Jezreel Valley Regional Project Archaeology and History of a Regional Landscape

JVRP White Papers in Archaeological Technology

Practical Uses for Photogrammetry on Archaeological Excavations

by Adam Prins and Matthew J. Adams

Ver. 1.0

12 December, 2012

What is photogrammetry?

Photogrammetry is a computerized process that produces spatially accurate images from ordinary photographs. With these georectified images, archaeologists can produce photographic plans of sites and their stratigraphy, take accurate measurements directly from the photo, and import photographic data into other computerized technologies for mapping and visualizing archaeological features. The production of a photogrammetric image involves the combination of a number of technologies: total station surveying, traditional archaeological photography, and geospatial rectification. By combining these technologies, we are able to produce a hybridized documentation technique that can serve many purposes.

 

What are the benefits?

Traditional photography, while essential for archaeological documentation, has a number of drawbacks. For one, a camera’s lens introduces significant distortion into its images, especially if a wide angle lens is used, therefore no reliable measurements can be taken from a standard photograph. Second, standard photographs do not contain any spatial coordinates, so they cannot be cross-referenced with any geographical data nor included in a Geographic Information System (GIS). Finally, a photograph without any additional meta-data is simply a 2D representation of a plane, its applications are limited. Photogrammetry eliminates these drawbacks and limitations and opens up new opportunities for how photography can benefit archaeological documentation and research by allowing for the creation of spatially accurate two-dimensional images - in both the horizontal (plan-view) and vertical (section-view) planes.

The importance of these spatially precise images is evident when considering their many applications. Comparing the original and georectified photos side by side helps illustrate how much effect the processing actually has on the photo (see Fig. 1). While there may not appear to be anything fundamentally wrong with the first image, the georectified image has a significant decrease in lens-distortion and the planar angle has been corrected. It would not be possible to conduct any accurate spatial analysis on the first image (Fig. 1a), but many types of analysis can be conducted on the second image (Fig. 1b).

 

   

 

 

Photogrammetric images not only provide the viewer with a real birds-eye view of an excavation, they give archaeologists a new way to efficiently document an excavation. Manual drafting is still an essential part of the archaeological process, but photogrammetry adds an additional layer of precision and detail to the archaeological record. Among a number of other uses, a properly georectified image can be digitally traced to produce a plan similar to one drawn by hand. The georectified images can be layered with these drawings to create a hybrid map of an archaeological site, in much the same way that online mapping services, such as Google Maps, layer street maps with georectified satellite imagery. Additionally, photogrammetry can be used to document sections and any other two-dimensional plane in a spatially accurate photograph (see, for example, photogrammetry used to record inscriptions by the Karnak Hypostyle Hall Project). Of course, once photogrammetry imbues an image with spatial correction and coordinates, these images may be imported into a wide variety of software for numerous applications (see other JVRP White Papers in Archaeological Technology for some of these applications).

 

The Process

The JVRP has incorporated photogrammetry into the recording of all architecture at the square level and the photographic documentation of all sections. An explanation of the workflow that the author (Prins) developed for the JVRP to document architecture will help illustrate some of the applications of photogrammetry.

 

1. Control Points

The photogrammetric recording of an architectural unit in a square is a straightforward procedure. First, a series of markers, or control points, are placed on important parts of the excavation unit, generally this is the architectural feature that is the subject of the photography and mapping. The markers should be brightly colored so they contrast enough with the subject of the photo to be easily differentiated, but small enough to not have a negative visual impact on the image if it were published at a commonly used scale (1:50, for example). The author uses either red or yellow duct tape torn into 1 cm square tabs. The markers should be distributed evenly across the entire area being photographed with the layout being roughly collinear and organized in a grid-like pattern. It is beneficial to place more markers on features such as architecture and bedrock (see Fig. 2). Fewer are necessary in “empty” spaces being photographed, though these spaces should still have collinear coverage. In open spaces within the square which consist only of earth or non-solid elements (like floors), the tabs can be put on the heads of nails pushed into the ground. The author has found surveyor’s Mag Spikes to be useful for this purpose. Because of the size of the head, they can even be used without the colored tab. They are particularly useful for photogrammetry of sections (see below).

