The outline is the bare-bones "template" that's used to write the essay.
The relationship between a skeleton and a human body
If 'S' is the relationship between actual and scale linear dimensions,then 'S2' is the relationship between actual and scale areas.
scale
Scale
Scale
The Scale.
Map scale
For book reports (as for most other kinds of writing), a sentence-outline has several disadvantages relative to a topic-outline. For one thing, a sentence-outline is more lengthy and thus more cumbersome to serve as a helpful outline for the actual writing. For another thing, a truly helpful sentence-outline itself will rely on a clear understanding of "topics" before the actual sentences of such an outline can be meaningfully organized.
I would extend this to any essay, whether in school, in business, in journalism, an article for a club magazine or a formal technical paper. The Outline is simply the basic framework or summary- perhaps a set of headings even if not included as such in the final work - of the Essay. My method sometimes is to write a list of headings with big spaces between them, and add a few notes in those spaces to amplify the headings. This gives me my framework. Then I write the text itself from that amplified list - either afresh or using those headings within the essay, according to context and purpose.
Is the relationship between the distance measured on the map to the actual distance on the ground
That depends on the situation. In many cases, there is no relationship whatsoever between the two.
E = mc^2
A scale represents the relationship between a model and the actual measurement if the real object.