Monday, October 12, 2015

Raster data analysis


RASTER DATA ANALYSIS
  • Raster data analysis is based on cells and rasters
  • Raster data analysis can be performed at the level of individual cells, or a group of cells, or cells within an entire raster
  • The type of cell value is an important aspect of raster data analysis
  • Various types of data are stored in raster format
  • Raster data analysis is software specific-raster data. Hence in order to use Digital Elevation Models(DEMs), Satellite images and other raster data in data analysis, they must first be processed and imported to software-specific raster data
  • The core of raster data analysis comprises of local operations that are cell-by-cell operations.
  • A local operation can create new raster from a single input raster or multiple input raster.
  • Converting a floating point raster to an integer raster is a simple local operation
  • Converting a slope raster measured in percent to one measured in degrees is also a local operation
  • Reclassification is a local operation that creates a new raster by classification. Reclassification is also called as recoding or transforming
  • Local operations with multiple rasters are also known as compositing, overlaying or superimposing maps
  • Map algebra is defined as local operations with multiple input rasters.
  • Local operations compute an output raster dataset where the output value at each location is a function of the value associated with that location on one or more raster datasets
  • Focal or Neighbourhood functions produce an output raster dataset in which output value at each location is a function of the value at a location and the value of cells in a specified neighbourhood around that location
  • Zonal functions compute an output raster dataset where the output value for each location depends on the value of the cell at the location and the association that the location has within a cartographic zone.
  • Global functions compute an output raster dataset in which the output value at each cell location is potentially a function of all the cells in the input raster datasets.
    • The two types of global functions are:
      • Euclidian distance and
      • Weighted distance
  • The raster calculator provides powerful tools in algebra syntax to perform mathematical calculations using operators and functions. It can also be used to set-up selection queries or type spatial analyst function syntax. Inputs can be grid datasets or raster layers, shape files, coverages, tables, constants and numbers.
  • The raster calculator provides four groups of mathematical functions: Logarithmic, Arithmetic, Trigonometric and Exponential.
  • The raster calculator allows boolean, relational and arithmetic operators
  • Raster data analysis also involves:
    • Terrain analysis
      • Contour
      • Slope
      • Aspect
      • Hillshade
      • Viewshed
      • Cut/Fill
    • Hydrologic analysis
      • Basin
      • Fill
      • Flow accumulation
      • Flow direction
      • Flow length
      • Sink
      • Snap pour point
      • Snap pour
      • Stream link
      • Stream order
      • Stream to feature
      • Stream shape
      • Watershed
  • Reclassification is replacing input cell values with new output cell values. The reasons to reclassify data are listed below:
    • To replace values based on new information
    • To group certain values together
    • To reclassify values to a common scale
    • To set specific cells to "NoData" or set "NoData" cells to a value