If you want to find the ‘x’-values where ‘y’ equals ‘thresh’, you need to reverse the order of ‘x’ and ‘y’ in the interp1 call. However with your data, you first have to find the approximate index of that value, since you cannot simply reverse the order of the entire vector.
Try this:
yval = find(y >= thresh, 1, 'last'); % Define Region To Interpolate
vq1 = interp1(y(yval-1:yval+1), x(yval-1:yval+1), thresh,'pchip') % Interpolate In Region
vq1 =
23.3818
