The POINT:SCALE parameter is a single floating-point value that stores the scaling factor that is applied to convert each of the signed integer 3D point values into the reference coordinate system values recorded by the POINT:UNITS parameter.  A positive SCALE factor indicates that the 3D data is stored as signed 16-bit integer values, while a negative SCALE factor indicates that the C3D file contains 3D points that have been pre-scaled and stored as floating-point values.

The POINT:SCALE parameter effectively documents the resolution of the stored 3D point values recorded in the C3D file and is used by both integer and floating point storage methods to generate each 3D co-ordinate residual value that documents the accuracy of each 3D measurement.  A copy of the SCALE parameter value can also be found stored in words 7-8 of the C3D file header.

The 3D scale factor is the maximum coordinate value present in the 3D data divided by 32000, thus a POINT:SCALE parameter value of 1.0 (or -1.0) indicates a 3D data resolution of 1mm.  This allows the 3D point coordinates to be recorded within the range of a 16-bit signed integer.  Since the POINT:SCALE value is required to interpret the 3D residual it is important that the correct SCALE value is calculated if the 3D information stored in floating-point format.  Failing to calculate the correct SCALE value can cause data loss if the file contains coordinates stored as floating-point values and is converted to an integer file for user application compatibility.

Note that if an integer formatted C3D file is converted to a floating-point C3D file then it is important to preserve the absolute POINT:SCALE value, as this documents the C3D data resolution and will allow the file to be transparently converted back into an integer form if needed at any time for other applications.  It is essential that the correct POINT:SCALE value is calculated and saved for all storage formats as it is used to scale the 3D residual information when a C3D file is stored any format.  Failing to calculate the POINT:SCALE parameter value violates the C3D format definition because the value is required if the C3D file is stored in integer format.

A negative POINT:SCALE value indicates that the file is already scaled and stored as  floating-point values.  The absolute POINT:SCALE value, ignoring the sign, is used to scale the 3D residual value for all formats but is only used to scale 3D data stored as signed integer values, it is not used to scale 3D floating-point data.  As a result, the POINT:SCALE factor must always be accurately calculated, and stored as a locked value, because any casual changes have the potential for 3D data corruption.

When C3D files with the POINT:SCALE value set to -1 are seen, it indicates that the application creating the file does not fully support the C3D file format and as a result the file cannot be saved as an integer format C3D file, and probably only contains processed values, not actual measurements.