Glossary

ArcGIS: ArcGIS is a GIS software used by some geographers to create maps and manage geographical data. It is developed by Esri Software and you must acquire an ArcGIS licence to use the software.

attributes: attributes are non-spatial information that are also stored in a layer (in addition to classic geographical coordinates). They can be accessed through the attribute table. For instance, a country layer may also include attributes about population numbers, administrative informations, and so on.

base layer: the layer you will use as the background to your whole map. Layers including coastlines and/or countries are usually a good starting point.

control points: when georeferencing a map, control points are precise points (with specific lat/long coordinates) you decide to add manually so the georeferencing tool can accurately turn an image into a georeferenced layer.

coordinates: coordinates are sets of values representing one particular geographical point. They are usually composed of two (x, y) or three (x, yz) dimensions, x representing a horizontal value, y a vertical one, and z elevation.

coordinate reference system (CRS): a CRS is a system corresponding to a certain projection and algorithm. Some of them are based on a latitude / longitude base but others may respond to different algorithms. Usually, each geographic entity has one or several CRS which will depict it best. The most common CRS used for global projections is WGS 84.

geolocating: adding specific geographical coordinates (x, yz) to any set of data.

georeferencing: turning an image (e.g. a scanned historical map) into a fully geolocated layer. This allows you to use them along with other layers.

geographic information system (GIS): the term GIS denotes the entire system used to create, edit, analyse and manage geographical data.

latitude: the latitude is a vertical coordinate, ranging from 0° (at the Equator) to 90° (North Pole) or -90° (South Pole). It corresponds to the y field in coordinates.

layer: a set of points, lines, and/or polygons which compose your map. Most maps will have more than one layer. For instance, a political map of Europe will generally have a layer for coastlines, one for national borders, one for capital cities, etc. See vector layer and raster layer below.

longitude: the lontitude is a horizontal coordinate ranging from 0° at the Greenwich Meridian to 180° (East) and -180° (West). It corresponds to the x field in coordinates.

pixel: a pixel is the smallest element that composed an image. In GIS, each pixel corresponds to a set of coordinates (x, yz). Raster layers (see below) are composed of multiple pixels: each pixel features a particular set of information (e.g. elevation, population density, etc.).

polygon: a polygon is a closed set of points and lines depicting a particular geographic entity (e.g. countries, regions, lakes, forests).

projection: a projection is the way geographical data will be rendered on a map. Some projections would be conformal, conic, cylindrical, etc. Since our planet is – nearly – round, there are numerous types of projections. Each projection is coded into one or several CRS (see above).

Quantum GIS (QGIS): QGIS is a free, open-source GIS software. The tutorials featured in this website are based on QGIS. Link to the QGIS project website

query: a query is a search for geographical data using particular criteria. For instance a query would allow you to only select Germany and France in a layer that includes all countries of the world. A query is usually in the shape of a formula similar to SQL, Java or Python programming languages.

raster layer: a layer composed of an entire surface instead of only points and lines. Used for instance for layers depicting relief.

vector layer: a layer composed of vectors (lines, points, or polygons).