I just found this information today in an Article I posted in the Resources tab, Guidelines for Collegiate Faculty to Teach Mathematics to Blind or Visually Impaired Students, by Maneki (2017):
The degree of resolution that the human eye can distinguish is much finer than that which human fingers can interpret. For a complex tactile drawing to be meaningful, it must be much larger than a drawing intended for the human eye. Furthermore, the sense of touch has no direct equivalent to the distinction of colors. The translation of visual graphics into tactile images means the production of line drawings, and this conversion requires human intervention. (para. 16)
For those of us in science working to make our content accessible, it is sometimes difficult to write informative alternative text (alt-text) for the plethora of figures, pictures, tables, and graphs used in our courses. Some instructors say that they put a short description in the alt-text, and indicate that a tactile image is available as a supplement. The paragraph copied above reminds that those tactile images must be large enough to convey the necessary information. Simply making a raised line drawing by tracing the actual image may not be adequate. Tools that may be useful for creating raised line drawings that are right side up, and that can be modified, are the inTACT Bundle and the Sensational BlackBoard.
Accessible Science 