Calculate the surface intensity for each pixel in the interior
        region of the ROI. Pixels are adaptively subsampled around the
        kernel centroid out to a radius of 'factor * max_pixrad'.

        Parameters:
        -----------
        factor : the radius of the oversample region in units of max_pixrad

        Returns:
        --------
        surface_intensity : the surface intensity at each pixel

def calc_surface_intensity(self, factor=10):
        """Calculate the surface intensity for each pixel in the interior
        region of the ROI. Pixels are adaptively subsampled around the
        kernel centroid out to a radius of 'factor * max_pixrad'.

        Parameters:
        -----------
        factor : the radius of the oversample region in units of max_pixrad

        Returns:
        --------
        surface_intensity : the surface intensity at each pixel
        """
        # First we calculate the surface intensity at native resolution
        pixels = self.roi.pixels_interior
        nside_in = self.config['coords']['nside_pixel']
        surface_intensity = self.kernel.pdf(pixels.lon,pixels.lat)

        # Then we recalculate the surface intensity around the kernel
        # centroid at higher resolution
        for i in np.arange(1,5):
            # Select pixels within the region of interest
            nside_out = 2**i * nside_in
            radius = factor*np.degrees(hp.max_pixrad(nside_out))
            pix = ang2disc(nside_in,self.kernel.lon,self.kernel.lat,
                           radius,inclusive=True)

            # Select pix within the interior region of the ROI
            idx = ugali.utils.healpix.index_pix_in_pixels(pix,pixels)
            pix = pix[(idx >= 0)]; idx = idx[(idx >= 0)]

            # Reset the surface intensity for the subsampled pixels
            subpix = ugali.utils.healpix.ud_grade_ipix(pix,nside_in,nside_out)
            pix_lon,pix_lat = pix2ang(nside_out,subpix)
            surface_intensity[idx]=np.mean(self.kernel.pdf(pix_lon,pix_lat),axis=1)

        return surface_intensity