3.3.3 The raster and vector data
Geographic data have two representation modes: the vector mode and the raster mode. These digital representation modes, thanks to their own characteristics, allow different treatments in terms of spatial analysis and representation.
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Why distinguish between the two? Depending on the data that you want to display / search online, I will focus on one or the other type of geographic data. We do not proceed to the same analysis according to the type of data.
To put it simply, the raster data represents the real world through an image. A raster consists of a matrix of cells, or pixels, organized in rows and columns (grid) in which each cell has a value representing real world information.
Vector and Raster Data
As in photography, a key characteristic of rasters in GIS is the resolution. This defines the size of the pixels and therefore the precision of the data.
Topographic data is an example of a raster data type. In this type of raster each pixel has a value corresponding to the elevation. Rasters can be used to represent data related to elevation or temperature. They can also be used to represent land cover data. In this case, the value of each pixel corresponds to a category (1 = forest, 2 = water, etc.).
What analysis with raster data
Raster data allows for analyses such as damage assessment before/after a disaster or population count via buildings visible in satellite imagery. The satellite image is used in this case.
Unlike vector data layers, raster data do not have an associated database (attribute component). They are georeferenced using the x/y coordinates of the pixels.
Vector data represents the real world using geometric objects (points, lines and polygons). To each of these objects are associated geographical coordinates allowing to locate them in space. To these coordinates is added an attribute component that allows to describe the data by providing additional information (name of the locality, number of inhabitants, type of locality, etc.).
What analysis with vector data
Vector data allows for analysis based on the geometry of the data:
- Crossing of data according to their location (buffer zone)
- Spatial statistics (counting of water points by zone).
- Surface calculations (density)
Illustrations of analyses with vector data