Depth 

Within a flat 2-D image such as a drawing depth refers to the illusion of distance or a third dimension within it. This can be achieve by employing a variety of different techniques such as overlapping the elements within, varying the their size as well as including a fore, mid, and background. The depth within your image can be further exaggerated by using a considered viewpoint. This is the angle at which elements within your image are being viewed (ie top down, bottom up) and can be used to create more interesting or harmonious compositions visually as well as reinforce ideas and motifs resent within (ie for a picture of David vs Goliath you would choose to have a bottom up view point from David towards Goliath so as to further exaggerate the size disparity of the two figures.

Finally when tackling depth(as well a composition in general) you also have to consider line of sight. This is the way in which the main lines of the elements within your image are arranged/interact with each other so as to direct your eyes unconsciously to the centre of interest. As it controls how your eyes move through an image, line of sight can naturally be used to create a sense of physical movement of the elements within your image. Depending on the intentions of the image maker and subject matter this can be used to create something rhythmic and harmonious or conversely something abrupt, jarring and full of tension.


Miroslav Sasek

Here overlapping of the figures, as well as varying their size is used to create a sense of depth and space of visual elements that would other wise be very flat. This is something that seems highly relevant to my own practice as I often draw in a very flat and graphic images and thus this could be used within my own work to better describe depth/space within my compositions.
As well as this several key lines of site are used to draw your eye your eye in a flowing and uniform manner(which mirrors that of the train tracks being depicted) to a vanishing point in the back. This coupled with the use of over lapping elements as well as the rule of thirds creates a very effective sense of depth within the image as well as an over all sense of harmony.


Moebius (Jean Giraud)

One of my all time favourites and a real master at composition and,in particular, creating a vary realistic sense of space and depth in otherwise highly surreal images. 
In this image this is achieved in a variety of different ways. Firstly there is a very clear fore (the foremost rock face), mid (bird,central set of rocks and waterfall) and background (furthest set of rock faces.
On top of this overarching of elements and varying of size is also used. To a lesser extent this is evident in the cliff faces and boats but is used much more clearly in the repetition of the mist clouds in the water fall that gradually get smaller and smaller.
Finally a carefully considered use of line of sight is used simultaneously to create a greater sense of depth and harmony, buy moving your eye from bottom left to top right, as well as draw focus to and make the man riding the bird the centre of interst by placing it in this line of sight.