In this paper we describe and evaluate new spectral line polarisation observations obtained with the goal of mapping the surfaces of magnetic Ap stars in great detail .
One hundred complete or partial Stokes IQUV sequences , corresponding to 297 individual polarised spectra , have been obtained for 7 bright Ap stars using the ESPaDOnS and NARVAL high resolution spectropolarimeters .
The targets span a range of mass from approximately 1.8 to 3.4 M _ { \odot } , a range of rotation period from 2.56 to 6.80 days , and a range of maximum longitudinal magnetic field strength from 0.3 to over 4 kG .
For 3 of the 7 stars , we have obtained dense phase coverage sampling the entire rotational cycle .
These datasets are suitable for immediate magnetic and chemical abundance surface mapping using Magnetic Doppler Imaging ( MDI ) .
For the 4 remaining stars , partial phase coverage has been obtained , and additional observations will be required in order to map the surfaces of these stars .
The median signal-to-noise ratio of the reduced observations is over 700 per 1.8 km s ^ { -1 } pixel .
Spectra of all stars show Stokes V Zeeman signatures in essentially all individual lines , and most stars show clear Stokes QU signatures in many individual spectral lines .
The observations provide a vastly improved data set compared to previous generations of observations in terms of signal-to-noise ratio , resolving power and measurement uncertainties .
Measurement of the longitudinal magnetic field demonstrates that the data are internally consistent within computed uncertainties typically at the 50 to 100 \sigma level .
Data are also shown to be in excellent agreement with published observations and in qualitative agreement with the predictions of published surface structure models .
In addition to providing the foundation for the next generation of surface maps of Ap stars , this study establishes the performance and stability of the ESPaDOnS and NARVAl high-resolution spectropolarimeters during the period 2006-2010 .