UNDERWATER PHOTOGRAMMETRY FOR ARCHAEOLOGY

The VENUS Project Framework

P. Drap

, D. Scaradozzi

, P. Gambogi

and F. Gauch

LSIS Laboratoire des Sciences de l'Information et des Systèmes UMR CNRS 6168, France

Interuniversity Centre for Integrated Systems for the Marine Environment (ISME) c/o DIST

Università di Genova, Italy

SBAT Soprintendanza per i Beni Archaeologici della Toscana, Firenze, Italy

COMEX, COmpagnie Maritime d’EXpertise Marseille, France

Keywords: Underwater, archaeology, photogrammetry.

Abstract: This article describes on-going developments of the VENUS European Project (Virtual ExploratioN of

Underwater Sites, http://www.venus-project.eu) concerning the first mission to sea in Pianosa Island, Italy

in October 2006. The VENUS project aims at providing scientific methodologies and technological tools

for the virtual exploration of deep underwater archaeological sites. The VENUS project will improve the

accessibility of underwater sites by generating thorough and exhaustive 3D records for virtual exploration.

In this paper we focus on the underwater photogrammetric approach used to survey the archaeological site

of Pianosa. After a brief presentation of the archaeological context we shall see the calibration process in

such a context. The next part of this paper is dedicated to the survey: it is divided into two parts: a DTM of

the site (combining acoustic bathymetry and photogrammetry) and a specific artefact plotting dedicated to

the amphorae present on the site.

1 INTRODUCTION

The VENUS project is funded by European Com-

mission, Information Society Technologies (IST)

program of the 6th FP for RTD. It aims at providing

scientific methodologies and technological tools for

the virtual exploration of deep underwater archaeo-

logical sites (Chapman et al., 2006).

Underwater archaeological sites, for example

shipwrecks, offer extraordinary opportunities for

archaeologists due to factors such as darkness, low

temperatures and a low oxygen rate which are fa-

vorable to preservation. On the other hand, these

sites cannot be experienced first hand and today are

continuously jeopardized by activities such as deep

trawling that destroy their surface layer.

The VENUS project will improve the accessibil-

ity of underwater sites by generating thorough and

exhaustive 3D records for virtual exploration.

The project team plans to survey shipwrecks at

various depths and to explore advanced methods and

techniques of data acquisition through autonomous

or remotely operated unmanned vehicles with inno-

vative sonar and photogrammetry equipment. Re-

search will also cover aspects such as data process-

ing and storage, plotting of archaeological artifacts

and information system management. This work will

result in a series of best practices and procedures for

collecting and storing data.

Further, VENUS will develop virtual reality and

augmented reality tools for the visualization of an

immersive interaction with a digital model of an

underwater site. The model will be made accessible

online, both as an example of digital preservation

and for demonstrating new facilities of exploration

in a safe, cost-effective and pedagogical environ-

ment. The virtual underwater site will provide ar-

chaeologists with an improved insight into the data

and the general public with simulated dives to the

site.

The VENUS consortium, composed of eleven

partners, is pooling expertise in various disciplines:

485

Drap P., Scaradozzi D., Gambogi P. and Gauch F. (2008).

UNDERWATER PHOTOGRAMMETRY FOR ARCHAEOLOGY - The VENUS Project Framework.

In Proceedings of the Third International Conference on Computer Graphics Theory and Applications, pages 485-491

DOI: 10.5220/0001100504850491

 SciTePress

archaeology and underwater exploration, knowledge

representation and photogrammetry, virtual reality

and digital data preservation.

This paper focuses on the first experimentation

in Pianosa Island, Tuscany, Italy.

The document is structured as follows. A short

description of the archaeological context, then the

next section explains the survey method: calibration,

collecting photographs using ROV and divers, pho-

tographs orientation and a particular way to measure

amphorae with photogrammetry using archaeologi-

cal knowledge. A section shows 3D results in

VRML and finally we present the future planned

work. .

2 THE UNDERWATER

ARCHAEOLOGICAL SITE OF

PIANOSA ISLAND

The underwater archaeological site of Pianosa,

discovered in 1989 by volunteer divers (Giuseppe

Adriani, Paolo Vaccari), is located at a depth of 35

m, close to the Scoglio della Scola, in front of the

east coast of the island. The site is characterized by

the presence of about one hundred amphorae of

different origin and epoch. The various amphorae

range from Dressel 1A (1st century B.C.) to Beltran

2 B and Dressel 20 , up to African models (3rd cen-

tury A. D.) The site has been surveyed in 2001 by

the Nucleo Operativo Subacqueo (MIBAC-SBAT)

divers. This survey, carried out by the SBAT,

proved that the site had remained untouched. And it

was necessary to start a first test of excavation to

know the exact nature of the archaeological site: this

was one of the aims that the October 2006 underwa-

ter mission has reached.

