogrammetry (UAVDP) to accelerate data collection,
provide comprehensive surface coverage, and simplify texture
creation. By mapping deterioration and monitoring the state of
conservation of historic structures, the methodology enhances the
preservation of cultural heritage sites for future generations.
This project builds upon the pioneering work done in the
documentation of other important heritage sites, such as the
Baptistery of San Giovanni in Florence (Bianchini et al., 2020),
the Athena Project (Bianchini et al., 2019), and the Cologne
Cathedral in Germany (Pritchard et al., 2017), the. By examining
and comparing these projects, the Aachen Cathedral project
offers valuable insights into the evolution of documentation
technology, highlighting the importance of continuous technical
innovation in cultural heritage preservation.
4.
THE DIGITISATION PROJECT
4.1 Integration of Capture Systems
Terrestrial laser scanning (TLS), terrestrial digital
skin.
4.2 Terrestrial Laser Scanning
The benefits of TLS in surveying and measuring architectural
structures have been well-established (Costa et al., 2016).
Whereas the Cologne project mostly used the Z+F 5010C
manufactured by Zoller + Frohlich (Z+F), Germany, the Aachen
Cathedral project used two terrestrial laser scanning systems, the
Imager 5010X and Imager 5016.
Figure 6: Rendering of point cloud (reflectance) - south elevation.
Figure 7: Rendering of point cloud (refectance) - cross section.
photogrammetry (TDP), and Unmanned Aerial Vehicle (UAV)
photogrammetry are highly effective technologies for capturing
and processing 3D data of objects, surfaces, and landscapes.
These tools and methods are well-established in documenting,
monitoring and conserving tangible cultural heritage
(Remondino, 2012; Fassi, 2013). On their own, both techniques
can generate exact full-colour 3D models; merging these two
methods of data capture makes it possible to create 3D models
that are both geometrically precise and visually realistic.
TLS provides fast and accurate point data of an object's surface
but with lower image resolution and is restricted to line-of-site
capture. However, TDP generates highly detailed and realistically
coloured points, although with lower metric accuracy and
requires proper lighting.
By integrating laser scanning with terrestrial and drone-based
photogrammetry, the weaknesses of each method can be
accommodated while benefiting from their combined strengths,
resulting in a more comprehensive surface capture and data
coverage, improved modelling accuracy, and better-quality
texture generation. (Galeazzi, 2017)
According to Rönnholm et al., there are four types of data
integration of terrestrial lidar and digital photogrammetry: objectlevel integration, photogrammetry aided by laser scanning, laser
scanning aided by photogrammetry, and tightly integrated laser
scanning and optical images. The Aachen project would be
considered colourised laser scanning aided by photogrammetry.
These two technologies complement each other well, so they
were utilised in the Aachen project. As an analogy, the scan data
acts as the dimensional skeleton, and the photogrammetry is the
All three types of scanners are phase-based measurement systems
that utilise a Class 1 infrared laser, which is invisible and
completely safe for both the operator and the public eyes. This
safety consideration is particularly relevant for scanning
activities at a World Heritage site with heavy tourist activity. The
5010X generates 360-degree point data based on a local
coordinate system with intensity values and has an approximate
range of 187 meters, while the 5016 has a range of almost double
that at 360 meters. The 5010X and 5016 have an acquisition rate
of 1.06 million points per second, with a linearity error of less
than 1mm within 20m from the surface.
All three scanning systems, the 5010C, 5010x and 5016, have an
integrated camera system, the CCD, positioned at the nodal point
of the scanning unit. On the 5010C/X, the camera is positioned
slightly off-centre, so a slight parallax is noticeable on the first
few meters but comparably small. This is not an issue with the
5016. The result is that during the post-processing, the imagery
sits precisely onto the point data without parallax, providing an
aligned, photorealistic scan dataset. The automated data
alignment avoids using a panoramic head or dealing with a
camera offset. Upon completing the scanning sequence, the
scanners initiate a series of 42 individual images that combine to
form an 80-megapixel panoramic image.
The other difference between the 5010C and the 5010X is that
the 5010X has on-site registration (tablet), an integrated GNSS,
IMU, altimeter and compass. The 5016 is approximately 30%
smaller in volume, 30% less in weight, has a parallax-free
camera, and 40% faster image acquisition. The 5016 also has
considerably less range noise.
