Seeing as neither the camera positions nor the 2D images are required to contain real world coordinates, the 3D point clounds that are generated through SfM photogrammetry are generated in a relative image-space coordinate system. This means that in order to be quantitative, the generated point clouds need to be aligned to a real-world, object-space coordinate system [WBG+12]. This transformation can be achieved using a 3D similarity transform based on either known camera locations, a small number of known ground control points (GCPs) with known object-space coordinates, or a combination of the two, which is covered in the upcoming sections of the Geo-SfM module.
Coordinate reference systems
A coordinate (or Spatial) reference systems (CRS) define specific map projections and the transformations that are needed between different systems. A CRS is always composed of a coordinate system and datum component. A coordinate system is a set of mathematical rules that specify how coordinates are to be assigned to points. A datum is a set of parameters that define the position of the origin, the scale, and the orientation of a coordinate system.
Luckily, these are pre-defined by the European Petroleum Survey Group (EPSG) for the most common CRSs in use [InternatonalAoOaGProducers19]. For example some codes often in use in Svalbard are:
EPSG:4326 corresponds to WGS 84 longitude-latitude (used by GPS).
EPSG:32633 corresponds to WGS 84 UTM-33N.
EPSG:40400 is a special wildcard CRS that has no real world meaning, e.g., useful for hand samples - though sadly not supported by Metashape (instead, use 9001 for Local Coordinates in metres).