  This paper presents a method of estimation of emission altitudes using
observational data - precise measurements of pulse profile widths at low
intensity level. The analysis of emission altitudes obtained using this method
for a large number of pulsars gives constraints that should be useful for
theory of coherent pulsar emission. It seems that radio emission originates at
altitudes of about few percent of the light cylinder and that they depend on
frequency, pulsar period and period derivative.
