During tracing of ray segments connecting two wavefronts, we obtain an estimate of the square of maximum error in distance per second traced ray. This quantity, ERR, has the unit of velocity squared (km²/s²). According to whether the user selects accuracy "Low", "Moderate", "High", or "Very High" the quantity ERR will be kept between predefined limits. The current limits set in the program are the following:

  • Low:
    • ERR must be <= 1.0e-2
  • Moderate:
    • ERR must be <= 2.5e-3
  • High:
    • ERR must be <= 1.0e-4
  • Very High:
    • ERR must be <= 1.0e-6

For velocity fields including relatively sharp contrasts it is recommended to use High or Very High to ensure that the ray tracer handles such contrasts properly. For smooth velocity fields, Moderate or Low may often be sufficient.