When reading numerical data values in the primary array or a table column, the values will be scaled automatically by the BSCALE and BZERO (or TSCALn and TZEROn) header keyword values if they are present in the header. The scaled data that is returned to the reading program will have
output value = (FITS value) * BSCALE + BZERO(a corresponding formula using TSCALn and TZEROn is used when reading from table columns). In the case of integer output values the floating point scaled value is truncated to an integer (not rounded to the nearest integer). The ftpscl and fttscl subroutines may be used to override the scaling parameters defined in the header (e.g., to turn off the scaling so that the program can read the raw unscaled values from the FITS file).
When writing numerical data to the primary array or to a table column the data values will generally be automatically inversely scaled by the value of the BSCALE and BZERO (or TSCALn and TZEROn) header keyword values if they they exist in the header. These keywords must have been written to the header before any data is written for them to have any effect. Otherwise, one may use the ftpscl and fttscl subroutines to define or override the scaling keywords in the header (e.g., to turn off the scaling so that the program can write the raw unscaled values into the FITS file). If scaling is performed, the inverse scaled output value that is written into the FITS file will have
FITS value = ((input value) - BZERO) / BSCALE(a corresponding formula using TSCALn and TZEROn is used when writing to table columns). Rounding to the nearest integer, rather than truncation, is performed when writing integer datatypes to the FITS file.