During the Cologne project, to illuminate dark spaces and avoid
external lighting systems, the 5010C and 5010X used an add-on
Z+F SmartLight. This accessory is mounted on the scanner
housing, resulting in a little contour shadow. The 5016 has a fully
integrated illumination with four LED spotlights around the
rotor's camera window. This system is parallax-free without any
shadowing in the imagery.
Although the Aachen project utilised digital photogrammetry,
adding colour to point cloud data can improve its visualisation
and help to identify different features or objects in the registered
data. Coloured point cloud data can help in better object
recognition and classification, such as different stone colours.
Coloured point clouds can also enhance the accuracy of 3D
mapping and modelling applications. Adding colour to the point
cloud data provides more accurate 3D models and better
represents the real-world environment.
4.3 Digital Photogrammetry
Digital photogrammetry has improved significantly over the last
decade due to advances in camera systems and processing
software (Giuliano et al.). With the advent of high-resolution
digital cameras, image quality has improved significantly,
providing more accurate and detailed data for photogrammetric
processing. Computer processing power has also increased
s ig n ifican tly, p ermittin g fas ter an d mo re complex
photogrammetric processing. Photogrammetric software such as
Reality Capture and Agisoft Metashape has become more
automated, reducing the need for manual intervention (hole
filling) and increasing processing speed. Creating fully textured
‘watertight’ 3D geometry is considerably faster and has a more
straightforward workflow.
The Aachen project incorporated two digital cameras for
terrestrial photogrammetry, a Nikon Z 7II (47.7 MP) mirrorless,
full-frame digital camera with a 24mm prime lens and a Sony
Alpha ILCE-A7 II (24.3 MP) full-frame with a 24mm prime lens.
Aachen Cathedral Documentation
Images
8145
Exterior Scans
128
Interior Scans
162
Cross-over/Threshold Scans
12
Data Bundle Error
0.005m
Overlap Percentage
44%
4.4. UAV Photogrammetry
UAV photogrammetry offers numerous benefits for recording
heritage buildings, including cost-effectiveness, noninvasiveness, safety, and detailed data analysis, making it an
excellent tool for conservation, restoration, and documentation
(Lo Brutto, 2012). The UAV can also go beyond the line of sight
of the terrestrial laser scanner or the terrestrial photogrammetry.
It does not require scaffolding, ladders, or other equipment that
could damage the building's structure or delicate surfaces.
Although UAV systems can carry high-resolution cameras,
thermal cameras and lidar systems, these tend to be heavy and
loud, requiring special permits. The operation of UAVs is subject
to the German Aviation Act (Luftverkehrsgesetz) and the German
Regulation on the Operation of Unmanned Aircraft Systems
(Drohnenverordnung). The regulations also require the
registration of all UAVs weighing more than 250 grams. There
are similar restrictions in the United Kingdom. Considering
these restrictions, the project used a DJI Mini 3 Pro drone
weighing 249 grams. The unit has a 1/1.3-inch CMOS camera
sensor providing 48 MP imagery and an f/1.7 24 mm lens.
The UAVDP and TDP utilise the same 24mm lens focal length to
aid the photogrammetric processing.
5.
AACHEN CATHEDRAL DOCUMENTATION
5.1 Project Planning
Given the complex nature of the architecture of the Aachen
Cathedral and the need for high-resolution data, the project
utilised short-distance scanning, multiple scan setups, and
extensive digital imaging. This approach ensured that every
detail of the building was accurately captured, from the
intricately decorated interior to various roofs and spires on the
exterior.
The project planning used existing 2D CAD drawings of the
cathedral to determine the positioning of the terrestrial scan
stations by drawing 50m diameter circles around the building to
establish setup locations, ranges and overlaps. This document
was also used to determine the necessary time on site, identify
any health and safety issues, and act as a communication tool for
the Cathedral management. These drawings were also used to
plan the flight paths of the UAV.
Like the Cologne project, the Aachen Cathedral project
considered several factors when positioning the terrestrial laser
scanners, including capturing the building's overall architectural
form and extensive exterior and interior surface area. To achieve
optimal scanning results, factors such as ideal laser range, data
resolution, data overlap, areas of occlusion, and visual
obstruction were carefully considered. The adjacent architectural
Strength
Cloud-to-cloud
0.004m
0.005
Figure 8: 3D Rendering of the Palatine Chapel, Aachen Cathedral
interior.
context, including adjacent buildings, was also included in the
project's archival dataset. The project encountered several
specific challenges, including the highly decorated and reflective
interior, various parapet levels, and the height of the central bell
tower, like those encountered during the Cologne Cathedral
project's planning stage.
