3D scans for better profile pictures
A guide for quick orthophotos
Summary
Speed and efficiency in recording information is the main task of any excavation.
The following quick guide describes the steps to generate orthophotos of a profile using a fast 3D scan. With this method it is possible to derive clean profile images even in the most difficult situations, without having to resort to complex layer measurements with total stations or traditional 2D photogrammetry images using measured nails and image rectification.
Typical problems when photographing profiles
Profiles can be complicated to photograph. If the pit is too deep or the trench too narrow, the opposite sides obscure the clear view of the exposed layers. Pipes or cables block the shot or the profile is simply too long to be captured cleanly in a picture. As we can rarely take pictures directly in front of the profile, profile shots are often heavily distorted and *tilt*.
At the same time, we record many things that are not part of the actual profile, such as the area above the profile or areas and plana of other features. This makes it more difficult for other people to work with existing documentation.
Last but not least, profile recordings are often shown to non-archaeologists in order to explain what has been found. However, experience has shown that they have difficulties in recognizing the actual profile between the various sands, planes and layers and in grasping and understanding the features cut into it. The unfamiliar perspectives are also of little help to the layman. Of course, most profiles are also recorded graphically, but a clean, distortion-free photo documentation can support the interpreted drawing better than a perspective-distorted image. In the following, I would like to show how such orthophotos can be created with minimal additional effort and outline an initial simple workflow.
On the excavation
Once the profile has been uncovered, it is prepared as normal for photographic and drawing documentation. This means that the surfaces are straightened, the corners are cleaned and any measuring nails for the elevation line are inserted and measured. The photo panel is pinned and positioned.
The profile is then photographed with the digital camera as usual.
After these photos for general documentation, additional photos are taken from as many perspectives as possible to create a 3D model. Perhaps change the memory card, as the images do not necessarily have to be included in the photo documentation lists. However, discuss this with the relevant heritage authority beforehand.
Vary the height of the camera, the location and the shooting direction. If more than one profile in the immediate vicinity is to be digitized, photograph these as well.
The entire profile does not have to be visible on every single image, only a good overlap between two images is advisable.
For a simple profile (single profile in a house connection trench), 10-25 recordings are sufficient. You will need about 5-10 minutes for this and then the data recording is already complete.
As always, “A lot helps a lot!” so you are welcome to take more photos.
Please remember to measure the profile as usual!
Model generation
At the end of the working day on the site or, if you have enough staff, also in conjunction with this, you process the images and create the model, derive the orthophoto and add this to your documentation.
Photogrammetry software
The choice of software to create the 3D model is relatively secondary. I personally prefer Agisofts Metashape, as it offers a good offline license and is relatively inexpensive.
Alternatively, you can also use Reality Capture
Reality Capture as a currently free alternative. However, you should read the terms of service. As the software incurs license costs above a certain amount of revenue.
Other alternatives are:
Agisoft Workflow
In this example, we use Agisoft and explain step by step how we can create a 3D model from simple images.
The embedded video below is in German but has English subtitles!!!
Workflow Summary:
- Upload the unprocessed data to your computer in a well–described working directory.
- Open Agisoft Metashape and import the images into a chunk.
- Calculate the camera positions of the loaded images. (Workflow -> Align Photos -> Standardeinstellungen)
- Inspect the reconstructed thin point cloud and adjust the reconstruction space if necessary.
- Create a 3D mesh. (Workflow -> Build Model -> Depthmap, mittlere oder hohe Auflösung)
- Trim the mesh to the required area.
- Generate the 3D texture. (Workflow -> Build Texture -> 8192 Px groß (Increase the number of textures for large profiles if necessary))
- Export the model as .OBJ or .FBX with texture to the working folder.
We now have a 3D model of the cut. We can now cut this directly in Agisoft or use it for further processing in Blender 3D.
Orthophotos of the house connection profiles from the tutorial video.
Of course, this may seem complicated at first, but almost all of these steps can be automated. This allows you to calculate and export a large number of models one after the other without having to stay at the PC yourself.
Results
With these simple workflow steps, profiles, planas and entire excavation sections can be quickly captured and cleanly mapped. With just a few minutes of additional effort, distortion-free images can be created to supplement or, in some cases, replace traditional documentation.
The size of the profiles to be digitized plays a minor role. If you intend to capture larger profiles, simply take more images. Whether it’s a fiber optic connection pit, a cut post hole, hundreds of meters of route profiles or the excavation of building foundations, you will always get clean profiles.
No possibility/desire to calculate the data yourself?
If you do not have the capacity to calculate the models yourself, I will be happy to help you out.
Just send me a short e-mail to Kontakt@praehist3d.de and we will find a way for you to take the photos yourself and get orthophotos and 3D models calculated by me.
You are also welcome to book a workshop to create your own 3D data!
