In the previous Learning path (vector Aanalysis) you were already introduced to the classification of analytical operations:
- Measurements
- Classification
- Retrieval
- Overlay
- Neighborhood functions
Parrallel to the vector analysis learning path, we will discuss the same topics for raster implementations. One extra topic is added (Surface Analysis). As Queries are normally not applied on raster data, we will skip this topic.
Measurement functions allow the calculation of distances, lengths or areas. All functions in this category are performed on a single raster data layer.
Reclassification functions allow the assignment of features to a class on the basis of attribute values or attribute ranges (definition of data patterns). On the basis of reflectance characteristics found in a raster, pixels may be classified as representing different crops, e.g. potato or maize.
Two types of reclassification operations will be discussed: - User controlled classification - Automatic reclassification
For automatic reclassification we will discuss two techniques: - Equal interval - Equal frequency
Overlay operations are one of the most frequently used functions in a GIS application. They combine two (or more) spatial layers, comparing them position by position and treating areas of overlap - and of non-overlap - in distinct ways. Many GISs support overlays through an algebraic language, expressing an overlay function as a formula in which the data layers are the arguments. In this way, we can find: * those potato fields on clay soils; * those fields in which potato or maize is the crop; those fields that do not have potato as a crop
Neighbourhood functions evaluate the characteristics of an area surrounding a feature's location. A neighborhood function "scans" the neighbourhood of the given feature(s), and performs a computation on it (them).
We will discuss three types of neighbourhood operations for raster data: * Basic Raster Proximity * Diffusion * Flow Computation
Diffusion assumes spread in all directions taking into account both distance and a resistance raster.
Where spread computation (diffusion) assumes that in principle the phenomenon spreads in all directions, the next concept (Flow) discusses movement along a given, least-cost path.
Continuous fields have a number of characteristics not shared by discrete fields. Since the field changes continuously, we can calculate slope angle and slope aspect.
The next concept discusses the Slope angle - the calculation of the slope steepness, expressed as an angle in degrees or percentage, for any or all locations.
Slope aspect is the calculation of the aspect (or orientation) of the slope in degrees ( between 0 and 360), for any or all locations.
Spatial data handling can lead to the propagation of error. In the next concept we will discuss a few examples, to raise awareness of this problem.