Isometric means "equal measurement". The true dimension of the object is used to construct the drawing. You get the true dimension from either orthographic views or by measuring the object. Because of the convenience of using actual measurements to create the isometric image, it has become the industry standard for parts manuals, technical proposals, patent illustrations and maintainance publications.
The height of the object is measured along vertical lines. The width and depth of the object are measured along the 30 degree to the horizontal plane.