QGIS / Map Layers / Overview
Map layers, also called spatial data layers, are spatial datasets that form the basis of GIS visualization and analysis.
The following sections are available in this documentation:
Spatial Data Layer Concepts
A spatial data layer at a fundamental level can be either a raster or vector layer.
A raster layer is used to represent data somewhat uniformly, where each cell or "pixel" has a constant size and has a data value. The raster "grid" typically represents some model space (such as a groundwater model grid), satellite imagery, processed data array, etc. The corners of the raster can be anchored to locations given a coordinate reference system, to allow projecting the raster to the display CRS. The "warping" of raster data layers onto projected representations of the earth is computationally intensive. To facilitate visualization, especially on the web, background layers such as satellite imagery and street maps are provided by web servers as tiled images using a standard CRS (such as Web Mercator, also referred to as WGS 84). Zooming into a map causes the needed tiles to be returned for display.
A vector layer (or feature layer) consists of a list of geometric shapes, such as points, lines, polygons, or other shapes. In many spatial data formats only one shape type can exist in a layer, although some newer formats and databases allow mixing shape types in a layer. The advantage of vector data is that shapes can be redrawn (rendered) at varying scale, although zooming in will reveal the distance between points and curves will become jagged.
Each shape also has attributes (also called properties), which are commonly described as an attribute table for all the shapes. In QGIS, the attribute table can be displayed by selecting a layer in the Layers panel, right-clicking, and selecting the Open Attribute Table menu, as shown below.
Menu to Open a Layer's Attribute Table (see full-size image)
An example attribute table is shown in the following figure.
Attribute Table View for a Layer (see full-size image)
Adding Layers to Map Project
Although concepts related to spatial data layers should transfer between different data formats, the design of data layers files and databases introduces differences that must be handled in software. This is why the QGIS software has multiple tools for adding spatial data layers. The following documentation sections provide information for different layer formats:
- Raster spatial data formats:
- Need to add.
- Tiled background layers
- Vector spatial data formats:
- GeoJSON - text format open specification