Eric Greenwell wrote:
How do they do this? Do they also record temperature along with pressure
altitude?

====
Sporting Code Section 1 - Aerostats
Annex 2 - Calculation of Geometric Altitude from Barometric Altitude:

3. Meteorological information must be obtained for a position and
time as close as possible to that of the flight. The surface
pressure should be obtained together with temperature and
(optionally) humidity for a range of heights up to the height
being measured. If the meteorological information is not
available the air must be assumed dry, the temperature the
coldest possible for that location and season, and the surface
pressure the lowest that could have been possible.

4. The claimed altitude must be adjusted for the effect of the
atmospheric data by a met hod which can be shown to be correct.
Calculations have been accepted using the following methods:

1) CALCULATION OF CORRECTED ABSOLUTE ALTITUDE by Hans Akerstedt
(Version 2/95 June 1995 effective date) - a method of manual
calculation.
2) CAMERON BALLOONS PROGRAM FOR FAI RECORDS (CBFAI version 97.3
and later). This is a program which gives a result which is
as precise as the data used, calculating the atmosphere layer
by layer.
3) Direct interpolation is possible using certain types of
meteorological data. The result must usually be converted
from geopotential to geometric metres.

Altitude calculations are very complex and procedures can differ
for different types of instrument and available meteorological
data. It is recommended that specialist help be obtained.
====

I believe they still primarily use barographs (an IGC-approved flight
recorder is acceptable as a barograph). I don't think they've
transitioned over to use of GPS derived geometric altitude as of this
moment.

Marc