 

2. Total Station Recording of Control Points

After the markers are placed, points are taken directly on them with a total station (see Fig. 3). This assigns spatial information to the markers in the form of x, y, z coordinates which can be directly viewed in programs like AutoCAD and ArcGIS.

    

 

3. Photography

After the points are recorded, a photograph of the area is taken from above, with the lens of the camera roughly parallel to the subject. For this step, the JVRP employs a modified painters’ pole with a heavy-duty camera mount (we use the Pole Pixie Professional Adapter + Tilt Mount) and a handheld radio remote control. This allows the photographer to position the camera up to 3 meters above the square and take the pictures immediately with the remote. A powerful digital camera is important for clear results. The JVRP uses a Canon EOS 60D Digital Single Lens Reflex camera for high resolution photographs. For basic photogrammetry any point and shoot camera will suffice, although photographs with a resolution of at least 5184x3456 pixels will produce better georectified results. See Fig. 4 for an example of a raw photograph taken with this setup.

 

4. Georectification

Back in the lab, the total station points are manually referenced to the visible points in the photograph using ArcGIS (see Fig. 5). The points from the total station are exported into a GIS-ready shapefile that can be viewed in ArcGIS and displayed as a layer in the program. Then the photograph is imported into ArcGIS. Using the Georeferencing tool, each total station point is matched to the corresponding tab in the photograph. This takes roughly 20 to 30 minutes depending on the number of control points recorded. When enough points have been matched (see Fig. 6), the technician uses ArcGIS to carry out a polynomial transformation on the image, which projects the photograph onto the spatial information gathered from the total station points, and warps the image to correspond to these points. For more information on cartesian coordinate transformations see Section 5.4, Subsection iii of this spatial referencing website.

The result of this transformation is a photograph with minimal lens-distortion placed within a real-world spatial context (see Fig. 7). This spatial context can be a site’s local grid, latitudinal and longitudinal coordinates, or in the case of the JVRP, the Israeli Transverse Mercator coordinate system. Now, this image can be dropped into any spatially aware software program and be placed automatically in spatial relationship with any other georectified images, maps, or plans. See the video below for an animation of this transformation process.

 

 

 

 

 

Digitizing Plans from a Georectified Photo

Perhaps one of the most beneficial applications of photogrammetry is the ability to digitize a feature directly from a georectified photo. This can potentially eliminate the need for manual hand-drawing, or at least reduce significantly the time involved in planning. The JVRP uses both traditional hand-drawn plans and digitization directly from georectified photos depending on expediencies in the field. The procedure for digitizing from a georectified photo is very simple and can be done in any vector drawing program like Adobe Illustrator (though this program can create scaled drawings, it doesn’t translate the spatial information) or CAD/GIS program like AutoCAD or ArcGIS (both of which encode spatial data in every point, line, and polygon drawn). The procedure varies slightly according to the program, but basically a “pen”-tool is used to trace directly over the image. The result is a digital drawing with accurate spatial information.

There are two distinct advantages of digitizing directly from the georectified photo. First, the hand drawing step is eliminated - even a hand drawing will eventually have to be digitized for publication. Eliminating this step reduces errors that occur when a drawing is made from a drawing. Second, the drawing from the georectified image is much more precise, avoiding the potential for overestimation and error inherent in taking hand measurements and translating those to drawing paper.

Below is an example of plans produced by the traditional hand-drawn process (Figs. 8-9) and again by using the above process (Figs. 10-11). Fig. 8 shows the hand-drawing of the architecture, and Fig. 9 is the completed digitized version of the hand-drawn plan produced with the digital “pen”-tool in AutoCAD. In the second set of figures, Fig. 10 shows a georectified image that was created using the JVRP’s photogrammetry workflow (above), Fig. 11 shows a hybrid plan made by digitizing the architecture directly from the georectified image. A more accurate plan was produced in a fraction of the time by the georectified image procedure with the end result yielding multiple visualization abilities: digital drawing, georectified image, and a hybrid image, all with spatial data allowing them to be imported into mapping software such as ArcGIS.

