Is there two Types of Scale Space in SURF algorithm? I couldn't figure out below paragraph on SURF paper and hope that someone can help me to understand it. 


  Bay H., Ess A., Tuytelaars T. Van Gool L. - Speed-Up Robust Features
  (SURF), page 4, column 2
  
  The large scale changes, especially between the first filter within these octaves (from 9 to 15 is a change of 1:7),
  renders the sampling of scales quite crude. Therefore, we
  have also implemented a scale space with a finer sampling
  of the scales. This first doubles the size of the image, using
  linear interpolation, and then starts the first octave by filtering with a filter of size 15. Additional filter sizes are 21,
  27, 33, and 39. Then a second octave starts, again using filter which now increase their sizes by 12 pixels, after which
  a third and fourth octave follow. Now the scale change be-
  tween the first two filter is only 1.4 (21/15). The lowest
  scale for the accurate version that can be detected through
  quadratic interpolation is s = (1.2(18/9)/2)=1.2.


is that mean they used two different scale space at same time or they used each one individually and choose the one with better result?
They implemented each one separately, then compared their results to each other.



FH-9  --->  it's a detector that it's initial filter size is 9x9

FH-15 -->  it's a detector that it's initial filter size is 15x15