The remarkable depth allows diving and the site

was chosen to make survey using both robotic

equipment and divers.

The experimental activity, under the supervision

of the archaeological team of MIBAC-SBAT, has

been carried out by CNRS for the photogrammetric

survey, ISME with its own ROV equipped with

camera from COMEX, and its georeferentiation and

positioning system.

The site had to be cleaned before surveying,

mainly because of the presence of dead posidonia.

This first operation was made in September 2006 by

SBAT including specialists from CH conservation:

Roberto Bonaiuti and Emiliano Africano.

3 PHOTOGRAMMETRIC

SURVEY IN PIANOSA

The survey is done merging several kinds of infor-

mation: bathymetry, DTM from photogrammetry,

artefacts measure with photogrammetry and theo-

retical model of artefact objects. The entire survey is

stored in a relational database and the geometry is

exported toward tools for Virtual Reality (see fig. 1).

This approach will allow archaeologists to see the

entire site, using immersive VR technologies, with-

out diving.(Drap et al., 2005), (Long, 1996).

3.1 Two Different Ways for Data

Capture

The photogrammetric survey in Pianosa is made by

a set of photographs with the right overlap (around

60%). The geometry is very similar to the technique

used in aerial photogrammetry; the main difference

is the distance to the seabed and the immersion in

water.

As we are sure that the seabed is more or less

flat, we can use a set of photographs with vertical

axis to make the survey.

The photographs are taken by strips with 60%

overlap for the consecutive photographs in a strip

and 20% overlap from one strip to another.

This first mission in Pianosa was an opportunity

to test and improve several ways to perform this

survey. As this site is 35m deep, we can use both a

survey with divers (CNRS partner), and start a sur-

vey by ROV, managed by ISME.(Conte et al.,

2006).

The diver has a Nikon™ D70 digital camera

with a 14 mm lens from Sigma™ and two flashes

Subtronic™. The digital camera was embedded in

Subal™ housing with a hemispherical glass.

COMEX brought its digital camera equipped for

connection to the ROV: a Nikon DH2, a 14 mm lens

from Sigma™ and two flashes Nikon™, SB800.

The housing and connector was made by COMEX

with a flat glass. (See fig.2)

A zone to be surveyed has been determined by

the team and equipped with 4 scale bar (2m) and a

set of 15 makers (cement block 15x15x10cm) in

order to define a network for a better ROV guid-

ance.

GRAPP 2008 - International Conference on Computer Graphics Theory and Applications

486

Archaeological

data

Photogrammetry

Photo

database

Photogrammetric

survey

Orientation

with Photomodeler

Photogrammetric

survey

Amphorae measure

Virtual reality generated from Database

A tool for ‘reading’ the site in laboratory,

revision, update …

Database and

new plotting

inconsistensy check

Data fusion

Bathymetry,

multi beam.

DTM for seabed

representation

DGPS

USBL

Theoretical model

Figure 1: Synoptic schema of surveying process.

Figure 2: Tthe ROV in water with digital camera and

flashes in their housing.

The ROV has made a survey on the zone delimited

by the markers. The pilot use a video camera located

on the bow. He can see the markers and pilots in

order to make strips. (Conte et al., 2007). The pho-

tographs were taken in two modes:

- Manually, an operator, looking thought the lens

by a small video camera to shoot the image.

- With a fixed frequency, decided according to

the ROV speed and altitude.

Figure 3: Two photographs from a strip made by the

ROV.

3.2 Multimedia Calibration

The camera calibration in multimedia photogram-

metry is a problem already identified since almost

50 years.(Bass and Rosencrantz, 1973), (ASP,

1980). You can refer to Hans-Gerd Maas (Maas,

1995-a) to have an overview of the state of art of

this field. The problem is not obvious, the light

beam refraction through the different diopters (wa-

ter, glass, air) introduces a refraction error which is

impossible to express as a function of the image

plane coordinates alone. (Maas, 1995-b) Therefore

UNDERWATER PHOTOGRAMMETRY FOR ARCHAEOLOGY- The VENUS Project Framework

487

the deviation due to refraction is close to those pro-

duced by radial distortion even if radial distortion

and refraction are two physical phenomena of dif-

ferent nature.

