3D recording of archaeological excavation

Transcription

1 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University 3D recording of archaeological excavation Stefano Campana UNIVERSITY of CAMBRIDGE Faculty of Classics & McDonald Institute for Archaeological Research Matteo Sordini ATS s.r.l. Digital Technologies for Cultural Heritage Siena - Italy Francesco Brogi UNIVERSITY of SIENA Landscape Archaeology & Remote Sensing LAB

2 Introduction - task Archaeological excavation is a destructive and not repeatable process: documentation of archaeological stratigraphy and relationships between layers is a fundamental and time consuming aspect of the work of archaeologist aimed at understanding the history of the context. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

3 Introduction - recording archaeological excavation This process is time consuming because drawing complex structures can take long time and the excavation activities are subordinate to documentation. Direct-survey and manual drawing are also affected by a low level of standardization and accuracy and this errors propagated in several years of archaeological campaign can lead the archaeologists in misunderstanding and mistakes. Traditional techniques like direct survey and manual drawing allow very narrow margin of errors in interpretation of the context during fieldwork: after the excavation (interpretation) the archaeologist records through manual measurement and drawings the shape of layers and their relationship, before proceeding to their destruction. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

4 Introduction - topographic instruments In the last decades this problems have been progressively reduced by the introduction of instrumental survey devices and digital imaging. Since the start of Pava excavation in 2004 we started to use topographic GPS, total station to increase accuracy of the documentation. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

5 Digital imaging In the beginning digital imaging was used for a massive qualitative documentation of the layers. Photographs did not replace traditional excavation recording but rather complemented them, adding a new form of documentation which in its turn required interpretation and sometimes graphical representation of the archaeological information present in the photographs. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

6 Digital imaging Hardware The creation of metric-images (ortho or rectified) was possible only with nadir images, acquired with mats, poles and towers. Metric images adds quantitative information to the images. The height but also inflexibility achieved with this tools limited this task to small areas as well as the needs of a large amount of ground control point. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

7 Aerial Imagery Since the beginning of Pava project in 2004, we started to experiment aerial imagery for documenting the site. During the first years, at the end of campaign, we acquired images using a Cessna light aircraft and digital SRL camera, taking picture of the site from many point of view (oblique aerial photography). 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

8 Aerial Imagery This kind of approach presents many limitations for the systematic use aimed at the detailed and the accurate documentation of archaeological excavation: l Flight altitude of Cessna aircraft was too high to record details and small objects (low GSD) l Flying systematically with Cessna was not cost effective and flexible. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

9 3D Laser Scanner Since 2007 we started to experiment laser scanner to perform 3D recording of excavation. While the quality of documentation was extremely accurate, the use of instruments was time consuming both on field (occlusions) and during post-processing tasks. Another limit of laser documentation was the complexity of the whole pipeline that always needed expert user and engineer support.

10 UAV We started to experiment the use of UAV in 2007 in cooperation with Zenit s.r.l. company and Fbk Foundation. We used the Microdrones MD 200 in order to produce 2D and 3D documentation of Pava excavation. This UAV was equipped with 4 rotors, carrying 200g of payload. MD-200 can fly by remote control or autonomously with a GPS navigation system.

11 UAV The experimentation revealed some hardware limitations (unreliability of drone) The MD-200 system was time consuming for setting up all equipment and to be ready to flight. It was strongly affected by atmospheric conditions. Furthermore the MD-200 can carry only some specific kind of compact camera: the images acquired with that hardware had a lossy compression and produced very poor images in most of cases unfitting the requirement needed for photogrammetric processing.

12 UAV In 2013 we were ready to give up (particularly after the key note of A.Gruen at S2P in Beijing at the end of 2012) Almost by chance we started the testing Aibotix X6 Hexacopter System. It can carry up to 2,5 KG payload allow to use a reflex or mirrorless camera producing a very high quality images. It can fly by manual control or autonomously with the integrated GPS navigation system. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

13 UAV This system is less affected by atmospheric condition, making the system more versatile in archaeological fieldwork. The system set-up is very fast, in few minutes the copter is ready to fly. The level of detail of surfaces, the repeatability of flights with identical parameters and the high speed in performing the photogrammetric coverage make this instrument particularly suited to the diachronic documentation of the excavation

14 Image Based Modeling! New software allow the three dimensional recording of objects from images, providing s e m i o r f u l l a u t o m a t i c processing steps and giving back a dense point cloud and, in some cases, the textured surface mesh model (Fassi et alii 2013).! The introduction of low cost automated photogrammetric packages based on Structure F r o m M o t i o n T e c h n i q u e represent a big (r)evolution in archaeological documentation, allowing the systematic use of 3D recording DIRECTLY M A N A G E D B Y T H E ARCHAEOLOGIST.

