Despite recent advances in visualization, the map is still the most elegant and compact medium for displaying spatial data. The main role of the map is to communicate spatial information to the user, such as location, size, shape, pattern, distribution and relationships among spatial objects [5
]. Designing a map is a complicated process. Initially, any map as an abstract representation of reality has a number of characteristics, such as spatial data to be portrayed, projection and datum, scale, generalization, annotation and symbolization that contribute to the result. Moreover, and according to good cartographic practices, in addition to the area portrayed, a map usually contains the following map elements [6
]: Title and subtitle, legend, scale, frame (border) and neat line, orientation/graticule, data sources, credits and insets. After all, a map is judged by its overall appearance in terms of design, visual balance and graphic communication characteristics. As a result, when it comes to map reviews, all the above issues should be taken into consideration.
Maps provided by online map services can be considered as general reference maps. General reference maps are simple maps that portray important natural and man–made features. They are usually easy to read and understand. Wall maps, most maps found in atlases and road maps belong to this category. Medium scale general reference maps of land area that portray terrain with contour lines are called topographic maps. Therefore, criteria for topographic map design can be identified as a basis for the review and adjusted to online map services specific characteristics.
3.1. Map Elements
In this subsection, web map elements are discussed. Map elements include the geographic area covered, cartographic projection and datum, frame and neatline, orientation, scale, legend, title and subtitle, insets, credits and the resulting map layout.
Geographic area: On online map services, the user may select an area of interest and acquire a map. They provide global earth coverage with some restrictions explained in the next paragraph. As the map area varies upon user choice, it cannot be emphasized in the visual hierarchy.
Cartographic projection and datum
: The coordinate reference system used by online map services is the WGS84 ellipsoid and a new version of the Mercator projection introduced by Google maps, called "Web Mercator". WGS84 is the right choice for global earth coverage and warrants compatibility to GNSS receivers. The Web Mercator projection causes significant area and distance deformations to the areas away from the Equator, so the Earth’s portrayal can be implemented up to 85° North. The use of a sphere instead of the ellipsoid and the spherical equations while the geographical coordinates refer to the ellipsoid, have a negative influence on conformality. This deformation is not noticeable in small scales but becomes apparent at larger scales. According to the National Geospatial–Intelligence Agency (NGA), an unacceptable risk to global safety of navigation activities and operations that require accurate and precise positioning and navigation information [8
] is possible. While the Web Mercator is "good enough" for most web-based mapping tasks, it is not necessarily advantageous for the appropriate display of different geographic areas as one zooms in and out of a web map or changes map area [9
]. These concerns were addressed, and the Adaptive Composite Map Projection was developed and suggested as an alternative to Web Mercator [10
At the smallest scale supported, HERE and Wikimapia display a world map (zoom level 3) and OSM displays an annoying repetition of the world map in order to fill the screen (zoom level 0). Recently (2018), Google came up with a new update, which is only available on the desktop interface. At smaller scales, the Earth is displayed as a virtual globe which can be rotated and zoomed and the arctic regions that were not visible so far are displayed. It looks like a perspective projection is used, e.g., orthographic; however, to the authors’ knowledge there is no official documentation of the projection. Google virtual globe cannot be really entrusted without specific reference to the projection used for each zoom level.
Frame (border) and neatline: No neatline, bounding the detail of the map and separating it from marginal information, is shown. The frame (border) and the neatline coincide with the window of the browser. In paper maps, the border adds stability to map design as it separates the map elements from the rest of the page and constrains the map user’s attention. This role is now assigned to the window border with questionable results. Due to the lack of neatline, no information about the geographical coordinates of the mapped area borders is provided.
Orientation: Traditionally, a grid is shown at large, and medium scale maps and a graticule appears at small scale maps, providing orientation and positional information. Most maps, that do not use a grid or a graticule as those provided by online map services, offer some indication of orientation through a North arrow. If no indication is provided, North is assumed to be at the top of the map which is true when the Web Mercator projection is applied.