For this reason we start to use standard photo-

grammetric calibration software and make a calibra-

tion of the set housing + digital camera. The distor-

tion corrects in a large part the refraction perturba-

tion. This was also shown by Kwon (Kwon, 1998)

(Kwon and Lindley, 2000).

But this approach is strongly dependent of the ul-

timate dioptr water/glass of the housing. To try to

minimize the refraction error we can found on the

market some housing with a hemispherical glass,

which is the case of Subal™ housing used with the

diver. For the other one, made by COMEX the glass

was plate and the refraction action is much more

important.

We have developed a method to compensate

separately refraction and distortion; this is published

as a deliverable of the VENUS project and is

downloadable from the VENUS web site.

3.3 The Reference System

The choice of a reference system to express the

measured data is very important. It’s depending of

the archaeological needs. Several cases can occur:

- We don’t have any way to get an absolute posi-

tion, or we don’t need it. In this case we have to

define the reference system on local, observable

geometry. For example something which defines

the axis of symmetry of the wreck (if there is

one); buoys to define the vertical axis; scale bar.

- We need an absolute orientation and we have

several ways to obtain it. For example a pipe line

as DGPS – USBL can give an approximation of

the ROV position, etc…

In Pianosa we will use an absolute reference

given in two modes: when it will be possible ISME

will associate for each photographs coming from the

ROV six parameters as: x, y, z, Omega, Phi, Kappa.

In the same time they will measure the absolute

coordinates of a set of markers seen on the photo-

graphs and used as control points.

3.4 Orientation Phase

More than three hundred photographs have been

taken by the diver. They cover an area of 20 x 20

meters. The orientation was done manually using

Photomodeler™.

The photographs orientation was done using

points on the seabed, except on the amphorae in

order to be used to define a DTM on the seabed. The

oriented photographs and the diver’s trajectory are

visible in figure 5.

Five markers, visible in figure 4, were used as con-

trol points.

3.5 Amphorae Plotting

Using the oriented photography a plotting phase,

driven by archaeological knowledge is processed to

obtain both 3D model representing the amphorae

and a database managing all the data of the project.

Figure 4: Oriented photographs visualised in VRML with

the non textured seabed.

A Method for Measuring Amphorae. After the

orientation phase we shall, in the next months, start

the amphorae plotting phase. This second step will

use archaeological knowledge to obtain a complete

representation of the measured artefact; it will be

articulated in three steps:

1) Development of the theoretical model: for

each identified object, a geometrical description

offers a whole of geometrical primitives, which are

the only objects to be potentially measured, and a

theoretical representation of the object. In our case

archaeologists have identified six amphora typolo-

gies and we shall produce a theoretical model for

each of them. This theoretical model is formalized in

a hybrid way, taxonomy of archaeological artefacts

and an XML representation for the Amphorae ty-

pology.

2) As photogrammetric measurements are highly

incomplete (the object is seen only partially or may

be deteriorated), an Expert System will determine

the best strategy to inform provide all the geometri-

GRAPP 2008 - International Conference on Computer Graphics Theory and Applications

488

cal parameters of the studied object, starting from

the measurement process and handling the default

data as defined in the archaeological model and the

geometrical model. The expert System used is Jess.

(http://herzberg.ca.sandia.gov/jess/)

3) The resulting object is thus based on a theo-

retical model, dimensioned more or less partially by

a photogrammetric measurement. During the exploi-

tation of the photographs the operator can choose

the number of attributes of the object which are

relevant to measure. The choice of attributes will be

revisable in time, as for example during a second

serie of measurements. The system can be used to

position in space some objects from a catalogue

after a scaling process.

All these development are done in Java and con-

nected to the Arpenteur photogrammetric toolbox.

(Drap et al., 2003), (Drap and Long, 2005).

Measuring Paradigm Amphorae. In order to use

the method describe above, the archaeologists have

taken up six amphorae from the site. These ampho-

rae will be used as paradigm to define the theoretical

model needed. The first step is to measure the am-

phorae and to define a geometrical model. Some

amphorae have been designed in a traditional way at

scale 1:1, for some others as for example the type

gauloise 3 we used the typology presented by our

partner ADS Archaeological Data Service, Univer-

sity of York, UK.

