The following article by Adam Prins and Matthew J. Adams of the Jezreel Valley Regional Project (JVRP) was originally published as the first installment of the JVRP White Papers in Archaeological Technology series. Bible History Daily updated and republished the article with the consent of the authors. All photographs and figures shown below are courtesy of the JVRP. Click here to learn more about participating in the JVRP Tel Megiddo East excavation, click here to read the publication on the JVRP website or scroll down to read a brief bio of the authors.
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 photographs 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.
Traditional photography, while essential for archaeological documentation, has a number of drawbacks. A camera’s lens introduces significant distortion into its images, especially if a wide angle lens is used; as a result, no reliable measurements can be taken from a standard photograph. Secondly, 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 two-dimensional representation of a plane with limited applications. Photogrammetry eliminates these drawbacks and opens up new opportunities for photography to aid archaeological documentation and research by allowing for the creation of spatially accurate, two-dimensional images—in both 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).
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 author Adam 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 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 1 cm square tabs of either red or yellow duct tape. 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, their exact locations are measured 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.
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 painter’s 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 three feet 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 5184×3456 pixels will produce better georectified results. See Fig. 4 below for an example of a raw photograph taken with this setup.
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. This image can now be dropped into any spatial software program and automatically placed into a spatial relationship with any other georectified images, maps or plans. See the video below for an animation of this transformation process.
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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 significantly reduce 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 (however, while this program can create scaled drawings, it doesn’t translate the spatial information) or a 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 to digitizing directly from a 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 another drawing. Second, drawing from a georectified image is much more precise because it avoids the potential for overestimation and the 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 using the process described above (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) and 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 using the georectified image procedure, with the end result yielding multiple visualization abilities: digital drawing, georectified image and a hybrid image, all with spatial data that allow them to be imported into mapping software such as ArcGIS.
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, making it a powerful tool for visualizing the stratigraphic record.
Along with architectural features, documenting the vertical stratigraphy 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 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, 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 either 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).
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 success with this technology reveal its great potential to effectively augment traditional archaeological documentation.
Adam Prins is a PhD candidate with the departments of Archaeology and Geography at Durham University. A JVRP field archaeologist and technology specialist, he has worked with archaeological projects and museums in Israel, Egypt, Cyprus and the United States and is a staff member of both the Jezreel Valley Regional Project and the Tel Megiddo Expedition. His current research involves developing new technologies for use in archaeology, emphasizing new applications for LiDAR (Light Detection and Ranging) and three-dimensional modeling. He has developed and deployed practical workflows for photogrammetric and three-dimensional imaging of archaeological sites, and works to integrate these and other new technologies into traditional archaeological documentation.
Matthew J. Adams, University of Hawaii, is the director of the Jezreel Valley Regional Project. Adams received his PhD in History from the Pennsylvania State University in 2007, specializing in Egyptology and Near Eastern Archaeology. He has more than 20 seasons of excavation experience at sites in Egypt and Israel. His primary research focus is on the development of urban communities in the third millennium in Egypt and Levant. In addition to directing the JVRP, he is also a member of the Penn State excavations at Mendes, Egypt, and the Tel Aviv University Megiddo Expedition. He is also President of the non-profit organization American Archaeology Abroad.
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