In case that the map reader is not familiar with the names shown on the map, the lack of graticule at small scale maps in conjunction with the absence of title and neatline, make it difficult if not impossible to perceive the geographic context of the map. A solution may be the reporting of the geographical coordinates of the map cursor. In Wikimapia, the geographical coordinates of the map center appear in the lower right part of the map. Since there is no reference to the map projection and the position of the cursor is reported in geographical coordinates, the novice user may consider that geographical coordinates can be portrayed on a plane (e.g., the screen) as such. Geographical coordinates of the map center are provided with one decimal digit in seconds. This means that the position is reported with precision which is not compatible with all Wikimapia map scales.
Scale: Online map services publish maps at a number of scales resulting from the zoom level selected by the user. The user is informed about the map scale through the graphical scale in Google, OSM and HERE. Graphical scale design is very poor. The scale appears on the lower part of the map and the user may not notice it, especially if the map background is vivid. The graphical scale is utilized traditionally only on large scale paper maps. On online map services, it is generally utilized, in order to assure that scale information adjusts to screen size and analysis and to depict projection deformation. The user is implicitly informed about the deformation caused by Web Mercator as he/she moves away from the Equator, since the graphic scale for the same zoom level changes as the latitude increases. Wikimapia does not provide a graphical scale but the zoom level information is descriptive, e.g., house, road, city, etc. This practice excludes any metric value from the map. On the contrary, although a distance measure tool is provided in Wikimapia and Google maps, distance measurements do not always correspond to true distances, due to great deformations away from the Equator.
The zoom level on web maps is an integer number from 0 (smallest scale) to 18 or greater (large scale). Each zoom level change alters the map scale by a factor of 2. Map scale depends on web map tiles, a hierarchical structure that relates the zoom level to the number of tiles utilized to portray an area. For example, for a 96 dpi screen resolution and tiles with 256 pixels, the scale ranges from 1:2257 on the Equator at zoom level 18 (zoom level—road) to 1: 591 658 710 at zoom level 0 (zoom level—world). Scales resulting from zoom levels and tiles do not comply with nominal map scales adopted in traditional cartography which are rather round numbers, e.g., 1: 2.500, 1:5.000, 1:50.000, etc., (especially in the metric system).
Previous studies have reported the absence of a legend on online map services [1
]. Legends or keys are indispensable to most maps since they carry the symbols explanations and any map symbol that is not self—explanatory should be included [11
]. Google, HERE and Wikimapia maps do not provide any legend. As a result, these maps can be used only for a general overview. Although some symbols are fairly intuitive and easy to understand, the information extracted about the descriptive attributes of the depicted features is inferential based on the users’ general knowledge and cannot be verified. Symbol meaning becomes more difficult when they express different categories of a certain feature type (e.g., Do types of roads at a nominal scale correspond to the motorway, dual carriageway, etc., or urban, rural, etc.? Do types of roads at an ordinal scale correspond to primary, secondary, tertiary, etc.?). Sometimes the user can acquire a brief description of a location or feature, but this functionality cannot replace the need for a clear map symbol explanation provided by a well-designed legend.
A map key is provided by OSM that adjusts to the map content in relation to the zoom level. However, the symbols included in the legend do not appear exactly as they look on the map, e.g., the width of road symbols is different (Figure 1
a) and some symbols included in the legend do not appear in the specific map area (Figure 1
The lack of legend may question map credibility. It might make one think that the meaning of map symbols is intentionally left intuitive in order to escape liability issues. Traditionally, the map-using community expects cartographers to conduct their business in a professional manner. Errors, due to wrong symbol interpretation, may at best cause inconvenience to map users and in extreme cases contribute to false decisions, e.g., the wrong interpretation of road category can lead to a car accident because of speed.
