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Photogrammetry, Orthophotos and Laser Scanning for Archaeology

Photogrammetry and Laser Scanning for Archaeology - 10 Advantages
Digital Orthophotos for Archaeology - 5 Advantages

Some Fields of Application:

1 - Prospecting and Landscape Archaeology
2 - Excavation
3 - Visualization

Photogrammetry and Laser Scanning for Archaeology

10 Specific Advantages:

1 - Photogrammetry documents objects without touching them. This is very often desirable with archaeological objects (cave paintings, mummies, etc.).

2 - In addition to maps, wall lines, stone by stone and "baulk" drawings, 2D/3D models, rendering of earth/organic remains, for example, photogrammetry also provides abundant and high-quality photographic documentation (with precise color and texture reference) of objects and sites. The analysis can be done at any time later on. In that way, a large number of important archaeological objects and sites can be documented in a short time at little expense. Analysis can be done, if desired or needed (for example, if the object is destroyed and should be rebuilt).

3 - The quality of conventional recording very often depends on the qualification, interest or condition of the documenting archaeologist. The photograph, on the other hand, is objective. Everything is recorded automatically, nothing is forgotten or overlooked.

4 - Especially true at excavations, features from upper layers have to be destroyed when digging down to the lower layers. If later on the documentation turns out to be insufficient or incorrect, the information is irretrievably lost. If the features are recorded using a photogrametric process, however, the documentation can be checked, verified or corrected at any time later on. This is extremely desirable, specially when the results have to be analyzed by someone who did not participate in the excavation.

5 - The accuracy and homogeneity of a photogrammetric analysis is, in most cases, far better than conventional recording techniques, affording known and assessed precision.

6 - Especially during rescue, emergency and salvage work, layers of stones, for example, can be very painstaking to document. Photogrammetry can dramatically accelerate the on-site recording procedure, with analysis being done at any time later on.

7 - All features can be viewed, drawn and mapped in 2D and/or 3D.

8 - Experience shows that if the features are recorded conventionally during an excavation, different people often have to draw them. This may result in different and non-homogeneous drawings. If photogrammetry is used during the whole excavation, the documentation will be standardized.

9 - All information, being in digital format (CAD) can be easily and readily used for print and web publication. Further, it allows computer rendering and animation showing the evolution of a site in time and facilitates its reconstruction.

10 - Finally, in most cases only a few archaeologists can handle photogrammetric instruments and techniques. Comparably, photogrammetrists who are not archaeologists, are not able to correctly interpret archaeological subjects. One of the advantages offered by Documenta Architectural Photogrammetry is its more than 30 years experience with photo-interpretation in archaeology and architectural heritage documentation.

Digital Orthophotos for Archaeology

Digital orthophotos and rectified photographs are by-products of digital photogrammetric surveys. They are distortion-free and true-to-scale photographs of predominantly two (2) dimensional objects, also either in digital or plotted formats.

5 Specific Advantages:

1 - They are objective; interpretations can be overlaid and checked by any archaeologist.

2 - An orthophoto makes all of the data transparent to the viewer, whereas a vector graphics (drawing) is an abstraction. An orthophoto, especially when it is overlaid with drawings, presents important information such as color and texture reference, in a very effective way, with clearer contexts that are easily understood.

3 - Image enhancement algorithms are applicable: ± contrast, ± definition, ± shadows, etc.

4 - It is appropriate for a combination with other complementary fields as geophysical prospecting and geo-magnetic, as well as other information including the land register, contour lines or interpretations of the composite image and new details, contributing to a better understanding of the archaeological site.

5 - It is a prerequisite to further contribute towards pattern recognition (earthen/organic remains, for example) and other expert interpretation.

Some Fields of Application of Photogrammetry, Orthophotos and laser Scanning

1 - Prospecting and Landscape Archaeology

• Analysis of vertical and oblique photographs: in most cases, archaeologists require exact and detailed mappings of archaeological information that can be seen in vertical and oblique photographs.

• Visualization of a site's topography: contour lines, digital terrain models or profiles of the surface can be depicted together with archaeological information from photos or from results of excavations.

• Digital orthophotos: a very important aid for archaeology. Structures showing up on the surface are depicted together with all of the image-information. Image enhancement, pattern recognition and subsequently expert systems are applicable with orthophotos.

• Combination of the data mentioned above with results from other (especially geophysical) prospecting techniques within a Geographical Information System - GIS.

• Landscape archaeologists are often in need of data obtained by satellites which is primarily used for "Predictive Modeling", to assess the likelihood of finding significant archaeological materials in given areas.

2 - Excavation - different kinds of excavations require different solutions:

• Rescue excavations: very fast solutions, accuracy is secondary.

• Stratigraphic excavations: 2D and 3D models of strata.

• Excavations of graveyards and burial sites: sometimes with several thousands of skeletons; standardized recording procedures.

• Excavations of settlements without stone walls: recording procedures can change depending on features; any system used must be adaptable.

• Excavations of settlements with stone walls: to go on with excavation, walls and pavements must be removed - they have to be documented first; (especially excavations in towns can be very complicated); documentation of wall-paintings.

• Excavations in moist areas: organic materials (tools, wooden pavements, wooden walls etc.) have to be recorded instantly.

• Excavations in remote areas without infrastructure (desert, jungle, mountain...).

• Excavations underwater: there are special problems which require sophisticated
solutions.

• Excavation of hunter-gatherer sites may also require different solutions.

Examples: • Recording of caves, mining activity and cave paintings • 3D models of caves and mines • Rectification and reconstruction of cave paintings, that should not be touched • 3D recording of mining traces • Recording and documentation of petroglyphs and pictographs • Recording of finds • Architectural finds • Ceramics • Metal finds • Jewelry • Recording of usage-traces • Mummies • Bones, stone and shell remains • Wood and other vegetation remains etc.

3 - Visualization

• An important factor for the interpretation and analysis of a site, and one which is often neglected by archaeologists, is its topography. It can be examined very easily out of photogrammetric data, and when mapping the site, it can be done with contour line intervals small enough to represent important archaeological details in the topography.

• The drawback of contour lines is that many people have problems "reading" them. It is much easier for them to look at the digital terrain model of the area.

• Visualization of the site can be done in different ways. The easiest way is to digitize contour lines from a map. The quality of the result will depend on the map where the contour lines were digitized from. If you have the ability to obtain terrain data yourself (either by geodetic measurements or by close-range photogrammetry), you certainly will get better results.

• Very impressive results can be obtained by "draping" an orthophoto on top of the digital terrain model. In this way you can give a realistic overview of the site's landscape. It is also possible to drape a thematic map (chronological themes, for example) on top of the digital terrain model making it easier to visualize archaeological mappings.

• Another possibility is to transfer the 3D model into rendering software (such as Autodesk's 3D-Studio), where it can be combined with modeled 3D reconstruction of the archaeological features. The result is a realistic image of the site and how it may have looked within its terrain. Furthermore, with all 3D photogrammetric information, it is easy to computer animate the model. Finally, the 3D reconstructed rendering can be integrated into photographs, giving an even more realistic feel to the image.

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