Terminology comparison between ArcGIS and GRASS GIS

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feature class or feature vector map or vector
layer map or map layer When talking about data, GRASS GIS uses term map in mathematical sense of associating (mapping) values from one space (geospatial coordinates) to another space (values specifying what's there). When talking about displaying data in the GUI, GRASS GIS talks about map layers (or just layers) as we are putting different layers on top of each other. Obviously, map can be also used in cartographic sense as a graphical output which combines different spatial data with attributes and says what is where (which is the same thing what, e.g. raster map is doing). Note that in GRASS GIS, one vector map can have different layers which are a way to associate the geometry features in the vector map with different sets of attributes.
Shapefile, shapefile, shape vector map or vector ESRI Shapefile format is of course still called the same. Here we talk about using Shapefile in sense of vector data (which is incorrect, yet common, usage).
(vector) feature vector feature
Table of Contents (TOC) Layer Manager, list of map layers
feature ID (FID), ObjectID cat, category The term feature ID (fid) is also used in GRASS GIS but it is an internal identifier which user does not have to be concerned about. In GRASS GIS, the term category (often shortened to cat) is used with both raster and vector maps.
polygon area, polygon Although most of the time area and polygon can be used as synonyms, the term area is more strictly defined in GRASS GIS than the term polygon. When speaking about types of vector features such as lines or points, the term area is used in GRASS GIS. In some more general or more technical context, the term polygon is used.
(current) workspace (current) Mapset and (current) working directory In GRASS GIS, newly created (raster and vector) maps are saved to the current Mapset (which is in GRASS Location in GRASS database directory). Other data (files) are saved to the path (directory) if specified, or the current working directory if only filename was provided. Note that every program (application) has current working directory, for GUI applications it is usually the user's home directory and they are often not using it.
database Mapset, Location, GRASS GIS database directory, or database (if used in general context) In GRASS GIS, geospatial data are stored in Mapset (which is in GRASS Location in GRASS database directory) as directories and files in GRASS GIS specific format. The attributes of a vector map are stored in a database.
tool module, command The functionality in GRASS GIS is divided into modules which are (technically) standalone programs. The modules can be used from GUI, Python or command line in very similar ways. Examples of module usages are usually written as command for command line and thus the modules are often referred to as commands which is a common practice in the command line world. Sometimes users use terms tool and function as well.
left, right, top, bottom west, east, north, south When specifying 2D extent (bounding box) of data or for computations, ArcGIS uses left, right, top, bottom to name the individual values, while GRASS GIS is using west, east, north, south. In GRASS GIS, top and bottom are used as well but to specify upper and lower limit of a 3D extent (bounding box).
Item description (item in Geodatabase or Data Catalog) metadata or information obtained from r.info, v.info and others Note that in GRASS GIS, there is also a new advanced metadata system available in GRASS GIS Addons repository.
NoData, Null, 0, -9999 NULL, null Missing or unknown data are referred to as NULL data or NULL values in GRASS GIS. Sometimes also other terms are used such as null() (in r.mapcalc), *, nv, or nodata. Because all numbers can have some meaning in their context, there is no numerical value which would represent NULL value.
map algebra, raster calculator raster (map) calculator, raster (map) algebra, r.mapcalc Note that there are other types of algebra in GRASS GIS, for example temporal algebra and different calculators, for example r3.mapcalc. When one need to be precise and unambiguous, one can use, e.g. r.mapcalc expression, rather then raster algebra expression.
Con, condition, conditional (in raster algebra) if statement
image raster (map) In GRASS GIS raster data are referred to as raster map (or just raster) except for the cases when we are talking about an actual image acquired by remote sensing or so. Note that in GRASS GIS, there are also natively supported 3D rasters.
mosaicking, creating mosaic patching, mosaicking, creating mosaic The GRASS GIS module used to create a mosaic of raster maps is called r.patch. There is also v.patch but in case of vector maps we don't talk about mosaicking but about merging, combining and patching.
hillshade shaded relief The main modules for shading are called r.relief, d.shade and r.shade. The term hillshade might be occasionally used as a synonym.
majority, most frequent (MAJORITY and MOST_FREQUENT in the dialog) mode In GRASS GIS, for example r.mode determines the value which appears most often in a raster map.
symbology or layer symbology layer properties and display command How the data are displayed (symbology, look) in GRASS GIS is for standard 2D display determined by d.rast, d.vect, d.shade and other display (d.*) modules in combination with color tables associated with each raster or vector map which are managed using r.colors and other modules. In the Layer Manager in GUI, the settings is accessible from the context menu item called layer properties.
color ramp color table, color rules In GRASS GIS, color table is a way to associate colors with values in a raster (or even a vector) map. A raster map can have a associated color table and there are predefined color tables (accessible e.g. through r.colors). When speaking about the actual definition, e.g. when stored in a file, the term color rules is used.
Output feature class (in a dialog) output, (name for) output vector map
Resampling Technique (in a dialog) resampling method For example, GRASS GIS module r.resamp.interp has an option method.
ArcScene 3D view, NVIZ, wxNVIZ, m.nviz.image In GRASS GIS, the 3D view is integrated into the main graphical user interface (GUI) while in ArcGIS suite, there is a separate tool ArcScene. The GRASS GIS library which is behind the 3D view is called NVIZ. The integration of NVIZ into GUI which is called wxGUI is called wxNVIZ. The 3D visualization is also available as a module called m.nviz.image (usable from command line or Python).
Identify tool Query raster/vector map(s) This applies to GUI. The underlying modules in GRASS GIS are r.what and v.what.

See also