External Geometry is information that the user knows about the model object or scene that is provided to PhotoModeler to help solve certain projects, to set up a coordinate system, add additional modeled details, or to check and verify results. External Geometry can be points, distances, or meshes.
A key use of External Geometry in PhotoModeler is to set up a coordinate system. After a project has been processed properly, the 3D model’s proportions will be correct. In other words, if the width of a door is half its height, PhotoModeler will calculate a value for the model door width that is half of its calculated height. On the other hand, the absolute value assigned to the width of the door will not be its actual value and the position and rotation of the door in 3D space may not match expectations. To correct the scale so that the width of the door matches its actual measurement, you must enter a known distance between two points in your model to 'scale' it. Similarly, the rotation and translation of the model may have to be defined to match the real world.
The modification of the coordinate system of the 3D model, which involves translation, scale and rotation, is called a coordinate system transformation. This transformation does not change the shape of, nor warp the model. The technical term is a Helmert Transform.
The coordinate system transform in PhotoModeler is not applied permanently to the model data, that way it can be changed at any time without affecting accuracy. The transform is applied before 3D Export, before displaying the 3D Viewer, and before displaying any data in Tables or Measure.
Control Points are 3D xyz points from an external source (survey, GIS, etc.) can be used to define a coordinate system as well but may change the shape or ‘warp’ the model.
A PhotoModeler project can be in the following states with respect to the coordinate system (translation, scale, and rotation):
• no applied coordinate transform (project will be in an arbitrary coordinate system after processing)
• no applied coordinate transform but the project was done with control points, so the project is already in the user’s desired coordinate system, and
• a combination of one, two or all components of translation, scale and rotation as defined by the settings described below.
You will usually want to get your project into states 2 or 3 but this depends on your needs. For distance measurement you will want to at least apply a scale.