What do people mean or think they mean by "not to scale?" [closed]

I can't comment on the specific case of the 'Angeles National Forest' map you mention, but in my experience, 'not drawn to scale' indicates your meaning '1', not meaning '2'.

One reason the phrase may be added to a map that appears to be scaled correctly might be as a disclaimer, to protect against litigation if someone gets lost, for instance. The phrase 'warning: contains nuts' on a packet of peanuts is a classic example of this kind of thinking.

The increasing prevalence of this practice is outlined in this 'Food Magazine' article.

The message means #1: all parts of the map are not necessarily scaled identically (or accurately).

"There don't seem to be any features on the map that are drawn out of proportion to other features."

There might not seem to be any, yet there might be some. That's what the message is supposed to draw your attention to. If it were obvious that things are not to scale then saying so explicitly would not be so useful.

The message should be taken in the sense of the map not necessarily being accurate and constant wrt scale.

And there can be some map features (topographic symbols etc.) that do not necessarily accurately reflect the current sizes of the things they stand for (width of a road, stream, lake, or building).

And inaccuracy & imprecision can reduce a map's fidelity, and these can reflect slight local differences in scale. Add distortions due to projection...

In engineering drawings, a numeric dimension can be labelled Not to Scale to indicate an inaccurate proportion, as in your case 1. My instructor likes to use the example of drawing a broom; the most informative drawing would use cut lines to shorten the handle to allow the grip and the head of the broom to be drawn larger. The dimension for the overall length would be marked Not to Scale to warn the viewer to pay attention to the misrepresentation of the objects actual proportions.

On plans, maps, and engineering drawings it means that you should not measure a length on the document and then expect to be able to calculate the “real life” distance from it. (Sometime there is a warning saying “Do not scale”, meaning do try to calculate the real life measure from what you have measured on the document.)

On a real estate listing, the plan shows how the rooms relate to each other, not their correct size. The room sizes may be listed in the description.

So for example the construction plan of a house is drawn to show the rough sizes of the rooms, then the dimension of the rooms are written on. The window that is shown in a room should not be measured on the plan to get its size, you should look the specification document for the window size and how far it’s edge is from the corner of the room.

Or on a map, if there are two side roads very close to each other, they will be moved apart a bit so as to make the map easier to understand, this is called Cartographic generalization.

Cartographic generalization is the method whereby information is selected and represented on a map in a way that adapts to the scale of the display medium of the map, not necessarily preserving all intricate geographical or other cartographic details. The cartographer is given license to adjust the content within their maps to create a suitable and useful map that conveys geospatial information, while striking the right balance between the map's purpose and actuality of the subject being mapped.

On some town maps, the centre of the town will be shown in more detail, as if you are looking down on the town with a “fish eye” lens. The user of the map is expected to be walking round the centre of the town, the rest of the town is just shown to allow the user to orientate themselves.