 

     

 

    

 

 

Site-wide Plans and Stratigraphical and Architectural Photo-mosaics

The procedures described above, produce both georectified spatially correct photos and plans of individual architectural elements. Since these products are encoded with spatial information in the form of coordinates in a known geographic coordinate system, they can easily be plotted together in programs such as AutoCAD and ArcGIS. This means with simple importing into these programs, one can create site-wide plans combining any of the photogrammetric products. Figure 12 shows the full extent of the excavated area at Tel Megiddo East from the 2011 and 2012 seasons. Additionally, photomosaic plans may be compiled from the orthophotos of each architectural element. Figure 13 represents a single stratum from Tel Megiddo East compiled from the appropriate orthophotos. Photogrammetry, therefore, is not just a means to an end (i.e. the speedy creation of traditional digitized architectural plans), but is also a valuable photographic record of the excavated remains and a powerful tool for visualizing the stratigraphic record.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Photogrammetry of Sections

Along with architectural features, documenting the vertical stratigraphy of archaeological sites is an essential component of archaeological recording. Just as photogrammetry documents horizontal surfaces, it can also be used to document vertical sections with undistorted spatially accurate photographs and section drawings digitized directly from these photographs. The process works under the same principles as the plan-view photogrammetry described above.

Traditionally, documentation of a fully exposed section is typically accomplished by a hand drawing (see Fig. 14) that is later digitized (as with the traditional architectural planning). However, a georectified photograph can be taken of the section, and a digitized drawing made from it, providing both a spatially accurate photographic record and a digitally-drawn record. In order to produce a georectified photograph of the vertical plane, markers similar to those mentioned above, are placed along the section. Typically the author affixes colored tape to the end of a nail and inserts the nail into the section in a roughly collinear pattern. After the markers are placed, their spatial information is collected with a total station. This is a good opportunity to use the total station’s reflectorless mode, which allows the points to be collected without using the prism. This mode is particularly useful when collecting points along a vertical section, since positioning the prism is often problematic. After the points are collected, the section is photographed from ground level, as directly as possible.

Because georectification software “thinks” in a topographical coordinate system, the x, y, z, values of the points on a section must be dealt with separately. If they are not, the georectification software will attempt to “look” at the image from the top - i.e. as though it were looking at a physical photograph from above with the photograph standing on its edge. The x, y, z, points, must be converted so that the georectification software “thinks” that looking directly onto the vertical photograph is the same as looking down at a horizontal plane - i.e. the z-value (up and down) should become the y-value (north and south) in the converted coordinate system.

This could, in theory, be done manually, converting and typing in the coordinates for each in the total station (or in the total station’s exported files), but this would be time consuming. JVRP team member and programmer Peter Ostrin developed a simple conversion utility called TS_Sectioner (TS standing for Total Station) for this operation (Ostrin 2012). One simply imports the total station output file into this application and the utility outputs a new total station file with the converted coordinates.

The new coordinate file and the photograph of the section can now be imported into ArcGIS and the points can be matched up to the visible markers in the photograph according to the technique of a horizontal photogrammetry job. This produces a georectified photograph (see Fig. 15). This georectified section photograph can then be used on its own as an accurate representation of the section, it can be used to add details to the hand-drawing, or if a drawing was not originally made in the field, the photograph can be traced to produce an original drawing. This tracing can be done digitally or by hand, using the same method as digitizing architecture from a georectified photograph. The digitized drawing can then be used on its own (see Fig. 16), or it can be visualized as a superimposed layer on the actual photograph by applying transparency to the digitized vector lines (see Fig. 17).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Summary

The Jezreel Valley Regional Project has successfully integrated photogrammetry into its daily documentation workflow during the 2011 and 2012 seasons of its Tel Megiddo East excavation. We have created complete photogrammetric architectural plans (see Fig. 13), as well as digitized some features from the rectified photographs (see Fig. 12). Our results have shown that this technology has great potential and can effectively augment traditional archaeological documentation.

 

Recommended Bibliography (will be updated as further materials are published)

Bitelli, G., et al. 2004. Low-Height Aerial Imagery and Digital Photogrammetrical Processing for Archaeological Mapping. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences 34, 55-9.