Title—Subtitle and Inset
: There is no title or subtitle and no insets are used. Map title [11
] reveals the map theme and/or the area covered by the map. Although the lack of title may be justified for a general-purpose map, at the same time, it hides information about the area covered by the map. Inset maps are used to add areas that are far away from the main geographical area of the map or to enlarge a portion of the map. However, it is advised to use them cautiously and, if possible, try to avoid them. However, in cases of countries like France or Portugal, overseas areas traditionally appear in insets. Due to the lack of insets on online map services, it is impossible to portray the country and the overseas areas without adopting a scale smaller than the one needed for the mainland.
Credits: Online map services utilize their own data sources. Therefore, the provider is the map publisher. As a result, the selection of an online map provider corresponds to the selection of a specific geographic database and a specific “view” of the world since there are no specifications to be followed as in traditional cartography. For example, topographic maps produced by NMA all around the world have many similarities in terms of content, layout and symbolization. Additionally, the data collection date is reported on paper maps but not on online map services. However, geographic data portrayed on online map services are continuously updated and thus it can be considered as always up-to-date. No information about the cartographic projection or the geographic reference system is given as in printed maps. As a result, only the expert user is aware of this information or may look for it on the web. Information that appears on online map services includes copyright information (Google, OSM, HERE, Wikimapia), terms of usage (Google, HERE), communication/feedback (HERE, Google). A reference in regional settings in Google maps (e.g., country name) may confuse the map reader who may assume that this is the geographic area displayed on the map.
The map body captures most part of the browser window. Credits always appear on the lower part of the map as footer. Other interactive tools that enhance the map user experience appear on the upper part of the browser window as a header, covering the whole window extend as in OSM (Figure A2
) and Wikimapia (Figure A4
) or part of it as in Google (Figure A1
) and HERE (Figure A3
). Additionally, navigation tools, such as zoom in, zoom out, pan, etc. are included. These tools are placed within the map area in a corner. Marginalia placement within the map frame is a common practice for thematic maps as well.
All the above, create a specific template that is applied in all maps published by an online map service. This is similar to the use of a specific layout by a map series and the adoption of its own cartographic style. Map series are usually produced by NMAs, e.g., topographic maps. They are used when an area, e.g., a country must be portrayed at a specific scale and thus must be spread over several sheets. These maps have the same scale, cover the same geographic extend, use the same database, the same symbols, etc., but they can also be used independently, as they have full marginalia. Maps by online map services can be considered as map series for a fixed zoom level as the geographic extent covered is also fixed for a specific device.
Practices followed by online map services regarding map elements are summarized and presented in comparison with traditional practices for paper maps in Table 2
, along with the consequences of certain choices. It seems that online map services generally comply with traditional cartographic best practices and in the case where erroneous decisions are made, e.g., cartographic projection, legend inclusion, etc., it is possible to adopt the appropriate tactic. On the other hand, online map services interactivity and accessibility create a valuable cartographic application.
3.2. Spatial Data
General purpose maps portray both natural and man-made features, such as coastlines, lakes, rivers, boundaries, settlements, roads, rail lines and others. In this context, thematic layers that appear on online map services are discussed. Indicatively, zoom levels 13 and 14, which are comparable to scale 1:50.000 of a printed map, are considered. Scale 1:50.000 is the basic scale for NMAs and thus provides comprehensive data coverage. Additionally, medium scale maps constitute the more complicated case, as large-scale maps provide enough space to facilitate map design whereas small scale maps design is simplified by the severe generalization degree.
Thematic layers of each online map service are presented in Table 3
. They all have more or less the same content. Differences appear in the portrayal of terrain and land use. Heterogeneous coverage is observed in Wikimapia, since thematic content in different areas at the same scale is not constant. Several areas are rather empty, due to deficient volunteer contribution and incompatibility is evident.
In order to comment on thematic layers completeness, the content of 1:50.000 scale topographic maps series was utilized. Compared to them, the majority of thematic layers are also portrayed on online map services (Table 3
). However, there is a lack of a number of point layers, which are traditionally portrayed on topographic maps, such as elevation spots, springs/fountains/wells, churches/convents/chapels etc. Online map services give emphasis to the portrayal of information that is more helpful to the citizen or the tourist in location discovery, e.g., points of interest, such as museums, parks, touristic attractions, castles, art galleries, etc., transportation information, e.g., metro/bus/tram stops and stations or the map users special interests (e.g., Google My places). As a result, it can be concluded that online map services content is as rich as that of a topographic map and capable to serve the role of general reference maps.
