Any alteration of a IGC flight data file with a text editor will cause
it to fail the (free) IGC Validation program that checks the file for a
number of things. An altered file will not be valid for any activity
that requires IGC-standards of flight data. This includes badge and
record flights, and competitions that use the IGC Validation check.

The alteration even of only one character in the flight data will cause
the Validation check to fail.

In terms of fix intervals, the Sporting Code makes the point that it is
the SETTING that the pilot that uses that matters. What happens in
flight may differ, particularly in conditions of poor GPS reception.
Short losses of GPS fixing should not invalidate a flight as long as it
is obvious that there was not time to carry out an intermediate landing
and re-launch. However, loss of fixes in an Observation Zone cannot be
remedied as there will be no evidence of reaching the point concerned.

Continuity of the flight record should be shown by a continuous
pressure altitude trace and, for motor gliders, a continuous engine
noise trace (even though no GPS fixes are being recorded, or, as often
happens, GPS altitude recording is lost for a time)

There is a significant difference between the pilot setting of fix
interval, and small variations that occur in the air due to adverse GPS
reception for short periods. Of course pilots should do everything
possible to ensure best antenna position, no kinks or breaks in the
antenna cable, adequate battery power to the GPS, and so forth. But
they should not be penalised by things out of there control when it is
perfectly obvious that no intermediate landing has occurred. IMHO, of
course!