Height at the withers
It is measured from the ground to the highest point of the shoulder blades, with the dog standing square on a flat, firm surface. It is the measurement used in breed standards and show regulations, where some breeds must be measured.
Body length and back length
In the FCI model standard, body length is measured from the point of the shoulder to the point of the buttock (the rear projection of the pelvis). Some standards use instead the distance from the withers to the base of the tail, and “back” in cynological vocabulary means the region between the withers and the root of the tail. The withers-to-tail-base distance (“back length”) is a valid measurement, but it is not the body length of the FCI standards.
When someone asks for the “length” for a coat or a harness, ask from where to where: manufacturers use their own conventions.
Chest girth
The location is not standardized: the Swedish measuring regulations take it around the deepest part of the chest, the FCI Poodle standard measures it “behind the shoulders”, and the Kennel Club glossary “immediately behind the withers”. The results differ, so a figure only makes sense together with the place it was measured. For harnesses, follow the manufacturer's instructions, not a general rule.
Neck girth
There is no standardized definition in the FCI, Kennel Club or veterinary sources consulted; it is a practical convention for collars and harnesses, usually where the collar sits.
What the atlas does not do
The drawing has no scale, so it does not calculate centimeters and does not recommend harness or coat sizes. Its purpose is to show the landmarks, so that your measurement is taken in the same place as the one in the standard or in the manufacturer's chart.