Use this map's own key, not a symbol meaning remembered from another map. Distinguish existing structures, proposed structures and operating access. Read the title, date, purpose and coverage. A plan can include existing places without proving that every proposed change was completed.
Worked example
A harbor plan uses a filled square for one existing pier and a hollow square for a proposed pier. A dashed crossing means a proposed ferry route. The two squares do not prove two completed piers or a running ferry service. A later inspection could help check what was actually completed.
The fictional North Path proposal
A park planner drew this North Path proposal in 1940. Filled diamonds mark existing shelters A, B and C. Hollow diamonds mark proposed shelters D and E. The dashed line means a proposed footpath, not confirmed open access. This diagram is not to scale and includes only places along this proposal, not every park shelter. No completion or opening record is supplied. A later newsletter claims that all five shelters and the path were open in 1940, using only this plan as its source.