# Relative scale in SfM

I'm using monocular visual odometry. As in this case absolute scale can't be recovered, we rather compute relative scale for subsequent transformations as in this tutorial

To do this, triple matches across three frames are required. We then triangulate two $3D$ points $X_{m}$ and $X_{n}$ from images $\{i,i+1\}$ and $\{i-1,i\}$ then the relative scale can be determined from the distance ratio between point pairs in subsequent image pairs as follows:

$r=\frac{\parallel X_{m,\{i,i+1\}}-X_{n,\{i,i+1\}}\parallel}{\parallel X_{m,\{i-1,i\}}-X_{n,\{i-1,i\}}\parallel}$

My question concernes the coordinate system of the triangulated $3D$ points, the points have to be expressed in the coordinate system of the first, second or third frame or it dosen't matter as we are using the norm?

The triangulation method that I'm using outputs a $3D$ point expressed in the first viewpoint. How to convert it to another viewpoint?