(http://ads.ahds.ac.uk/catalogue/archive/amphora

_ahrb_2005/details.cfm?id=135)

The Plotting Interface. The diversity of the objects

handled by the archaeologists and the geometric

complexity of their surfaces led us to search for

stable morphological characteristics of the objects

where diagnostic measurements could be taken. A

series of simple geometric primitives are used to

approximate these morphological characteristics and

are used as an interface between the photogrammet-

ric measurement and the underlying model.

In the case of amphorae we define four measur-

able zones, rims, handle, belly, bottom, and we use a

set of geometrical primitives computed by least

square method onto the measured points. For exam-

ple a circle on the rim or belly points, a line on bot-

tom point and center of these two circles.

This interface allows the user (generally an ar-

chaeologist) to

- Recognize the amphora type on the photo-

graphs,

- Choose the amphora type in the interface

combo box (The site was already studied in

collaboration with archaeologists to define the

typology),

- Measure a set of points on the zone where

measure is allowed,

- Add archaeological comments and observa-

tions,

- Compute the object, using the measured points

to construct a new instance of amphorae,

- Insure consistency between observations and

theoretical model,

- Store this new instance in the remote database.

Figure 5: VRML representation of reconstructed ampho-

rae. Also visible the measured points on amphorae, a

marker and two scale bars. The seabed is textured using

the oriented photographs.

4 MERGING RESULTS

We have merged the data coming from the bathym-

etry mission, conducted by Geosystem Parma, Italy,

the photogrammetric campaign ie, a survey of the

seabed at large scale with a good quality texture and

a survey, driven by archaeological knowledge of all

the amphorae and fragments of the site with a direct

link from the Amphorae representation to the data-

base. (see fig 9 to 11).

UNDERWATER PHOTOGRAMMETRY FOR ARCHAEOLOGY- The VENUS Project Framework

489

All these data now are stored in a relational data-

base (MySql) and a set of java tools allows to wrap

objects from the database and to produce a VRML

representation.

The VRML file produced contains a link for

every amphora to the database via a PhP interface.

This interface allows the user to see, check and

modify the archaeological values regarding the am-

phorae. Of course the user has access to all the data,

i.e. measuring points, photos and photo orientation

used to measure the artefact, but these data are read

only through this interface.

4.1 Seabed Representation

The textured seabed is obtained by triangulation of

the points used to orient the photographs. We have

developed a tool to link each triangle to a set of

possible photographs for texturing with the current

used photograph mentioned. The result (3D points,

triangle, oriented photographs) are written in XML

with possible transformation to X3D and VRML.

This way is very convenient to change the photo-

graph used to texture a triangle or a zone.

4.2 Accuracy

The bundle adjustment precision for all the photo-

graphs is around 2 cm and the relative uncertainty of

measurement is less than 5mm when the signal is

good. The absolute accuracy is around 40cm (using

the control points given by the acoustic measure-

ment from the ROV).

5 CONCLUSIONS AND FUTURE

WORK

Archaeologists need to explore and make an inven-

tory of deep wreck sites unreachable by divers as

these sites may be jeopardized by deep trawling in

the very next few years. The digital preservation

aspect is one of the main goals of this project.

We have presented here the underwater survey

process from taking photographs to the site recon-

struction, merging acoustic and optical data, and a

theoretical model based on archaeological knowl-

edge for amphorae. In the framework of the VENUS

project a work is in progress to define ontologies for

underwater archaeology and more precisely for

amphorae present on the site. (Jeansoulin and Pap-

ini, 2007)

The measured object are stored in a database and

wrapped in Java Objects able to generate their mor-

phology in VRML.

In addition of the site survey presented here we

plan to immerse archaeologists inside a virtual uni-

verse depicting a reconstructed archaeological site,

for example a shipwreck, and allow them to work on

this site as naturally as possible. The digital model

generated by the survey will then be used, with the

help of virtual reality and mixed reality, for con-

structing immersive, virtual environments that en-

able archaeologists and general public to experience

an accurate and fully immersive visualization of the

site.(Chapman et al., 2006).

The next VENUS mission in open sea is sched-

uled in October 2007. It’s about a shipwreck, lo-

cated at Sesimbra, near Sado river mouth, south of

Lisbon, lying at a depth of 55m. It is yet possible to

dive and we will use both divers and ROV to fi-

nalyse our survey procedure. The ultimate mission

in France, scheduled in October 2008, will be done

completely diverless.

