geotrellis.raster.mapalgebra.focal.Aspect.scala Maven / Gradle / Ivy

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GeoTrellis is an open source geographic data processing engine for high performance applications.

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package geotrellis.raster.mapalgebra.focal

import geotrellis.raster._
import geotrellis.raster.mapalgebra.focal.hillshade.{SurfacePoint, SurfacePointCalculation}
import geotrellis.raster.mapalgebra.focal.Angles._

/** Calculates the aspect of each cell in a raster.
  *
  * Aspect is the direction component of a gradient vector. It is the
  * direction in degrees of which direction the maximum change in direction is pointing.
  * It is defined as the directional component of the gradient vector and is the
  * direction of maximum gradient of the surface at a given point. It uses Horn's method
  * for computing aspect.
  *
  * As with slope, aspect is calculated from estimates of the partial derivatives dz / dx and dz / dy.
  *
  * If Aspect operations encounters NoData in its neighborhood, that neighborhood cell well be treated as having
  * the same elevation as the focal cell.
  *
  * Aspect is computed in degrees from due north, i.e. as an azimuth in degrees not radians.
  * The expression for aspect is:
  * {{{
  * val aspect = 270 - 360 / (2 * Pi) * atan2(`dz / dy`, - `dz / dx`)
  * }}}
  *
  */
object Aspect {

  def apply(tile: Tile, n: Neighborhood, bounds: Option[GridBounds], cs: CellSize): Tile = {
    new SurfacePointCalculation[Tile](tile, n, bounds, cs)
      with DoubleArrayTileResult
    {      
      def setValue(x: Int, y: Int, s: SurfacePoint) {
        resultTile.setDouble(x, y, degrees(s.aspect))
      }
    }
  }.execute()
}