Online map services should reconsider the portrayal of terrain, land use and sea area. The terrain is portrayed with shading in Google (upon user selection “Terrain” or at smaller scales) and HERE maps. Spot heights are depicted in OSM and no terrain information appears in Wikimapia. As a result, in Google and HERE, the user is generally informed about the terrain as needed. Especially for OSM, partial terrain portrayal (only with spot heights) is acceptable for maps of urban areas at large scale but the addition of shading would be most welcome for rural area maps or at smaller scale maps. Land use is portrayed with more categories in OSM, whereas Google, HERE and Wikimapia just indicate green areas. The problem is evident in Google maps where empty areas appear due to the lack of terrain portrayal. Regarding sea area, the depiction of sea routes completes the transportation networks information portrayed on the land. The enhancement of web maps with the coloring of bathymetric zones may be considered.
Another very important aspect of a map, which influences map reliability, is map consistency. According to “Vertical integration between layers” criteria [7
], maps in Google, HERE and OSM generally exhibit map consistency. However, only a thorough review of different geographic areas and scales can verify this statement. On the contrary, a number of obvious inconsistencies are observed in Wikimapia, e.g., roads and settlements in sea area, etc. Online map services are open to users’ complaints about the map content and thus consistency errors are usually spotted and edited.
The objective of generalization is to portray spatial data to a content and detail level corresponding to the information necessary for correct geographical reasoning. The proper degree of generalization is related to map scale and purpose. Since online map services encompass dynamic scale change, a number of different scale maps are provided for the same geographic area. Ιf maps of the same geographical area are compared across online map services, it becomes evident that they differ in generalization degree and information density. OSM presents the highest information density and diversity compared to Google and HERE. OSM also offers a very dense map that does not permit the portrayal of additional layers, such as POI as on other maps like Google. However, at larger scales the depiction of POI in OSM is very detailed. Generally, data are very scarce in Wikimapia.
At first glance, a number of problems regarding generalization are detected in online map services. The degree of detail is often excessive in relation to the scale level (Figure 2
a road network into town, Figure 2
b very small buildings are portrayed). Problems are observed between layers, due to a different degree of generalization, e.g., river network in comparison with land use (Figure 2
c) and onthe same layer, e.g., between different land use types (Figure 2
c). In certain cases, overlay problems between layers are observed because of lack of generalization or erroneous degree of generalization, e.g., road, land, sea (Figure 2
e), road, urban area, park (Figure 2
d). On the contrary, there may be a lack of detail for several layers, e.g., the road network (Figure 2
d) that does not have the detail required at this scale. Often there is no homogeneity in the degree of detail of the portrayed entities. This is due to the lack of design scope of the map and as a result information hierarchy cannot be established (Figure 2
A more systematic review across web mapping services for the zoom levels 13–14, which have also been reviewed for content, based on the review criteria for “Generalization” [7
] is performed. A number of problems, such as “Line shapes too complex (or too simple)”, “Areas too small to suit scale (outline closes in area fill)” and “Area shapes too complex (or too simple or unrealistic)” exist. Due to the absence of legend for online map services, categories or hierarchy of geographic features are not clear. As a result, it is difficult to see whether the type and the number of categories are appropriate to the scale.
With respect to terrain generalization, it is noted that terrain is balanced as hill shade is neither too jagged nor terraced. However, HERE hill shading seems more realistic and Google (option “Terrain”) is extremely enhanced.