ACKNOWLEDGEMENTS

The Venus sea trial operations in Pianosa have been

made possible thanks to the voluntary support of a

number of different Institutions. In particular, the

Italian Ministery of Internal Affairs - Corpo Nazion-

ale dei Vigili del Fuoco - Direzione Regionale Vigili

del Fuoco della Toscana - Nuclei Sommozzatori

(Diving team of the Tuscany Fire Brigade) has made

available the ship and diving teams for assistance in

all the stages of the activities. The Italian Ministeries

of Justice (Casa di reclusione di Porto Azzurro,

Polizia Penitenziaria), of Transportation and Navi-

gation, of the Environment (Parco Nazionale Arci-

pelago Toscano) have given the necessary permits

and the required logistic assistance. The Cooperativa

Ormeggiatori Piombino, Studio Archeologico The-

tys and Geosystem Parma have contributed in the

data gathering process and with diving assistance.

VENUS is partially supported by the European

Community under project VENUS (Contract IST-

034924) of the "Information Society Technologies

(IST) programme of the 6th FP for RTD".

The authors are solely responsible for the content

of this paper. It does not represent the opinion of the

European Community, and the European Commu-

nity is not responsible for any use that might be

made of data appearing therein.

GRAPP 2008 - International Conference on Computer Graphics Theory and Applications

490

REFERENCES

Paul Chapman, et al. VENUS, Virtual ExploratioN of

Underwater Sites. in VAST/CIPA The 7th Interna-

tional Symposium on Virtual Reality, Archaeology and

Cultural Heritage. 2006. Nicosia, Cyprus: Archaeo-

lingua, Budapest, Hungary.

Pierre Drap, et al. Vual Reality in underwater archae-

ology: First results on the case study “L’Anse des

Catalans”, Marseille. in VSMM2005 Eleventh Inter-

national Conference on Virtual System and Multime-

dia. 2005. Ghent, Belgique.

Luc Long. L’archéologie sous-marine à grande

profondeur: fiction ou réalité. in Archeologia

Subacquea, Come opera l’archeologo sott’acqua,

Storie dalle acque, VIII ciclo di lezioni sulla ricerca

applicata in archeologia, Certosa di Pontignano, .

1996. Siena, Italia.

Giuseppe Conte, et al. Robotics tools for underwater

archaeology. in ECSAC07 VII International Confer-

ence on Science, Arts and Culture. Science for Cul-

tural Heritage. 2006. Veli Lošinj, Croatia.

Giuseppe Conte, et al. Data gathering in underwater

archaeology by means of a remotely operated vehicle.

in XXI CIPA international Symposium. 2007. Athens,

Gree: The CIPA / VAST, Nicosia, Cyprus.

Georges F. Bass and Donald Rosencrantz. L’utilisation

des submersibles pour les recherches et la cartogra-

phie photogrammétrique sous-marine. in

L’archéologie subaquatique, une discipline naissante.

1973. Paris.

ASP, Manual of photogrammetry, Fourth Edition. 1980:

American Society of Photogrammetry( Asprs Pubns).

Maas, H.-G. New developments in Multimedia Photo-

grammetry. in Optical 3-D Measurement Techniques

III. 1995: Wichmann Verlag, Karlsruhe.

Hans-Gerd Maas. New developments in Multimedia Pho-

togrammetry. in Institute of Geodesy and Photogram-

metry, Swiss Federal Institute of Technology. 1995.

Zurich.

Kwon, Y.-H. Refraction at the Water-Air Interface. 1998

(cited; Available from: http://kwon3d.com/theory/

dlt/refr.html).

Kwon, Y.-H. and S. Lindley, Applicability of 4 Localized-

calibration Methods in Underwater Motion Analysis.

XVIII International Symposium on Biomechanics in

Sports., 2000.

Pierre Drap, Julien Seinturier, and Luc Long. A photo-

grammetric process driven by an Expert System: A

new approach for underwater archaeological survey-

ing applied to the ‘Grand Ribaud F’ Etruscan wreck.

in Applications of Computer Vision in Archaeology

ACVA’03 2003. Monona Terrace Convention Center,

Madison, Wisconsin, USA.

Pierre Drap and Luc Long, Photogrammétrie et archéolo-

gie sous-marine profonde. Le cas de l’épave étrusque

Grand Ribaud F XYZ, 2005. N°103, partie 1 et N°

104, partie 2.

Robert Jeansoulin and Odile Papini.

Underwater archaeo-

logical knowledge analysis and representation in the

VENUS project: a preliminary draft. in XXI CIPA in-

ternational Symposium. 2007. Athens, Greece.

UNDERWATER PHOTOGRAMMETRY FOR ARCHAEOLOGY- The VENUS Project Framework

491