  The Euclid mission will conduct an extragalactic survey over 15000 deg$^2$ of
the extragalactic sky. The spectroscopic channel of the Near-Infrared
Spectrometer and Photometer (NISP) has a resolution of $R\sim450$ for its blue
and red grisms that collectively cover the $0.93$--$1.89 $\micron;range. NISP
will obtain spectroscopic redshifts for $3\times10^7$ galaxies for the
experiments on galaxy clustering, baryonic acoustic oscillations, and redshift
space distortion. The wavelength calibration must be accurate within $5$\AA to
avoid systematics in the redshifts and downstream cosmological parameters. The
NISP pre-flight dispersion laws for the grisms were obtained on the ground
using a Fabry-Perot etalon. Launch vibrations, zero gravity conditions, and
thermal stabilisation may alter these dispersion laws, requiring an in-flight
recalibration. To this end, we use the emission lines in the spectra of compact
planetary nebulae (PNe), which were selected from a PN data base. To ensure
completeness of the PN sample, we developed a novel technique to identify
compact and strong line emitters in Gaia spectroscopic data using the Gaia
spectra shape coefficients. We obtained VLT/X-SHOOTER spectra from $0.3$ to
$2.5$ \micron;for 19 PNe in excellent seeing conditions and a wide slit,
mimicking Euclid's slitless spectroscopy mode but with 10 times higher spectral
resolution. Additional observations of one northern PN were obtained in the
$0.80$--$1.90$ \micron range with the GMOS and GNIRS instruments at the Gemini
North observatory. The collected spectra were combined into an atlas of
heliocentric vacuum wavelengths with a joint statistical and systematic
accuracy of 0.1 \AA in the optical and 0.3 \AA in the near-infrared. The
wavelength atlas and the related 1D and 2D spectra are made publicly available.