It was decided that at the project's outset, the 5016 would handle
all interior scanning, and the 5010X would provide all the
exterior scanning. The main reason is that although the 5010X
has the capability of an external illumination system, the 5016
has four built-in 700-lumen LED spotlights providing the ability
to capture imagery without an external light source.
5.2 Data Capture and Registration
After five days of scanning and photography, there were 301
individual TLS scans with the 5010X and 5016 and
approximately 8,145 images with the TDP and UAV.
The data was organised per interior, exterior and roof levels for
production purposes. Due to the high number of overlapping
scans, the registrations were successful. There were also 12
cross-over scans connecting interior and exterior datasets.
The individual interior and exterior scan files were brought into
Z+F Laser Control (Sapienza) and Leica Register 360 (RGU).
Data from the registered dataset were then exported as individual
E57 format files, and like the Cologne Cathedral project, into
Autodesk Recap 360 (v. 2023) for further data clean-up.
Although there is an optimisation and reduction of the original
data when importing into Recap, the ease of real-time navigation
for data quality review and client presentation purposes has
proven exceptionally beneficial. For further development, RCP
files can be quickly brought into other Autodesk applications,
such as Autodesk AutoCAD and Revit.
5.3 Visualisation
The acquired TDP and UAV images were organised into interior,
exterior and roof-level groups for production. All the raw files
Figure 9: Rendering of the 3D model of the Palatine Chapel.
were colour balanced and converted to JPEG. The registered E57
files and images were combined in Agisoft Metashape (Sapienza)
and Reality Capture (RGU).
an issue. Also, due to the proprietary nature of the temporary files
and rigid file structure, exporting to a long-term archive would be
problematic.
The UAV photogrammetry filled in missing areas due to
occlusions or hard-to-reach surfaces that the laser scanners may
have missed. Combined with the terrestrial imagery, this
coverage method provided a comprehensive dataset of the
cathedral’s exterior. The UAV was not flown inside the cathedral,
and the interior dataset consisted of laser scans and terrestrial
imagery.
A point cloud dataset of a historic structure can be used to create
accurate measurements and detailed documentation of its
condition and brought into many 3D and CAD packages. Point
cloud data is valuable for documentation and preservation, while
3D mesh models are more useful for visualisation and
presentation. In contrast, the 3D mesh models are cumbersome
for CAD purposes but can be used to create photorealistic virtual
representations for interpretation purposes.
A DJI RS 3 Pro gimble was used to operate the cameras to
remotely speed up the imaging process. The gimble method
worked in the Aachen project, and the laser scans provided the
meshing data, but the images could only be used for texturing.
An additional benefit is that having the system on a high tripod,
the camera was above the heads of the occasional tourist. It is
important to point out that this data capture method is not
recommended for non-laser scanned photogrammetric projects as
there is no parallax between the images. Had the photos only
been used to build the 3D model, it likely would not have
registered correctly.
The Reality Capture software was able to generate an exceptional
photorealistic 3D model. Once exported into an application like
3D Studio Max, generating images was slow but straightforward.
Future development of this project will include integration into
the Unreal Game Engine.
One of the challenges of the 3D photogrammetric approach is
that even with decimation, the size of the meshed model can be
challenging on average-specified computers. There is also an
issue with a strict file structure system, making interoperability
A secondary benefit of a photogrammetric project is that it
requires comprehensive imaging of a heritage site's primary and
secondary spaces. If the images are correctly organised, they can
provide an additional, valuable building and artefact collection
information source. If a cultural heritage property were ever
damaged or destroyed, the images would be used for
reconstruction efforts. If such an unfortunate situation occurred,
no one would likely complain about having too much data.
The acquired data was used to analyse the chapel and is reflected
in the subsequent paper: The Vaulting System of the Palatine
Chapel: The Aachen Cathedral World Heritage Site
Documentation Project.
6.
