Data sets for proper motions in the Galactic planebcsv

In the context of Nina getting the best data sets for doing proper motion in the Galactic plane,
various discussions (Nina, David, Zeljko) have occurred on what data sets to combine with PS1
data for the most productive proper motion constraints. It seems there should be two sets of criteria:

-- what does the data set add in terms of  dmu ~ FWHM / (S/N) / Depoch

-- how painful are systematic issues, such as photo-plate boundaries etc..


SDSS-overlap areas
  -- take SDSS as an additional epoch
      * SDSS positions are based on the r-band; check what information arises
          from the g band position measurements
  -- take mu(SDSS-POSS) as a prior, rather than deal with (a,d)(POSS) itself
       * work out how to deal with the fact that mu(SDSS-POSS) may have systematics
UKIDSS: could be very interesting the galactic plane:
Questions:
  -- when are the observation epochs of the survey
  --  practical access: get the whole darn thing and add to LSD?
  -- some links
       http://www.astrogrid.org/agpython.html
       http://surveys.roe.ac.uk/wsa/theSurveys.html

Areas that have neither UKIDSS nor SDSS?
   -- take 2MASS, or is it not worth it?
   --