Using maps as historical sources
Maps are vital historical evidence but must be interpreted cautiously and carefully. They should not be used in isolation. Related documents may give information about accuracy, purpose, method of production, uniqueness, or may confirm map details. Such documents may be essential for full use of the map. For example, sea charts could need sailing directions to be understood; boundary maps may need legal documents; estate plans may need a key.
When was the map made?
When was the survey made?
The date of map information relates to the date information was collected, not necessarily the publication date.
What is the publication history of the map?
Is this the earliest version of the map, or is the information copied or compiled from previous maps?
What is the printing date?
Is it a reprint of an earlier map?
If the map is undated or the date is ambiguous -
Is there any external date evidence?
For example: a dated watermark in the paper; scientific dating of vellum, parchment, paper, ink or paint; evidence of wear in the copper plate.
Is there any internal date evidence?
For example: style of decoration or cartouche; known date of an event (a new building, a new road or bridge); the publisher's address; a dedication.
How was the map made?
Was the map made from an original field survey?
Several people might be involved in making an early map. A surveyor measured and collected information; a draftsman selected information and drew a manuscript map by hand; an engraver engraved a flat metal plate, usually of copper; a printer impressed the copper plate onto paper; and a publisher distributed and sold the printed map. Sometimes a mapmaker might perform all these tasks, but they were often done by different people. Each stage of the map production process could introduce errors.
What surveying methods and instruments were used?
Accuracy (in mathematical terms) depends upon survey methods and instrument precision. However, lack of mathematical accuracy need not hinder the use of a map as a historical source, if the early map is used together with a more accurate modern map.
Was the map copied from other maps?
Some maps may be copied from a single earlier map; other maps may be compiled from several sources. Copying may lead to transcription errors. These errors may be useful in dating map copies, if the error is continued in later versions.
Did the engraver introduce errors?
Engraving involves preparing a mirror (reversed) image on a copper plate. Errors are especially likely if the engraver is foreign (language difficulties) or unfamiliar with terrain (how could a Dutch engraver imagine the Scottish mountains?).
Did the mapmaker re-use an existing copper plate?
Engraved copper plates were costly to produce and the mapmaker made the most of his investment. Information could be altered (by hammering out a section of the plate, then re-engraving the section) or the plate might pass to another mapmaker, whose name would be inserted as the new publisher, but the map information could be unchanged.
Why was the map made?
What was the purpose of the map?
For whom was the map intended? No map can show all available information. Mapmakers select information to suit the purpose of the map.
What criteria did the mapmaker use to select his information?
For example, did he show only towns of a certain size? Did he concentrate on roads and omit trees?
Is the mapmaker consistent with this selection?
For example, a placename may be missing for various reasons. Perhaps it did not fit the mapmaker's criteria (eg. village size), or there was not enough space to include it (or it was put elsewhere), or it was left out by accident.
What bias might the mapmaker have?
Has the map been prepared to make a political point? Maps make a powerful statement (for example, to show imperialist intentions or to establish nationhood).
Is there deliberate omission?
For example, industrial sites, military installations or boundaries might be omitted for military or political reasons - or deliberately put in the wrong place.
What was the market for the map?
What was the social and intellectual climate? For example, 'gentlemen's seats' might be shown to increase the market for the map, or antiquities shown because of fashionable interest.
Does the mapmaker use conventional symbols?
Are they used consistently? Do they correspond to the use of a similar symbol by another mapmaker? Could symbols be misinterpreted because of modern assumptions about symbols?
Does the map show what actually existed?
Is it a plan of proposed changes or developments? Did these changes ever happen?