I am reposting something from six years ago: To begin a drawing done on location, we must first select an advantageous viewpoint that conveys a sense of place and frame the composition to fit on the page. Then, a crucial step is establishing the “bones” of the drawing—its basic structure—with the first lines we draw. For some views only a few lines may be necessary while for others, more might be required. 

It is essential to understand that once this structure is established, changes can still be made to calibrate scale, improve proportional relationships, and adjust the positioning of elements. Drawing these first few lines is simply a way to block out the essential relationships on a page quickly, before expending too much time on a drawing only to find out that a portion might be misplaced or is out of proportion to the rest of the composition.

Here is an example—a very quick outline of a view of the Campo in Siena. With more time and better weather, I might have finished it but I think it is possible to see and visualize the space even in this incomplete state.