Cyfrowa fotogrametria bliskiego zasięgu El Fuerte de Samaipata

Bartłomiej Ćmielewski, Izabela Wilczyńska, Ciechosław Patrzałek, Jacek Kościuk

doi:10.37190/arc200206

Streszczenie

Wpisane na Listę Światowego Dziedzictwa UNESCO El Fuerte de Samaipata jest świadectwem bogatej, wielokulturowej, prekolumbijskiej historii rzeźbienia tej wielkiej piaskowcowej skały. Opisano zastosowanie wielu sposobów precyzyjnej dokumentacji całej skały, w tym klasyczne metody geodezyjne, nowoczesne pomiary w wysokiej rozdzielczości (naziemne skanowanie laserowe) i fotogrametrię bliskiego zasięgu. Fotogrametria bliskiego zasięgu jest techniką niskokosztową, a dostarczona przez nią szczegółowa dokumentacja RGB pomaga w badaniach architektonicznych i archeologicznych. W artykule przedstawiono wyniki zastosowania fotogrametrii bliskiego zasięgu w różnych pasmach promieniowania elektromagnetycznego (widzialnym, podczerwonym i termicznym). Zastosowano kamery Sony ILCE-7RM2, Parrot Sequoia i Flir Tau2. Autorzy zebrali ponad 50 tysięcy obrazów RGB i ponad 27 tysięcy obrazów multispektralnych, co pomnożone przez cztery rejestrowane pasma dało w sumie ponad 100 tysięcy pojedynczych obrazów do przetworzenia. Dane multispektralne i termiczne były szczególnie przydatne przy opracowaniu map ryzyka w celach konserwatorskich.

Podgląd pełnego artykułu jest możliwy wyłącznie na większych ekranach.

2020

2(62)

DOI: 10.37190/arc200206

Introduction

El Fuerte de Samaipata is an archaeological site in the

department of Santa Cruz in Bolivia, located on a rocky

hill at an altitude of about 1950 m above sea level. The site

was constructed and occupied by the Mojocoyas culture

in the fourth century, and it was during this time that the

rock began to be carved. In the 15

century, the site was

conquered by the Inca, who made it a provincial capital.

The strategic location of the site was also recognised by

the Spaniards, who made it an important staging post on

the road from Asunción and Santa Cruz to La Plata (mod-

ern Sucre), Cochabamba, and Potosí (where silver mines

were located) [1].

The archaeological site consists of two parts: the cere-

monial part (14

–16

centuries), and the area to the south

– the residential and administrative part. The ceremonial

part is a sculpted rock that is 250 m long and 60 m wide,

which makes it one of the largest collection of petroglyphs

in the world. The sandstone was carved to create niches,

canals, and zoomorphic designs such as snakes and wild

cats like pumas and jaguars [2].

Bartłomiej Ćmielewski*, Izabela Wilczyńska**,

Ciechosław Patrzałek***, Jacek Kościuk****

Digital close-range photogrammetry of El Fuerte de Samaipata

Cyfrowa fotogrametria bliskiego zasięgu El Fuerte de Samaipata

In this article, we present the results of using a non-met-

ric camera as a tool in terrestrial close-range photogram-

metry for documentation of El Fuerte de Samaipata as

part

of the project “Architectural examination and complex

documentation of Samaipata (Fuerte de Samaipata/Bolivia).

The place on the World Heritage List”, funded by The Na-

tional Science Centre Poland.

Methods

Nowadays, 3D data are extremely useful for perma-

nently storing and documenting important sites in digital

form. Sometimes, they may be the last form to be passed

to the next generations (for example, as heritage may be

destroyed in wars). With technical progress, a large num-

ber of projects now have complete digital models [3]–

[8]. Remote sensing technologies and methodologies are

common for cultural heritage 3D documentation and mo -

deling [9]. They allow very realistic 3D models (in terms

of geometric and radiometric accuracy) to be generated.

Such models can be used for many purposes, including

historical documentation [10], [11], preservation and con-

servation [12], [13], cross-comparisons, monitoring shape

and colour, virtual reality applications [14], [15], 3D re-

positories, web-based Geographic Information System

(GIS), computer-aided restoration [16], multimedia exhi-

bitions [17], and simple visualisations.

