3D Photogrammetry Used During Rescue Excavations Of Highway D11

About

Hello, my name is Štěpán Kravciv and I work at the University of Hradec Králové in the Czech Republic as a member of a team responsible for 3D and 2D documentation of archaeological excavations. More specifically of documentation of one of the largest rescue excavations, which was conducted in the modern history of the Czech Republic.

The Project

The team consisted of 4 people—2 GIS specialists, one 3D specialist, and one geodet, but all of them were trained to be able to replace other members if necessary.

The excavations were conducted in advance of construction of highway D11 between Hradec Králové and Jaroměř in the eastern part of the Czech Republic (Figure 1). The total length of this highway section is 25 km and it took roughly a year and a half, 30 archeologists, and hundreds of workers to finish the excavations. Thousands of archaeological features were found there, and 3D photogrammetry was used for documentation of features with a high density of artefacts such as storage pits (D11 06-8 obj 1500, above) or ovens (D11 07-4 obj 2183, below) and for documentation of graves (animals or humans).

Every archaeological feature was accurately measured with GNSS rover or with total station. On top of that, every grave had its skull, pelvis and lower limbs measured with geodetic leveling. This approach allowed us to georeference 3D models, if necessary, and to create plans of excavated areas.

Challenges

The total length of this excavated area brought challenges:

• a daily commute between individual parts of excavations in all kinds of weather;
• sometimes it was necessary to work quickly because the anthropologists were waiting on us before removing the bones from graves.

Figure 1. Highway D11 between Hradec Králové and Jaroměř

Tools

The process of creating 3D documentation can be quite time-consuming. Therefore, we were looking for a simple solution which would provide accurate results. In this case, we selected the Agisoft Metashape (formerly known as PhotoScan) software. This software creates image-based photogrammetry with the help of SfM algorithms (structure from motion) and is user-friendly. Its only limitation is the number of photographs. More photographs require more RAM for processing, but also provides more accurate results. Therefore it is necessary either to buy a computer with sufficient RAM or to create less complex models.

Workflow

The workflow itself is easy. The first step is to take enough photographs in the field of the selected feature. It is necessary for these photographs to overlap (at least 30% overlap in each pair of photos is recommended) and to cover the whole feature. It is also necessary to avoid moving objects and shadows. During the first attempts, we had trouble with moving shadows due to the clouds and it was necessary to use canvas to avoid this. The amount of photographs depends on the complexity of the archaeological feature. Generally, it varies between 40 and 130 photographs for one feature. We were using a Canon D60, but frankly, any digital camera—even a phone—would suffice.

The second step is to load photographs into the software and align them. Agisoft automatically determines the alignment of these images and creates tie points between them (Figure 2).

Figure 2. Photo alignment in Agisoft Metashape, blue rectangles represents positions of individual photographs

In the next step, Agisoft builds a dense cloud at the selected quality (Figure 3) and, in the last step, it creates a 3D mesh (Figure 4) that can be covered with texture created from photographs.

Figure 3. Generated dense cloud of a human grave

Figure 4 3D model without texture

Results are generally exported in OBJ format with texture and imported into Autodesk Meshmixer to repair errors and to simplify the model before uploading it to Sketchfab. The whole process takes roughly a couple of hours for each model but doesn’t require any user inputs during most of the processing.

I highly recommend the use of Agisoft Metashape software in cultural heritage. It has proven to be a powerful tool for creating 3D models from photographs. Although it is not available for free, there are huge discounts for cultural institutions and students.

Sketchfab

At this time we have uploaded almost 180 models to Sketchfab in a dedicated collection and more are on the way. Sketchfab was used due to its ability to upload large models and ability to share models simply across all platforms. Archaeologists were, therefore, able to use models, instead of standard photographic documentation, for their reports.

Štěpán Instagram / Štěpán on ResearchGate