Below is a short excerpt on comparative measuring taken from one of my portrait drawing classes. This lesson covers how to develop the “envelope” using width to height measurements. A portrait has many angles and shapes to it, and if we understand the principles of comparative measuring, we can find solutions to the potential proportional problems that may arise. Every line and shape within the drawing can be verified with a measurement. This technique relies on vertical and horizontal measurements and may be used to construct the entire drawing, or simply to double check the proportions at various stages throughout the process.
When using this technique, remember to take your time. With practice this will eventually become a quick and convenient way to verify your judgements in any stage of the drawing process. If you are interested in another video on this topic, click here, https://riveraportraits.com/2018/10/12/sight-size-vs-comparative-measuring