Metal Detecting Research and Exploration - A Complete Guide, page 27
Basic Calculations Used in Map Research
DETERMINING THE SCALE OF A DIGITAL HISTORIC MAP - TUTORIAL
You can determine the scale of a digital historic map by either using a specific software (described on page 32) or by methods described on previous pages if at least one scale of any type is present on a map you are analyzing. In many cases it is the opposite.
For example, the value of a Swedish mile ("mil"; "Mÿl" - on historic Swedish maps) used to have a few different lengths, ranging from 10.688 meters (6.64 miles) to 10,972.8 meters (6.81 miles), in different parts of Sweden until 1699 (today in metric system, 1 "mil" = 10 kilometers = 6.2 miles). The discrepancies between the lengths of the same length unit come into play when a few historical maps are used for research, and this may greatly affect the results.
To avoid this problem, it is essential to determine the scale of each historic map. Described below is an example of how to figure out the scale of a digital historic map, i.e. when it is displayed on your computer monitor, if no scales are shown on the map, i.e. none of the methods described on a previous page can be used.
This simple method is based on a few calculations done with two sets of distance measurements taken from both a historic map and corresponding modern topo map. To obtain distance measurements, I use two sets of corresponding control points: one set originates from a modern reference map (topo map), while the points of another set are positioned on the old map.
For my reference map, I use the modern topographic map of a large scale, such as 1:50,000 or larger, which would be close to the scale of the historic map. The distance measurement between two control points on my reference map is considered to be perfectly accurate. To acquire the most precise ground measurement between these points, I use the web-based Google Earth application.
On the topo map, I choose the landmarks that would be mapped at the same spots on a historical map. And here is a trick: to serve this purpose, these landmarks cannot be man-made because they might have not remained at the same spots in the past four centuries. Basically only permanent and well-defined landmarks can be used for this project.
For that reason, I do not choose villages, roads, bridges, taverns and other structures to be my control-points or tie-points. Neither I can use some natural landmarks, such as springs, brooks, ponds, etc. And now I am facing another problem: there is a lack of static landmarks that are available on the historic non-topographical map. If it was a topographic map, for example, I would easily find a couple of large single-standing boulders - perfect permanent landmarks (they are always present on a modern topo map), and measure the distance between them.
For the sake of this map analysis tutorial, I am going to use any two suitable points I can find on historical map. Point A is a spot at which the original road used to cross the creek 300 years ago. On topo map, it is marked with abbreviation "бр." = "ford." Point B is a sharp turn of the road that I know has been there since the late 1600s. It is also important to choose the points that are at least at the same elevation level.