OverviewTeaching: 5 min Exercises: 0 minQuestions
What is a raster?
What sorts of information does a raster typically model?
What are the major characteristics of a raster dataset?
What assumptions does the format imply?Objectives
Understand the raster data model
Unlike vectors, where features have discreet boundaries (which is useful for storing data like country borders, land parcels and streets), rasters are useful for storing data that varies continuously. At its heart, a raster is:
In the 1950’s raster graphics were noted as a faster and cheaper (but lower-resolution) alternative to vector graphics.
|Bands in Landsat 7 (bottom row of rectangles) and Landsat 8 (top row)|
|Graphic created by L.Rocchio & J.Barsi.|
|Landsat 8 Band 1 (“Ultra Blue”)||Landsat 8 Band 3 (“Green”)||Landsat 8 Band 9 (“Cirrus”)|
A raster is just an image in local pixel coordinates until we specify what part of the earth the image covers. This is done through two pieces of metadata that accompany the pixel values of the image:
Spatially-aware applications are careful to interpret this metadata appropriately. If we aren’t careful (or are using a raster-editing application that ignores spatial information), we can accidentally strip this spatial metadata. Photoshop, for example, can edit GeoTiffs, but we’ll lose the embedded CRS and geotransform!
Examples of common raster datasets include:
Gridded data that vary in space and time are common in many geospatial applications
Specialized tools are needed to accommodate the complexity and size of many raster datasets
Some image formats have been adapted to store spatial metadata