Wikipedia:Using maps and similar sources in Wikipedia articles

This is an essay.

It contains the advice or opinions of one or more Wikipedia contributors. This page is not an encyclopedia article, nor is it one of Wikipedia's policies or guidelines, as it has not been thoroughly vetted by the community. Some essays represent widespread norms; others only represent minority viewpoints.

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WP:MAPCITE

This page in a nutshell:

Maps and line charts used as illustrations follow the same rules as pictures that are used as illustrations
Maps and line charts may be used as sources, but the core content policies Verifiability, and No original research still apply.
It is not original research to extract information from a map or line chart using standard techniques, particularly if those techniques are common to two or more disciplines.
The core content policies of Verifiability, and No original research apply to user-made maps.

Any map or line chart used in Wikipedia should conform with Wikipedia's policies of neutrality, reliability and verifiability. Maps can be used for two purposes in Wikipedia articles, as sources or as illustration. Regardless of the point of using a map in an article, the following points should be kept in mind:

The presence of an object on a map is not sufficient by itself to show notability of a subject.
Even maps produced using reliable sources (such as GIS data) can have minor errors as a result of errors in the underlying database, or the cartographer trying to overlay data from two non-linear sources. Minor map errors are common, for that reason any map detail that is key to the article should be confirmed with a separate map from a different publisher. Rarely are map errors notable, even if they have gone uncorrected for decades.
Many cartographers intentionally introduce minor errors in inconspicuous locations as a form of copyright trap. Likewise, rarely are such introductions notable.
Many companies publish a yearly edition of a map, and include the year as part of the title (e.g. 2009 Road Atlas). As these have to be drafted in advance, often the cartographer will draw projects under construction as completed, or proposed projects as under construction. This is an effort to increase the shelf life of the map, so as to not be obsolete before the year on the edition name has finished. (e.g. a highway expected to be completed by 2009 may be drawn as complete in the 2009 edition, even though it was still under construction when the map was drafted) This can introduce another source of error in an otherwise reliable map, should a project be cancelled or significantly changed. Such errors can be detected by comparing multiple editions of the same map.
Any cultural bias apparent in a map should be clearly explained in a neutral fashion. For example, Wikipedia's article about the Falkland Islands sovereignty dispute features side-by-side maps showing the rival naming conventions used by Argentina and the United Kingdom for the named features of the islands, without implying one is correct. In cases where it is not practical or possible to show maps from each POV in a boundary or sovereignty dispute, a caption should explain which details of the map are disputed.
Editors who use maps should be aware that maps covering large areas almost always have one type of distortion or another, depending on the map projection used. Care should be taken to select a map whose projection is appropriate for the purpose to which it is being put.

Illustration

When a map is used for illustration, the map should follow the same principles as photos used for illustration. Maps that are classed as primary sources may be used as illustrations provided that the accompanying text identifies their authorship.

Editor-created maps should be careful to only depict details supported by reliable sources in the article and common information that would appear on any published map relating to the subject. For user-created maps based on GIS or satellite images, it is acceptable to use details already present in the database used to create the map. Before cloning or adding details to third-party created maps, ensure that the derivative work complies with copyright laws. When uploading the image to Wikipedia or Commons, list all sources used on the file description page.

Sources

Usage

Maps are used not only in geography, but also in many branches of mathematics, science and engineering where gradients are calculated in areas such as vector calculus, electric fields, fluid dynamics and so on. Geographical features such as saddle points are often discussed in a mathematical rather than a geographic context. However, topological assessments made from maps follow the same rules in all of these subjects and are often first introduced at secondary school level geography. Making topological assessments using techniques that are standard in two or more disciplines is a reasonable way of judging if a map has been properly used as a source.

Remember that people who have good spacial visualisation ability have no problem using maps, but people who have poor spacial visualisation ability may find it difficult to relate to maps. This is often the underlying reason for disputes as to what constitutes original research and what does not.

From a mathematical point of view, line charts are nothing more than one-dimensional maps, so the same rules regarding the extraction of information apply.

Reliability

Similar to other types of sources, maps should not be self-published sources, and the reliability of the publisher should be considered before use. Maps directly derived from government surveys, GIS data and aerial or satellite images are generally reliable. Maps made by commercial interests for promotion of business may not be reliable. Online maps should be treated the same as web sources, listing both the date the cartography was completed (if known) and the date the information was accessed, as online content can frequently change.

Relevance

Maps can be used to source specific details about the subject, consistent with the intended purpose of the map. For example, It would be appropriate to use a geographical map to source elevations, common names or relative locations, but not necessarily historical timelines. Maps depicting the historical growth of a city are appropriate for historical information, but may not be accurate in other details, such as geographical names. Road maps may be appropriate to source distances and routes, but not land ownership, etc.

Original research

Use of a map to source information beyond the intended use of a map can violate Wikipedia's policies, such as no original research, verifiability or reliability. For example, if an object does not appear in the 1950 and earlier editions of a map, but does appear in the 1951 and later editions of a map, this does not prove the year of construction was 1950, unless there is additional information in the map, as this is using the map in a different manner from its intended purpose. There are two potential flaws with using maps in this manner. First commercial maps are sometimes drafted in advance of the edition date. Second, commercial maps sometimes draw projects expected to be completed during the edition year as complete. Government and non-profit maps are more likely to list cartography and effective dates and are less likely to include anticipated details in a map verses commercial maps. In any case, it would be acceptable to say "The freeway first appeared on the department of transportation's map in 1951" or "The freeway was constructed by 1951" (assuming the source has a reputation for accuracy in its map updates) in the above case. Some maps are totally unreliable if used in a manner for which they were not designed. For example, maps of mass transit systems are usually not drawn to a scale, as they are drawn to show the interconnections of a system in such a way that the map will fit in a limited space. This makes such maps unsuitable for other uses. For example, the London Underground Map is an unreliable source for estimating the distance between two stops, but it is an excellent source for the number of stops on a given route.

When using maps as a source for topographic descriptions, care should be taken not to read more into the map than is already there. It is quite in order to state the “Valley X is ‘U’ shaped with glacial moraine at its entrance”, assuming that is what the map shows; the words “glacial moraine” should be explicitly stated on the map. However it is original research to state that “Valley X is glacial in origin” unless the map actually states that (e.g., in a caption), because there might be some other explanation for the valley's origin or the presence of the glacial moraine.

Description

Maps are often used to provide a description of linear features, such as a waterway, mountain range, road, railroad, or pipeline, etc., when doing so a single map should not be used the sole source used to provide the description. While a map is useful to source details in a general description section, other types of sources should also be used, if available. For some subjects the only sources available will be maps and it will be necessary to have the entire general description sourced from maps. Keep in mind that different types of information will be visible depending on the type of map used (i.e. road map, political map, topographical map, or recreational map). Ideally a map of each of those types should at least be consulted in drafting a description, if not used. For example if only a road map is used, the summary would consist almost exclusively of road features, and neglect geographical or political considerations. Using different maps of the same type (e.g. two road maps from different publishers) would likely not enhance the description, however may provide value to confirm a suspected map error or controversial detail. If a description only contains details from one map, the reader could save time by just looking at the map to get this information. The author should remember that a good article will describe an object above and beyond what is visible in a map.