COMPARISON OF PROJECTS
Regarding the evolution of laser scanning recording methods at
Aachen and Cologne, there was a slight increase in scanning and
imaging speed with the 5016 vs the 5010X. The acquired data
from the 5016 seemed to have less noise, but hard to determine
as it was mainly used indoors. Outdoors, both systems provided
well-balanced colour imagery, but the built-in LED light helped
illuminate dark interiors. Still, the real change was the ability to
review and register the scans via a wifi-connected tablet.
Ergonomics is often an underappreciated consideration when
purchasing a TLS. The design of the 5016 is different from the
5010X and includes two fixed handles and an anchor point,
which is highly beneficial when setting up the scanner on uneven
surfaces on a heritage structure. These features are essential when
lifting, repositioning or securing the scanner. The reduced size
and weight of the 5016 vs the 5010X add to the system's
versatility.
The most significant change between the Cologne and Aachen
project is the integrating of TLS, TDP, and UAV digital
photogrammetry. The combined systems will increase the
documentation time at a heritage site and possibly require
certified UAV staffing. However, the increased recording
capabilities and quality of data are significant.
7.
CONCLUSION
The Aachen Cathedral documentation project is a collaborative
effort between the Sapienza Università di Roma, Rome, Italy,
Robert Gordon University, Aberdeen, Scotland, and in
partnership with RWTH Aachen University and the Dombauhütte
Aachen that aims to comprehensively document the cathedral
architecture and constituent materials using advanced
technologies. The project offers practical insights into the
evolution of documentation technology, highlighting the
importance of continuous technical innovation in cultural
heritage preservation. It was an opportunity to gauge the
development of recording methods from the Cologne project by
using a more current laser scanner and integrating digital
photogrammetry and UAV technology into the production
system.
The project provides a digitally integrated methodology that
simplifies texture creation and accelerates data collection by
combining terrestrial laser scanning, terrestrial digital
photogrammetry, and aerial digital photogrammetry. Despite
encountering specific challenges during the project, the team
carefully considered multiple factors to achieve optimal scanning
results, including the building's overall architectural form,
extensive exterior and interior surface area, and adjacent
architectural context. Ultimately, the project and documentation
methodology enhances the conservation of cultural heritage sites
for future generations.
ACKNOWLEDGEMENTS
The authors of this paper would like to thank the contributions
and assistance from Dipl.-Arch. Bruno Schindler, Lehrstuhl für
Architekturgeschichte, RWTH Aachen University, Dombauhütte
des Aachener Doms, and Dr. Jan Richarz, Dombaumeister,
Aachen Cathedral.
REFERENCES
Allegra V., di Paola F., lo Brutto M., Vinci C., Scan-to-Bim for
the management of heritage buildings: the case study of the
castle of Maredolce (PALERMO, ITALY), Int. Arch. Photogr.,
Remote Sens. Spatial Inf. Sci. 43 (B2) (2020) 1355– 1362,
doi:10.5194/isprs-archives-XLIII-B2-2020-1355-2020.
Aicardi, I., Chiabrando, F., Lingua, A.M., and Noardo, F. 2018.
Recent trends in cultural heritage 3D survey: The
photogrammetric computer vision approach. Journal of Cultural
Heritage 32, 257–266.
Bassier, M., Vincke, S., Hernandez, R. de L., and Vergauwen, M.
2018. An Overview of Innovative Heritage Deliverables Based
on Remote Sensing Techniques. Remote Sensing 10, 10, 1607.
Boehler, W., Bordas Vicent, M. and Marbs, A., 2003.
Investigating laser scanner accuracy. In: Proc. of XIXth CIPA
Symposium, Antalya, Turkey, 30 Sept. – 4 Oct. 2003.
Bolognesi, C. Villa D. (eds.), From Building Information
Modelling to Mixed Reality, Springer Tracts in Civil
Engineering, DOI: 10.1007/978-3-030-49278-6_2
Bianchini, C., A Methodological Approach for the Study of
Domes. Nexus Netw J 22, 983–1013 (2020).
Bilis, T., Kouimtzoglou, T., Magnisali, M., Tokmakidis, P., 2017.
The International Archives of Photogrammetry. Remote Sensing
and Spatial Information Sciences XLII-2/W3, 2017 3D Virtual
Reconstruction and Visualization of Complex Architectures, 1–3
March 2017, Nafplio, Greece.
Buchkremer J., 1955, Der Dom zu Aachen. In: Beiträge zur
Baugeschichte. III: 100 Jahre Denkmalpflege am Aachener Dom.
Aachen.
