WISE            –                  wise       w1 w2 w3 w4
  [**South**]{}                                 
  Survey          sub-surveys        mnemonic   filters
  BCS             –                  bcs        g r i z
  DECam           –                  decam      g r i z
  VISTA           VHS                vista      J H K
  SSDF            –                  irac       36 45
  GALEX           MIS AIS GI         galex      fuv nuv
  WISE            –                  wise       w1 w2 w3 w4

  : Photometric surveys and filters used for counterpart association. The mnemonic for surveys (all lowercase) and filters (capitalisation as shown) is used to name the columns listed in Table \[TabMulti\]. The IRAC filter mnemonics ‘36’ and ‘45’ correspond to the $3.6\mu$m and $4.5\mu$m filters. The CFHT filter labelled ‘y’ is an i-band replacement filter. []{data-label="TabSurv"}

Summary of photometric surveys\[SecPhotoSumm\]
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The XXL-N area is covered by the following surveys:

-   Most of XXL-N was covered by the CFHT Legacy Survey (CFHTLS) Wide1 (W1) and Deep1 (D1) fields in five filters ($u,g,r,i,z$); we used the T0007 data release [@2007AAS...210.6202V; @2012yCat.2317....0H]. Observations were obtained with the 3.6m Canada-France-Hawaii Telescope (CFHT), using the MegaCam wide field optical imaging facility. The northernmost part was observed at CFHT in the $g,r,z$ bands (PI: M. Pierre) and the relevant fields are labelled WA, WB, WC in Table \[TabSurv\].

-   The Sloan Digital Sky Survey (SDSS) 