A wavelength calibration technique is described, which is based on a combination of a Fourier transform wavelength meter and a distributed feedback laser locked to a molecular transition as a frequency marker in the spectrum. The technique provides a reliable wavelength scale to be used in high resolution continuous wave cavity ringdown spectroscopy without need for stabilization of the probe laser and accurately known molecular transitions in the scanned wavelength range. Due to a continuous reference measurement, ambient influences on the laser sources are effectively suppressed. As an example, we measured highly resolved cavity ringdown spectra of N2O isotopomers and determined the line strength of several absorption lines at a wavelength around 1687 nm. A near infrared wavelength precision of 6×10-8 and an absolute accuracy on the order of 1×10-7 was readily achieved. The general concept is easy to implement and can be further refined by using additional reference lasers, thus holding the potential of even higher wavelength accuracy.