Map symbolization is one of the most interesting and challenging aspects of cartography. Clearly, symbolization is critical to any map’s success and to online map services as well. In Table 5
, symbolization is analyzed for each thematic layer and the visual variable applied is noted. Generally, symbolization in on-line map services is in accordance with the cartographic practice. Absence of legend for Google, HERE and Wikimapia does not permit a detailed symbology review as the attributes to be portrayed are not evident and consequently the use of visual variables cannot be commented. It is observed that areal and linear symbols cover most of the map area, whereas point symbols are rare at the scale examined (zoom levels 13–14). Color is the predominant visual variable utilized in order to portray differences at the nominal scale. For a detailed review of color see Section 4
. Symbol sizes are well selected to support legibility.
At larger scales, Points of Interest portrayal utilizes pictorial point symbols based on shape and color. Pictorial symbols are suitable as they are easy to understand. All point symbols in Google maps use the well-known Google Maps pin as a background and a pictorial symbol on top. The Google Maps pin is the inverted-drop-shaped icon that marks locations on Google Maps. It is protected under a U.S. design patent as “teardrop-shaped marker icon including a shadow” and it is considered as “a product of pure function that has evolved into a cultural phenomenon” [12
]. Google maps popularity has probably appointed this symbol as the most well-known map symbol and as a result the users’ default selection for their own maps. This symbol has never been used on maps before Google maps advent and never before have people utilized massively and internationally the same symbol, without adopting a standard, except maybe for the north arrow symbol. This uniformity of Google map symbols, which are always based on the pin marker, results in an immediate recognition of the map provider and a sense of familiarity for the user. Pictorial symbols in the other on-line mapping services are more in agreement with the ones used on general reference maps.
The coastline symbolization is also very important as it bounds the land area. Coastline is not portrayed with a linear symbol in online map services and in OSM it coincides with an administrative line symbol. Road symbolization on web maps should be revised taking into account the category they belong to, provided that this information would be available through the legend.
At large scales, settlements are portrayed without symbol and their position is implied by their name. Only Wikimapia utilizes a point symbol for settlements. At smaller scales Google, HERE and Wikimapia utilize point symbols for settlements. In traditional cartography, it is possible to use geographical names to denote a feature and this is the case for capes, gulfs, seas, mountains, etc.
The use of color in symbols will be discussed in detail in Section 4
. Regarding the review criteria for “Symbol appearance (point, line, area)” [7
], rendering is performed correctly and no point form, line form or area casing is jagged. Problems referring to point symbol appearance are not observed. Concerning the “Poor multilayer point combination (e.g., outline, shape within shape)” issue all Google point symbols use an opaque background symbol layer and do not face this problem. However, OpenStreetMap, HERE and Wikimapia point symbols can be enhanced with the addition of an opaque background layer. With respect to line symbols no issues are observed in Google maps and HERE. In OpenStreetMap, due to generalization issues sometimes roads are too narrow and thus difficult to read at medium scales. In addition, OSM line symbols for roads with casing are not always fully drawn. Line symbols for roads in Wikimapia are poorly designed. Too thin lines are depicted, the only color is used for the symbols, different categories are not distinctive, the width is too wide resulting in overlays and road network information becomes illegible. As for area symbols and although only hue and value are utilized in Google, HERE and Wikimapia, symbols are distinguishable. Even though no symbols are too similar to other area symbols as the number of categories is small, one may notice poor pattern choice and multilayer pattern combination. Administrative area symbolization with a continuous line border in Wikimapia, creates confusion problems in map composition. The portrayal of administrative areas at middle scales in not justified for a reference map. OpenStreetMap has more area symbols as land use portrayal is more detailed and thus comprehensive. In this case, both hue and pattern are utilized successfully, symbols are distinguishable, patterns choice and multilayer pattern combination are performed correctly.
Terrain portrayal in Google maps and HERE is based on shading and no contours are shown. Hillshade colors are appropriate and logically positioned. Hillshade or contours should be included in OSM and Wikimapia as well. In Google maps, the terrain is displayed upon user’s selection “Terrain”. This causes a change in some other symbols as well, e.g., roads and new land use areas appear as well. At large scales in Google maps, the terrain is portrayed inside a building block, which is very misleading.