Photogrammetry

Photogrammetry is a technique that uses photographs for measurements, such as determining the positions of geographical elements or creating 3D models. Photogrammetry can be performed from a distance, such as by using drones, or up close with a regular camera. A popular branch of photogrammetry is Structure-from-Motion (SfM), a technique that generates 3D models from two-dimensional image sequences. 

Through photogrammetry, 3D models are created from photographs taken from multiple angles. This often results in large datasets. If the image collection is not well documented, it may be difficult for others to understand how the images relate to one another. Therefore, all information that makes it easier to understand the image material should be documented, including metadata for geographical locations.  

What to preserve?

Sometimes it is important to measure the exact geographical location when photographing. If you want to determine the position of a photographed object, you need to integrate reference points into the project. Reference information can be ground control points (photographable points with known coordinates), lengths of photo-identifiable objects (e.g., checkerboard paper sheets), and/or angles between photo-identifiable objects. A map of the location, where the object and control points are indicated, should be included in the documentation.

Photogrammetric models often lack scale information, and in such cases, this needs to be added by the data creator. For example, you can measure an easily recognizable length of the object or place a ruler or scale indicator (such as a stick or cube of known size) in front of or beside the object. Such reference information should be documented.  

To easily determine the orientation of the photogrammetric model, you can place a north arrow next to the photographed object. For more precise orientation, 3D control points are required, i.e., identification marks with known coordinates that are either directly on or close to the object. All such reference data should also be documented.  

Data management in photogrammetry

If you have created a 3D model through photogrammetry, the model is likely the most important product. However, to allow photogrammetric data to be reused, you should make at least the photographs accessible. Other data products are also useful, but the photographs are essential for generating a new model. It is also desirable to make point clouds and other files that capture various stages of the photogrammetry process accessible, as this saves time and simplifies the process of generating the model again. Additionally, calibration images and coordinates for control points may be needed if the photogrammetric model contains spatial reference information.

Typically, photogrammetry software collects all related data in a project file. The project file with the final model can also be made accessible. The project file may include a list of all uploaded photographs, layers and masks applied to the photographs, information about the camera(s) position in relation to the object, point clouds, mesh models, and texture reconstructions. The project file often links to data stored elsewhere, so it is important to ensure that the file path is correct.

Photogrammetric project files are often in a proprietary format, which can complicate reuse of the files. If possible, data should be exported to a more common, open, and well-documented file format. If exporting the entire project file is not possible, you can still facilitate secondary users by exporting point clouds and mesh models as separate files.

Recommended file formats for sharing and long-term preservation

The following formats are suitable for sharing and long-term preservation of mesh models but should only be used when X3D format is not available:

• COLLADA (.dae)
• Stanford PLY (.ply). The ASCII version is preferred for long-term preservation
• Universal 3D Format (.u3d)
• VRML (.vrml).

For many 3D models, the OBJ and PLY formats have the best potential for preserving geometry and visual surface properties, but they are not suitable for more complex scenes that require light sources or interactivity.

Filmbox File/FBX file (.fbx) is a commonly used format that supports embedded textures and assembly/animation data. It is easy for secondary users to reuse shared FBX files, but since the format is proprietary, it is not recommended for long-term preservation in archives or similar.

For simpler 3D data, CSV (for point clouds) or STL (STereoLithography format, for triangular facets) can be used for sharing and long-term preservation.