Observational drawing is the art of putting a mark on the page that represents something you are seeing. In order for the mark to be "accurate" it should be the right LENGTH, in the right DIRECTION or ANGLE, and be in the right POSITION on the page. How do we draw accurately? Measuring. You should spend a fair amount of time measuring before you start drawing. Determine the size of what you are drawing and where it needs to fit on the page. A couple exercises to help with measuring:
The dot quiz- These dots are on an 8.5X11 page. Use measuring to find relationships between the dots and plot them accurately on your own paper. Don't trace-
Use a pencil and your thumb with an outstretched arm to find measurements. Move your thumb up and down on the pencil for different sizes. Compare measurements (widths, heighths, lengths, etc.)
Use this image to draw from.