Bitelli, G., et al. 2004. Low-Height Aerial Photogrammetry for Archaeological Orthoimaging Production. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences 34, 55-9.

Farjas, M. 2009. Digital Photogrammetry: 3D Representation of Archaeological Sites. Ingeniería Cartográfica, Geodésica y Fotogrametría [Online]. Available: http://ocw.upm.es/ingenieria-cartografica-geodesica-y-fotogrametria/3d-scanning-and-modeling/Contenidos/Lectura_obligatoria/photogrammetry1.pdf.

Matsumoto, K. & Ono, I. 2009, Improvements of archaeological excavation efficiency using 3D photography and Total Stations. In: 22nd CIPA Symposium, October 11-15, 2009, Kyoto, Japan.

Osrin, Peter. 2012. TS_Sectioner. Megiddo Software Applications.

Skarlatos, D. & Rova, M. 2010, Photogrammetric Approaches for the Archaeological Mapping of the Mazotos Shipwreck. In: 7th International Conference on Science and Technology In Archaeology and Conservation, 7-12 December, 2010, Petra.

 

Online Resources Linked

Adobe, Illustrator Product Page

Autodesk, AutoCAD Product Page

Autodesk, AutoCAD Map 3D Services & Support. Exporting to a Shapefile

Brand, Peter J., Jean Revez, Janusz Karkowski, Emmanuel Laroze, and Cédric Gobeil. 2011. Karnak Hypostyle Hall Project, Report on the 2011 Field Season for the University of Memphis & the Université de Québec à Montréal

Canon U.S.A., EOS 60D Product Page

ESRI, ArcGIS Product Page

ESRI, ArcGIS Resources. Fundamentals of georeferencing a raster dataset

ESRI, ArcGIS Resources. Georeferencing tools

Forestry Suppliers, Mag Spikes

Google Maps

Google Maps, Satellite Imagery of New York, NY

Jezreel Valley Region Project, White Papers in Archaeological Technology

Knippers, R.A and Hendrikse J. 2001. Coordinate transformations. Kartografisch Tijdschrift, KernKatern 2000-3.

Machine Control Online. Hahn, Paul F. 2010. The Bottom Line: Reflectorless Total Stations & Their Applications

Osrin, Peter. 2012. TS_Sectioner. Megiddo Software Applications.

Pole Pixie

Virtual Terrain Project, Coordinate Systems and Map Projections

Wikipedia, Geographic Information System

 

Figure 1a: Photo before georeferencing. Note the distortion. The subject of the photo is at an oblique angle to the viewpoint.

Figure 1b: Photo after georeferencing. Note that the subject of the photo is now perfectly parallel to the viewpoint.

Figure 2: Red markers on an architectural feature.

Figure 3a: Points being taken with a total station.

Figure 3b: The total station's prism being held on a point.

Figure 4: Photograph taken from above with a modified painters' pole and a radio remote-control.

Figure 5: Visible points being matched with total station points.

Figure 6: All the control points matched in ArcGIS.

Figure 7: A completed georectified photograph after a polynomial transformation was applied in ArcGIS.

Figure 8: A hand drawn plan from JVRP's 2011 Tel Megiddo East season.

Figure 9: A standard digitized plan from JVRP's 2011 Tel Megiddo East season.

Figure 10: A georectified photo from JVRP's 2011 season.

Figure 11: Hybrid plan produced by digitizing directly from the georectified photo.

Figure 12: Complete digitized plan of Area C from the JVRP's Tel Megiddo East excavations. This plan was produced using a combination of digitized hand-drawn plans and digitization done directly from the photogrammetry.

Figure 13: Complete photogrammetric plan of Area C from the JVRP’s Tel Megiddo East excavations.

Figure 14: Hand-drawing of a section from JVRP's 2011 season.

Figure 15: Georectified photo of a section from JVRP's 2011 season.

Figure 16: Section digitized directly from georectified photo.

Figure 17: Hybrid plan produced by digitizing directly from the georectified photo.

 

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