Does there exist an isometry from a Manhattan plane $A$ to a Euclidean plane $B$? I.e. a function $\varphi:A \to B$ that suffices $\|\varphi(a)\|_B = \|a\|_A$ for all $a \in A$, where $\| \cdot \|_A$ is the Manhattan norm and $\| \cdot \|_B$ the Euclidean norm.
I actually had the same question, and the answer is that there is no isometry, but you can find a relation between both norms. It takes to add a second variable to the norm, that is the number $N(a)$ of minimal paths to cover $a$ on the manhattan plane.
$||a||_B = \phi(||a||_A,N(a))$ or $||a||_A = \psi(||a||_B,N(a))$
You can verify easily on a few examples that it works, and if you compute the number of paths, you will find out that as long as you are just after the euclidean norm. If you compute the number of paths, you'll find explicit forms for $\phi$ and $\psi$.