Clemen P., 1916, Die Kunstdenkmäler der Stadt Aachen – Das
Münster. In: Die Kunstdenkmäler der Rheinprovinz. 10. Band,
I.2 Das Münster. Düsseldorf, L. Schwann.
Costa, E., Balletti, C., Beltrame, C., Guerra, F., Vernier, P.,
Digital survey techniques for the documentation of wooden
shipwrecks The International Archives of the Photogrammetry,
Remote Sensing and Spatial Information Sciences, XLI-B5
(2016) XXIII ISPRS Congress, 12–19 July 2016, Prague, Czech
Republic
Fassi, F., Fregonese, L., Ackermann, S., De Troia, V., 2013.
Comparison between laser scanning and automated 3d modelling
techniques to reconstruct complex and extensive cultural heritage
areas. In: The International Archives of the Photogrammetry,
Remote Sensing and Spatial Information Sciences, Vol. XL-5/
W1, 2013 3D-ARCH 2013 - 3D Virtual Reconstruction and
Visualization of Complex Architectures, 25 – 26 February 2013,
Trento, Italy, pp. 73-80.
Faymonville K., 1916, Die Kunstdenkmäler der Stadt Aachen.
In: Die Kunstdenkmäler der Rheinprovinz. Band 10, I. Das
Münster zu Aachen. Düsseldorf, L. Schwann.
Galeazzi, F., 3D recording, documentation and management of
cultural heritage, International Journal of Heritage Studies, 23
(2017) 671-673.
Gawronek P., Noszczyk T., Does more mean better? Remotesensing data for monitoring sustainable redevelopment of a
historical granary in Mydlniki, Kraków. Herit Sci 11, 23 (2023).
https://doi.org/10.1186/s40494-023-00864-0
Gonzalez-Aguilera D., et al., Terrestrial laser scanning for
cultural heritage documentation: Analysis of state-of-the-art
techniques and tools (2020).
Gormans A., Markschies A. (ed.), 2012, Venite et videte.
Kunstgeschichtliche Dimensionen der Aachener
Heiligtumsfahrt (= Aachener Beiträge zu Pastoral- und
Bildungsfragen. Band 27). Aachen, einhard verlag.
Pfalzkapelle zu Aachen. Berlin, Geymüller Verlag für
Architektur.
Giuliano, M.G., 2014. Cultural Heritage: an example of graphic
documentation with automated photogrammetric systems. Int.
Arch. Photogramm. Remote Sens. Spat. Inf. Sci. 40, 251–255.
Reinoso-Gordo, J.F.;Gámiz-Gordo, A.; Barrero-Ortega, P.,
Digital Graphic Documentation and Architectural Heritage:
Deformations in a 16th-Century Ceiling of the Pinelo Palace in
Seville (Spain).ISPRS Int. J. Geo-Inf.2021,10, 85. DOI:
10.3390/ijgi10020085
Grimme E. G., 2000, Der Dom zu Aachen. Aachen, einhard
verlag.
Guerra F. et al., Terrestrial laser scanning for cultural heritage:
First experiences and new prospects, (2018).
Hugot L, 1988, Der Dom zu Aachen. Aachen, einhard verlag.
Inzerillo G., et al, Documentation of built heritage with terrestrial
laser scanning: A review of recent advances (2020).
Knopp G., Heckner U. (ed.), 2012, Die gotische Chorhalle des
Aachener Doms und ihre Ausstattung. Baugeschichte,
Bauforschung, Sanierung (= Arbeitshefte der Rheinischen
Denkmalpflege. Band 58). Petersberg, Michael Imhof Verlag.
Konnegen L., 2016, Die antiken Säulen des Aachener Domes und
ihr Schicksal in der Französischen Zeit. In: KarlsvereinDombauverein (ed.): Schriftenreihe des KarlsvereinDombauvereins. Band 18: Der Aachener Dom in Französischer
Zeit 1794 bis 1815. Aachen, Thouet.
Julin A., Kurkela M., Rantanen T., Virtanen J. P., Maksimainen
M., Kukk, A., & Hyyppä, H. (2020). Evaluating the Quality of
TLS Point Cloud Colorization. Remote Sensing, 12(17), 2748.
https://doi.org/10.3390/rs12172748
Konnegen L, 2015, Sanierung Nikolauskapelle – Dachstuhl und
Außenwandflächen: Ein historischer Überblick (= Schriftenreihe
des Karlsverein-Dombauvereins. Band 17). Aachen, Thouet.
