| Auxiliary map layer |
An auxiliary (or non-data-driven) map layer is a visual representation of geographic information that does not come from a spatial object, but uses the map’s coordinates. Examples: tm_grid()/tm_graticules() and tm_basemap(). |
| Chart |
A small non-spatial data visualization. In tmap for several purposes: 1) an addition to a legend tm_polygons(fill = "var", fill.chart = tm_chart_histogram()), 2) a glyph (see glyphs), 3) a custom chart plotted as an inset. |
| Data-driven map layer |
A data-driven map layer is a visual representation of a spatial object. E.g., tm_polygons() or tm_raster(). |
| Facets |
Multiple maps in one plot. Also called ‘small multiples’. |
| Features |
Elementary spatial data objects. For instance, a row in an sf data.frame or a single raster cell. |
| Glyph |
Mini charts that are used as proportional symbols. See the extension package tmap.glyphs. |
| Inset |
A visual object that is plotted on a specific (pre-defined) location. It can be a small map (minimap) or a chart. |
| Layout |
All aspects that specify the plot apart from the map layers and map components. Examples: margins, background color, aspect ratio, font sizes, etc. These can be set via tm_layout. These layout options form a subset of all tmap options (see options). |
| Map component |
A visual plot object with a position independent of map coordinates. Examples: a legend, a compass, a title. |
| Map layer |
A map layer is a visual representation of geographical information. We distinguish two types: data-driven and auxiliary map layers. The former requires spatial objects whereas the latter only the geographic information (bounding box and coordinate reference system). |
| Options |
In the context of tmap, we refer to the options as settings, which can be configured using tm_options. These can be layout options (see layout) or otherwise (e.g., 'show.messages'). |
| Proportional symbols |
Proportional symbols are symbols that are drawn at geographic locations and that are sized proportionally with a data variable. The result is known as a ‘proportional symbol map’. These symbols are usually bubbles (filled circles), but can also be small charts, called glyphs. |
| Scale function |
A scale function determines how to scale a data variable to either a visual variable or a transformation variable. Examples: tm_scale_continuous, tm_scale_categorical. |
| Shape (object) |
‘Shape’ is a nickname for a spatial data object. It is used in tm_shape(). |
| Shape (visual variable) |
The shape is a visual variable for some map layer functions, most prominently tm_symbols(). It determines the shape/design of the symbols. |
| Style |
The overall layout of the map. Similar to ggplot2’s ‘theme’ (see theme). |
| Theme |
tmap stands for ‘thematic maps’, where theme refers to the topic of the data plotted on a map. Note that in ggplot2, a ‘theme’ refers to the overall layout, e.g. ggplot2::theme_minimal(). In tmap, we use style for this. |
| Transformation variable |
A variable of a data-driven map layer that determines a transformation of the spatial object. (TO DO: exact definition). For instance, tm_cartogram(size = "var"). |
| Visual variable |
A variable of a data-driven map layer that determines a visual aspect. This can be data-driven, e.g. tm_polygons(fill = "var"), where var is the name of an sf column, or a constant value, such as tm_polygons(fill = "blue"). |
| tmap element |
A tm_ object that can be stacked with the + operator. These are: tm_shape(), map layer functions (such as tm_polygons()), facet specification function tm_facets(), map components (such as tm_compass()), and layout/option specification functions, such as tm_layout(). |