Question

What is the measured value always positive from one point to another

Answer 1

It is called "distance". In fact, distance measures how far apart two points are from each other, and it is always a positive number.
In mathematics, for instance, the distance between two points of coordinates (x1,y1) and (x2,y2) on a xy-plane is defined as
$d= \sqrt{(x_1-x_2)^2 + (y_1-y_2)^2}$
and by definition, this number is always positive.