15 Santa Marta Excavation! During Santa Marta excavation (2012 ongoing) we developed a pipeline for archaeological documentation based on 3D recording and digital drawing of the contexts, features and layers.!!

16 Topography The first important aspect of our pipeline is the topographic approach: we use a topographic GPS with real time differential correction for absolute coordinate measurement and a total station to measure GCPs during fieldwork. Absolute coordinates of each GCP have been calculated using topographical GPS with real-time differential correction, with 30 minutes of static measurement. TS is oriented using GCPs on the field in gauss boaga coordinate system, so we get all survey points directly in absolute coordinate system and no post-processing tasks are needed. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

17 Aerial Images Fast documentation and survey of large areas 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

18 Aerial Images Fast documentation and survey of large areas 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

19 Aerial Images Fast documentation and survey of large areas 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

20 Terrestrial Images fast documentation and survey of layers 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

21 Image based Modeling Ground Sample Distance Resolution can be managed during image acquisition by adjusting the distance between camera and target-object. It can be estimated by this formula: pixel size x distance focal length In case of low distances between camera and object, it can be adjusted during the processing of data-sets, rescaling images or decimating point cloud.

22 Image based Modeling To process our image datasets we choose a low cost automated photogrammetry package, Agisoft Photoscan Pro. The extraction and matching of features in Photoscan are made with an algorithm similar to the well known SIFT (Scale-invariant feature transform)

23 Image based Modeling In archaeological excavation some layers have very homogeneous texture. In this cases SIFT algorithm may have problems to detect and match features properly. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

24 Image based Modeling To overcome this limit is important to use control point measured with TS and match them a priori in the images

25 Image based Modeling During recording layers of archaeological excavation it's important to proceed keeping an high level of accuracy in order to produce reliable documentation in which the spatial relationships between layers are properly expressed High level of accuracy is also needed to integrate different diachronic models

26 Image based Modeling After orientations Photoscan can extract dense point cloud from the dataset matching images throught different alghoritms according to the requirements of the user. It's important to choose appropriate GSD to describe context: all significant details must be recorded without create redundant points that can increase dimensions of the dataset and making hard to manage it.

27 Image based Modeling Dense point cloud can be processed with Photoscan or, if needed, with external software like Meshlab to create a polygonal mesh.

28 Image based Modeling The poisson surface algorithm used by meshlab allows user to customize meshing parameters Meshlab allows to visualize geometry with different shaders and light position Radiance scaling shader can be useful for enhanced surface depiction

29 Image based Modeling The resulting model can be textured with Photoscan using different methods and performing a color correction that reduce the chromatic differences between images

30 4D Modeling The data sets obtained with IBM shows different situation of parts or whole excavation For practical reasons, the excavation of large areas cannot show a diachronic phase at same time September 2013 October 2013 July th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

31 4D Modeling 3D modeling can be a powerful tool (and sometime the only way) to show diachronic phases of whole excavation. A diachronic model can be obtained by removing point from older datasets that overlaps with new. All dataset are in same coordinate system: no alignment is needed The global mesh is generated using poisson algorithm and textured in photoscan disabling older images for each new area

32 4D Modeling

33 Digital drawings The excavation data is managed inside a geographic information system where are recorded alphanumeric and spatial information. The digital drawing of each layer is performed in post processing through Ortho-images extracted from 3D datasets

34 Digital drawings

35 Digital drawings The digitizing can be performed at more detailed scale and in non-comparable faster time/accuracy than traditional/direct drawing: in few time is possible to digitize large areas and representing all small/articulated details that in manual drawing takes long time to be reproduced as well as manual-drawing skills.

36 Digital drawings

37 Digital drawings

38 Approximate time effort in a typical UAV-based photogrammetric workflow Remondino, Nex 2013 UAV for 3D mapping applications: a review, Applied Geomatics

39 Conclusions The recording of archaeological features in excavations goes beyond the graphical representation involving the essence of the archaeological process based on the understanding and the interpretation of the contexts. 3D recording is the best way of measuring and documenting archaeological features in terms of detail and accuracy, that need we to be performed by archaeologist. Standardization of archaeological documentation on excavation The role of drone should be considered crucial reducing dramatically on site documentation timing and therefore allowing archaeologists keeping on excavation. The traditional/manual survey/drawing methods on site can be considered totally obsolete both because time consuming and due to quality/ accuracy/precision issue. 5 th International Conference Remote Sensing in Archaeology The Age of Sensing October Duke University

40 thank you for your attention