Raster Data Explained - Open Source Geospatial Tools: Applications in Earth Observation - page 50

Chapter 3
Raster Data Explained
In this chapter, we explain some basic terms on raster data. We will deal with
coordinate systems, single and multi-band images and different encoding schemes
to store raster data in files. Knowing how raster data are stored can be of great help
to better understand the principles behind the different utilities discussed in this
part. Also when you start writing your own utilities using the GDAL application
programming interface (discussed in Sect. 14.3 ) this is important.
In its simplest form, raster data (images) are constructed of values that are ordered
in rows and columns in a two dimensional plane. Each cell, or pixel, is defined by its
row, column and value. Multi-band images contain multiple values for each pixel. We
aremost familiar with digital photographs, which are in fact multi-band raster images.
Color images are commonly stored with three values for each pixel, representing the
intensities for red, green and blue color components (other representations also exist
with four components, e.g., cyan, magenta, yellow, and black).
In the context of this topic, we will deal with georeferenced raster data. This
means that each pixel has map coordinates in a defined coordinate system attached
to it. As a result, georeferenced data can be viewed and analyzed with respect to
other spatial datasets. A georeferenced raster image can be overlaid with features
stored in vector data that represent objects or locations on Earth. This way, the pixel
values can provide information, such as temperature values, on the features in the
vector file that are covered by the georeferenced raster image.
Unlike vector data, where a location must be stated for each vertex of a feature,
the position of all pixels in a georeferenced raster image is defined by knowing the
position and size of a single reference pixel. Usually this information is stored for
the upper left pixel of the image. It can be stored in a separate file or embedded
within the image file as metadata. Just as for the vector data, various file formats
are in use for raster data. Many of them are supported by the GDAL command line
utility programs. Supported raster formats can be listed using gdalinfo (or any
other GDAL utility) with the general option -- formats 1 :
1 For all supported raster formats, check http://www.gdal.org/formats_list.html .