The documentation and conservation of cultural herit-

age depends on the availability of means and resources.

Data repositories allow the present state of a site to be de-

scribed and provide metrical information, which is useful

for direct restoration of objects. Nowadays, digital pho-

togrammetry is used in many applications, especially in

* ORCID: 0000-0002-1035-3905. Faculty of Architecture,

Wrocław University of Science and Technology, e-mail: bartlomiej.

cmielewski@pwr.edu.pl

** ORCID: 0000-0002-1397-8118. Faculty of Environmental

Engineering and Geodesy, Wrocław University of Environmental and

Life Sciences.

*** ORCID: 0000-0002-0853-5494. Faculty of Environmental

Engineering and Geodesy, Wrocław University of Environmental and

Life Sciences.

**** ORCID: 0000-0003-0623-8071. Faculty of Architecture,

Wroc ław University of Science and Technology.

36 Bartłomiej Ćmielewski, Izabela Wilczyńska, Ciechosław Patrzałek, Jacek Kościuk

cartography and mapping. It is also used for precise 3D

documentation of cultural heritage [5], [11], [18]–[20],

reverse engineering, monitoring and deformation analy-

ses of structures [21], human movement analyses [22], in-

dustrial measurements [23], urban planning, and forensic

analyses [24]. Innovations in the eld of photogrammetry,

especially matching algorithms, increase of the quality of

spatial data obtained by images.

Sensors

Remote sensing involves the sensing of energy in parts

of the electromagnetic spectrum (Fig. 1), which is the con-

tinuous range of electromagnetic radiation from gamma

rays, through visible light, to radio waves.

To create documentation for archaeological, architec -

tural, and conservation purposes, it is common to use multi-

spectral or hyperspectral sensors that provide images from

satellites. These are commonly considered in literature to

be successful [26]. Other solutions are based on terrestrial

cameras, which also provide good results [27]–[29].

While RGB orthoimages are used for documentation

purposes, another indicator, the Normalised Dierence

Vegetation Index (NDVI), is commonly used in remote

sensing measurements and allows the developmental state

and condition of vegetation to be determined. Vegetation

interacts with solar radiation, weather conditions, and

the nutrients and water content on its roots (usually from

soil). The NDVI can show where soil has more moisture

(which can indicate underground constructions like hid-

den walls) or places that have been degraded by vegeta-

tion – like in the case of Samaipata. The NDVI is based on

the contrast between the largest reection in the near-in-

frared band (which vegetation strongly reects) and the

red band (which vegetation absorbs). The indicator was

used for the rst time by John W. Rouse et al. in 1973 [30]

and is calculated according to the following formula (1):

(1)

where:

VIS – reection in the red band,

NIR – reection in the near-infrared band.

Vibrant, green plants absorb photosynthetically active

solar radiation in the range of 0.4–0.7 μm. Radiation with

longer wavelengths (0.7–1.1 μm) is absorbed to a small

extent. The NDVI ranges from –1 to 1. Higher index val-

ues correspond to a higher reection in the near-infrared

range and a smaller reection in the red range. A high val-

ue of the indicator corresponds to areas covered with lush

vegetation in good condition [31].

Materials and methods

RGB camera

For the close-range photogrammetry, we used an

ILCE-7RM2 DSLR camera from Sony. It has a full frame

(35.9 × 24.0 mm) CMOS Exmor R

sensor and eective

resolution of 42.4 megapixels (image resolution 7952 ×

5304). The pixel dimension is 4.53 × 4.53 µm, which pro-

vides good sensitivity.

Multispectral camera

For multispectral analysis, we used a Parrot Sequoia

camera equipped with four monochrome sensors and one

RGB sensor. The monochrome sensors (global shutter,

1.2 megapixels) collect data in discrete spectral bands:

green (550 nm with 40 nm bandwidth), red (660 nm with

40 nm bandwidth), red edge (735 nm with 10 nm band-

width) and near infrared (790 nm with 40 nm bandwidth).

The RGB sensor (rolling shutter) has a 16 megapixel res-

olution (4608 × 3456) with a focal length of 4.88 mm. It

is complemented by a sunshine sensor with four spectral

sensors (the same lters as the body), and GPS, IMU, and

magnetometer sensors for direct geotagging.