Lo Brutto, M., Garraffa, A., & Meli, P. (2014). UAV
PLATFORMS FOR CULTURAL HERITAGE SURVEY: FIRST
RESULTS. ISPRS Annals of Photogrammetry, Remote Sensing
& Spatial Information Sciences, 2(5).
Liang H., Li W., Lai S., Zhu L., Jiang W., Zhang Q., The
integration of terrestrial laser scanning and terrestrial and
unmanned aerial vehicle digital photogrammetry for the
documentation of Chinese classical gardens – a case study of
Huanxiu Shanzhuang, Suzhou, China, J. Cult. Herit. 33 (2018)
222– 230, doi:10.1016/j.culher.2018.03.004.
Maintz H.,2007, Sanierung Anna- und
Matthiaskapelle (= Schriftenreihe des KarlsvereinDombauvereins. Band 9). Aachen, Thouet.
Meng X., et al., Terrestrial laser scanning for documentation and
monitoring of historic structures: A review, by Xiaolin (2021).
Nieddu E., et al., Terrestrial laser scanning and digital modelling
for the documentation of Tangible Cultural Heritage: The case
study of the Sanctuary of Santa Maria delle Grazie in Mantova
(2016).
Pritchard, D., Sperner J., Hoepner S., Tenschert R., Terrestrial
laser scanning for heritage conservation: the Cologne Cathedral
documentation project, ISPRS Annals of Photogrammetry,
Remote Sensing and Spatial Information Sciences 2017, DOI:
10.5194/isprs-annals-IV-2-W2-213-2017
Pieper J., Schindler B., 2017. Thron und Altar, Oktogon und
Sechzehneck. Die Herrschaftsikonographie der karolingischen
Remondino F., Heritage recording and 3D modelling with
photogrammetry and 3D scanning. Remote Sens.
2011;3(6):1104–38. https://doi.org/10.3390/rs3061104.
Remondino F., Terrestrial laser scanning for heritage
documentation: the Saint-Antoine abbey as a case study (2014).
Schein K., Wentzler R., 2008, Hoffnung und Gewißheit. Aachens
Dom und Domschatz in Kriegs- und Nachkriegszeit. Dokumente
und Berichte (= Schriftenreihe des KarlsvereinDombauvereins. Band 8). Aachen, Thouet.
Siebigs H.-K , 2000, Die Ungarnkapelle am Dom zu Aachen.
Bauliche Sanierungsmaßnahmen an der Ungarnkapelle des
Domes zu Aachen in den Jahren 1991–1994 (= Schriftenreihe des
Karlsverein-Dombauvereins. Band 3). Aachen, Thouet.
Siebigs H.-K , 1997, Die Chorhalle des Aachener Doms.
Geschichte und Sanierungsmaßnahmen (= Schriftenreihe des
Karlsverein-Dombauvereins. Band 2). Aachen, Thouet.
Soler F., Melero, F. J., Luzón, M.V., A complete 3D information
system for cultural heritage documentation, Journal of Cultural
Heritage, 23 (2017) 49-57.
UNESCO World Heritage: https://whc.unesco.org/en/list/3bis
(accessed 28/04/23)
UNESCO World Heritage Centre: https://whc.unesco.org/en/
periodicreporting/ (accessed 28/04/23)
Virtanen J. P., Daniel S., Turppa T., Zhu L., Julin A., Hyyppä H.,
& Hyyppä, J. (2020). Interactive dense point clouds in a game
engine. ISPRS Journal of Photogrammetry and Remote Sensing,
163, 375-389. https://doi.org/10.1016/j.isprsjprs.2020.03.007
Virtanen, J., Julin, A., Handolin, H., Rantanen, T., Maksimainen,
M., Hyyppä, J., & Hyyppä, H. (2020). Interactive
Geoinformation in Virtual Reality – Observations and Future
Challenges. International Archives of the Photogrammetry,
Remote Sensing and Spatial Information Sciences, 44(4/W1).
https://doi.org/10.5194/isprs-archives-XLIV-4W1-2020-159-2020
Zwegers, B., Cultural Heritage in Transition, Studies in Art,
Heritage, Law and the Market 4, DOI:
10.1007/978-3-030-93772-0_4