I have bathimetry data in .shp format. The points (picture below shows one of multiple groups/profiles) do not have ID numbers and are randomly dispersed through the attribute table.

[![enter image description here][1]][1]
[![enter image description here][2]][2]


I have selected each group and added group ID to it. Now each point has "Sort" attribute that serves as the group ID. I still need each individual point to have ID number within the group. 



I need these points numbered starting with the northernmost point in group (Field2 in attribute table is northing) and ending with the southernmost. So, a point with highest "Field2" value gets value of 1 in "point_num" field.

Basically, what I need to  do is sort selected points according to value of "Field2" attribute. How can I do this with (preferably) QGIS field calculator or some other tool?


  [1]: https://i.sstatic.net/WTUks.png
  [2]: https://i.sstatic.net/UtrS4.png