  We present results from the largest contiguous narrow-band survey in the
near-infrared. We have used WIRCam/CFHT and the lowOH2 filter (1.187$\pm$0.005
$\mu$m) to survey ~10 deg$^2$ of contiguous extragalactic sky in the SA22
field. A total of ~6000 candidate emission-line galaxies are found. We use deep
ugrizJK data to obtain robust photometric redshifts. We combine our data with
the High-redshift Emission Line Survey (HiZELS), explore spectroscopic surveys
(VVDS, VIPERS) and obtain our own spectroscopic follow-up with KMOS, FMOS and
MOSFIRE to derive large samples of high-redshift emission-line selected
galaxies: 3471 H$\alpha$ emitters at z=0.8, 1343 [OIII]+H$\beta$ emitters at
z=1.4 and 572 [OII] emitters at z=2.2. We probe co-moving volumes of >10$^6$
Mpc$^3$ and find significant over-densities, including an 8.5$\sigma$
(spectroscopically confirmed) over-density of H$\alpha$ emitters at z=0.81. We
derive H$\alpha$, [OIII]+H$\beta$ and [OII] luminosity functions at
z=0.8,1.4,2.2, respectively, and present implications for future surveys such
as Euclid. Our uniquely large volumes/areas allow us to sub-divide the samples
in thousands of randomised combinations of areas and provide a robust empirical
measurement of sample/cosmic variance. We show that surveys for
star-forming/emission-line galaxies at a depth similar to ours can only
overcome cosmic-variance (errors <10%) if they are based on volumes >5x10$^{5}$
Mpc$^{3}$; errors on $L^*$ and $\phi^*$ due to sample (cosmic) variance on
surveys probing ~10$^4$ Mpc$^{3}$ and ~10$^5$ Mpc$^{3}$ are typically very
high: ~300% and ~40-60%, respectively.
