| 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()`. |
| 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`. |
| 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`. |
| 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()`. |
| 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")`. |
| 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. |
| Inset |
A visual object that is plotted on a specific (pre-defined) location. It can be a small map (**minimap**) or a **chart**. |