Thermal camera

For thermal analysis, a Flir Tau 2 324 VOx Microbolo -

meter, f/13 mm, 9 Hz camera was used. It has a pixel size

of 25 µm with a better performance than 50 mK at f/1.0.

The sensing spectral band is from 7.5 to 13.5 µm and

advanced radiometry was also enabled. The TeAx Ther-

malcapture module was attached to the camera for direct

Fig. 1. Electromagnetic (EM) spectrum (after: [25])

NDVI =

—————

(NIR + VIS)

(NIR – VIS)

Digitalclose-rangephotogrammetryofElFuertedeSamaipata / CyfrowafotogrametriabliskiegozasięguElFuertedeSamaipata 37

geotagging (from GPS times and coordinates) of images

that were loaded onto an external USB memory.

Surveynetwork

The rst step was to perform a survey, so we used

a Leica TCRP1203 total station with an angular accuracy

of 3 arcsec and a distance error of 5 mm + 2 mm. Next,

we processed the data in local coordinates, and later these

were adjusted to the network. The nal step was to trans-

form the coordinates from the local to the global refer-

ence frame using isometric transformation to the WGS

84/UTM zone 20S, EPSG:32720 coordinate system and

GPS points FS01, FS02, FS03, and FS04. After adjust-

ment, the worst point average square position error was

6.8 mm and the worst point average square height error

was 2.9 mm. Using these adjusted and transformed coor-

dinates, the photogrammetric network was measured and

adjusted. The total number of photogrammetric points

was over 1500, and their distribution on the site is pre-

sented in Figure 2.

Photogrammetryprocessingresultsandworkflow

All acquired data was processed in one of the best piec-

es of software on the market – Agisoft Photoscan Profes-

sional. The workow is presented in Figure 3.

The rst step after importing images was to calculate

the image quality and remove the lowest quality images.

Next, photos were aligned. For high RGB resolutions,

the key point limit was set to 40,000 and the tie point

limit was set to 3,000. For multispectral low-resolution

images, the key point limit was set to 80,000 and the

tie point limit was set to 10,000. Such settings allowed

sustainable image quality and cloud computing power

to be used. After alignment, the markers were loaded,

and the tedious process of pointing markers in pictures

started. Next, optimisation was applied to recalculate

the lens equitation and camera location. After this step,

the error value was checked and the gradual calculation

of the reconstruction uncertainty value of tie points was

calculated. After removing values over 100, optimisation

was conducted again. This step was repeated a few times

Fig. 2. Survey point distribution (elaborated by B. Ćmielewski)

Fig. 3. Workflow for image

processing

(elaborated by B. Ćmielewski)

38 Bartłomiej Ćmielewski, Izabela Wilczyńska, Ciechosław Patrzałek, Jacek Kościuk

with the additional action of adding common points in

places where the tie point cloud and image location were

“strange” – meaning that the reconstructed location of im-

ages could not have been in such a position, or the point

cloud was divided, which happened in particular when the

multispectral low-resolution images were processed.

After achieving satisfying results – at point cloud and

low error value on check and control points – we created

a digital elevation model (DEM) and later an orthoimage

of the rock. The main statistics of processing both RGB

and multispectral images are presented in Table 1.

There were over 1500 applied markers. Some of them

were used as controls and some as check points, and the

selection of both types of points were specially selected

to evenly cover the rock area. The error values are sum-

marised in Table 2. The average 3D error both control and

check points for the RGB project was around 1.5 cm, and

for the multispectral project, 3 cm. The photogrammetric

project allowed the archaeological site to be reconstructed

with an accuracy of centimetres and a very high resolution

of the nal orthoimage at the level of 0.3 mm. This reso-

lution, as presented in Figure 4, allowed individual blades

of grass to be counted.

Using multispectral cameras with NDVIs, we can high-

light the places where vegetation is accelerating the ero-

sion process (Fig. 5).

