The models which are prepared with the same scale ratio in different directions are known as Geometrically true models.
However, sometimes it is not possible to use a true model. So different convenient scales are used in different directions.
Such models which are prepared with different scales in different directions are known as geometrically distorted models.
Important examples of this type of model may be of a very wide and shallow channel. When prototype is a very wide and shallow channel, then there is a huge difference between horizontal and vertical dimension. So, different convenient scales in different directions are used according to requirements. Such a model is called as distorted model.