Preliminary drawings provide the means to identify and correct errors in measuring. Once drawing has begun, errors in wall alignment, window placement, roof heights, for example, quickly become apparent, especially when plans are overlaid, elevations are matched with adjacent elevations and corresponding sections, etc.
Corrections are of course more easily made on pencillings than on ink drawings.
Fort Sumter National Monument, Sullivan's Island, South Carolina. A number of factors may contribute to the misalignment of drawings. Adjacent elevations will not align if, for example, the end point of an eave line is visible on one elevation, but on the adjacent elevation the eave line is hidden by a gutter and not measured. Also, improperly established or recorded datum lines and the use of different brands of measuring devices and drawing scales lead to mismatches. Finally, variations in temperature and humidity cause vellum to change shape and size, so drafting film is recommended when highly accurate preliminaries are required.