Photo-alignment RGB Multispectral

Camera stations 51,128 110,684 (4 × 27,671)

Camera altitude 2.36 m 3.34 m

Coverage area 0.0292 km² 0.0213 km²

Ground resolution 0.292 mm/pixel 3.16 mm/pixel

Tie points 23,071,218 51,607,684

Projections 118,482,378 634,582,165

Reprojection error 0.647 pixels 1.14 pixels

Max reprojection error 125.92 pixels (36,77 mm) 78.69 pixels (248,66 mm)

Key points 161.48 GB 259.761 GB

Matching time 22 days 7 hours 25 days 14 hours

Alignment time 3 days 11 hours 1 hours 46 minutes

Optimisation time 37 minutes 53 seconds 5 hours 11 minutes

DEM RGB Multispectral

Size 14,267 × 5,642 pixel 19,744 × 7,359 pixel

Processing time 10 minutes 20 seconds 30 minutes 57 seconds

Resolution 1.91 cm/pixel 1.22 cm/pixel

Point density 0.274 points/cm² 0.673 points/cm²

Orthoimage RGB Multispectral

Size 931,971 × 368,254 pixel 76,075 × 28,216 pixel

Processing time 5 days 21 hours 1 days 0 hours

Colours 3 bands, uint8 4 bands, uint16

Raster Transform Expression – ((B4-B2)/(B4+B2))+0.25

Table 1. Summary of photogrammetric projects (elaborated by B. Ćmielewski)

Thermal documentation

We had problems with thermally documenting the

whole archaeological site, as it was not possible to cap-

ture the entire site at one time. The thermal colours on

the images depend on temperature, which depends on en-

vironmental conditions. We were not able to y a drone

over the whole site due to very strong wind. We could

only produce an orthoimage for part of the site captured

in a relatively short period of time. The example given in

Figure 6 presents part of a vertical wall facing south from

sector S08

Conclusions

The results of these remote sensing studies are not only

important documentation showing the current state of El

Fuerte de Samaipata, but also form the basis for further

studies. The detailed survey network established for this

project was also used during the 3D laser scanning of the

Samaipata rock.

Cf. J. Kościuk, G. Oreci, M. Ziółkowski, A. Kubicka, R. Muñóz

Risolazo, Description and analysis of El Fuerte de Samaipata in the

lightofnewresearch,andaproposaloftherelativechronologyofits

main elements, in the same issue of “Architectus”.

Digitalclose-rangephotogrammetryofElFuertedeSamaipata / CyfrowafotogrametriabliskiegozasięguElFuertedeSamaipata 39

The high-resolution orthoimage was, together with

the results of 3D laser scanning, the basis for drawing up

a detailed CAD plan of all the terraces, niches, reservoirs,

canals, and petroglyphs densely covering the entire rock

surface. As both the photogrammetric studies and 3D

laser scanning results were embedded in an analogous

coordinate system that guaranteed their correlation, they

were extremely important when preparing the vector plan.

Some details of the complex were better visible either on

the orthoimages made on the basis of photogrammetric

studies or on orthoimages obtained from terrestrial laser

scanning (TLS). In a few cases, the best source for inter-

preting the most eroded features were digital terrain model

(DTM) images rendered with hill-shading algorithms.

Multispectral orthoimages also proved to be extremely

useful – they allowed the current state of the spreading of

lichen, fungi, and algae attacking the surface of the rock to

be documented. In the future, if such a study is repeated,

the orthoimages produced in this project will allow the

speed of progressing biological erosion to be determined.

Fig. 4. View of the Samaipata rock followed by images zoomed into the central part of the main orthoimage (elaborated by B. Ćmielewski)

40 Bartłomiej Ćmielewski, Izabela Wilczyńska, Ciechosław Patrzałek, Jacek Kościuk

Fig. 6. Juxtaposition of the RGB and thermal images

(elaborated by J. Kościuk)

RGB project accuracy Multispectral project accuracy

x error

[cm]

y error

[cm]

z error

[cm]

xy error

[cm]

total

[cm]

x error

[cm]

y error

[cm]

z error

[cm]

xy error

[cm]

total

[cm]

Control points RMSE 0.934 1.076 2.628 1.426 2.989 2.447 1.705 2.527 2.983 3.910

Check points RMSE 0.855 1.112 3.560 1.403 3.827 2.654 1.956 3.532 3.297 4.832

Table 2. Accuracy of control and check points in photogrammetric projects (elaborated by B. Ćmielewski)

Fig. 5. Comparison of RGB (A) and multispectral (B) orthoimages

(elaborated by B. Ćmielewski)

A B

Digitalclose-rangephotogrammetryofElFuertedeSamaipata / CyfrowafotogrametriabliskiegozasięguElFuertedeSamaipata 41

If a restoration project ever starts, the results obtained by

us will be an important reference material to monitor the

eectiveness of actions taken.

