code: def estimate_hsr(self,R,z=0.,dR=10.**-8.,**kwargs):
          
          Rs= [R-dR/2.,R+dR/2.]
          sf= numpy.array([self.sigmaR2(r,z,use_physical=False,
                                        **kwargs) for r in Rs])
          lsf= numpy.log(sf)/2.
          return -dR/(lsf[1]-lsf[0])
docs: NAME:
  
             estimate_hsr
  
          PURPOSE:
  
             estimate the exponential scale length of the radial dispersion at R
  
          INPUT:
  
             R - Galactocentric radius (can be Quantity)
  
             z= height (default: 0 pc) (can be Quantity)
  
             dR- range in R to use (can be Quantity)
  
             density kwargs
  
          OUTPUT:
  
             estimated hsR
  
          HISTORY:
  
             2013-03-08 - Written - Bovy (IAS)