An important limitation and drawback of the research

was the choice of season in which the measurements were

taken. Theoretically, June and July should be the months

of the lowest precipitation in Samaipata. However, the

2016 season was unusual in this respect – during our 60-

day stay, we recorded only 20 days without rainfall. In

addition, unusually strong winds did not allow us to use

the drone, which in turn forced the team to take almost

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40,000 photos from ground level. This number of photos

obviously had to result in longer photogrammetric calcu-

lations.

Periodic monitoring of the state of preservation of El

Fuerte using the methods presented here seems worth rec-

ommending. However, the choice of future dates for this

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day of ights.

42 Bartłomiej Ćmielewski, Izabela Wilczyńska, Ciechosław Patrzałek, Jacek Kościuk

Abstract

The El Fuerte de Samaipata site inscribed on the UNESCO World Heritage List presents a pre-Columbian, multicultural history on the surface of a big

sandstone rock. There are several ways of creating precise, high-resolution documentation of this rock, including classic geodetic surveys, modern

high-denition surveying (terrestrial laser scanning), and close-range photogrammetry. Close-range photogrammetry is a low cost technique, and the

detailed RGB documentation provided by it aids architectural and archaeological research.

This paper presents the results of the application of close-range photogrammetry in dierent light bands (visual, infrared, and thermal). Sony

ILCE-7RM2, Parrot Sequoia, and Flir Tau2 cameras were used. The authors obtained over 50 thousand images and over 27 thousand multispectral

images (multiplied by four bands, which gave over 100 thousand single band images). The multispectral and thermal data enabled risk maps to be

created for conservation purposes.

Key words: photogrammetry, Samaipata, heritage documentation, rock art

Streszczenie

Wpisane na Listę Światowego Dziedzictwa UNESCO El Fuerte de Samaipata jest świadectwem bogatej, wielokulturowej, prekolumbijskiej historii

rzeźbienia tej wielkiej piaskowcowej skały. Opisano zastosowanie wielu sposobów precyzyjnej dokumentacji całej skały, w tym klasyczne metody

geodezyjne, nowoczesne pomiary w wysokiej rozdzielczości (naziemne skanowanie laserowe) i fotogrametrię bliskiego zasięgu. Fotogrametria

bliskiego zasięgu jest techniką niskokosztową, a dostarczona przez nią szczegółowa dokumentacja RGB pomaga w badaniach architektonicznych

i archeologicznych.

W artykule przedstawiono wyniki zastosowania fotogrametrii bliskiego zasięgu w różnych pasmach promieniowania elektromagnetycznego

( widzialnym, podczerwonym i termicznym). Zastosowano kamery Sony ILCE-7RM2, Parrot Sequoia i Flir Tau2. Autorzy zebrali ponad 50 tysięcy

obrazów RGB i ponad 27 tysięcy obrazów multispektralnych, co pomnożone przez cztery rejestrowane pasma dało w sumie ponad 100 tysięcy

pojedynczych obrazów do przetworzenia. Dane multispektralne i termiczne były szczególnie przydatne przy opracowaniu map ryzyka w celach

konserwatorskich.

Słowa kluczowe: fotogrametria, Samaipata, dokumentacja zabytków, sztuka naskalna

Acknowledgements /Podziękowania

Thepresentedworkisapartoftheresearchprojectsponsoredbythe

grantgiventotheWrocławUniversity ofScienceandTechnologyby

thePolishNationalScienceCentre(grantNo.2014/15/B/HS2/01108).

Additionally, the municipality of Samaipata, represented by Mayor

FalvioLópesEscalera,contributedtothisresearchbyprovidingtheac-

commodationduringtheeldworkinJuneandJuly2016,aswellasin

July2017.TheMinistryofCultureandTourismofBoliviakindlygrant-

edallnecessarypermits(UDAMNo.014/2016;UDAMNo.060/2017).

TheresearchwasconductedinclosecooperationwiththeCentrefor

Pre-Columbian Studies of the University of Warsaw in Cusco. Spe-

cialists from many other universities and research centres also joined

the project.

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