Add dynamic ZFactor for slope calculation

docs/CHANGELOG.rst

@@ -9,11 +9,13 @@ API Changes & Project structure changes
 - ``geotrellis-raster``
 
   - **Change:** ``geotrellis.raster.summary.polygonal.[Multi]TilePolygonalSummaryHandler`` replaced with ``geotrellis.raster.summary.polygonal.PolygonalSummary. Users should expect to implement concrete subclasses of ``geotrellis.raster.summary.GridVisitor`` and pass those to the new polygonalSummary methods. There are a number of default GridVisitor implementations provided for simple operations in `geotrellis.raster.summary.visitors``
+  - **New:** Created the ``geotrellis.raster.mapalgebra.focal.ZFactor`` to improve slope calculations (`#3014 <https://github.com/locationtech/geotrellis/pull/3014>`_).
 
 - ``geotrellis.layer``
 
   - **Change:** Replaced ``Mergable`` with cats' ``Semigroup``
   - **New:** A sparseStitch method on ``geotrellis.layer.stitch.SpatialTileLayoutCollectionStitchMethods``. Note that ``SpatialTileLayoutCollectionStitchMethods`` now has the additional constraint ``geotrellis.raster.prototype.TilePrototypeMethods`` on type ``V``. This should be transparent for users of the ``geotrellis.raster.Tile`` and ``geotrellis.raster.MultibandTile`` types.
+  - **Change:** The ``slope`` focal method now requires a ``ZFactor`` instance.
 
 - ``geotrellis.util``, ``geotrellis.store``, ``geotrellis.store.s3``
 
@@ -70,6 +72,7 @@ API Changes & Project structure changes
     - ``SpatialComponent``
     - ``TemporalComponent``
   - **Change:** Polygonal summaries on raster RDDs of ``RDD[(SpatialKey, T <: Grid[Int])] with Metadata[TileLayerMetadata[SpatialKey]]`` can now be performed with far less boilerplate using the same visitor pattern as the new raster polygonal summary API. See ``RDDPolygonalSummary.scala`` for additional details.
+  - **Change:** The ``slope`` focal method now requires a ``ZFactor`` instance.
 
 - ``geotrellis.spark.tiling``
 

layer/src/main/scala/geotrellis/layer/mapalgebra/focal/CollectionFocalOperation.scala

@@ -16,12 +16,13 @@
 
 package geotrellis.layer.mapalgebra.focal
 
-import geotrellis.layer.{SpatialComponent, TileBounds}
-import geotrellis.raster.buffer.BufferedTile
+import geotrellis.layer._
+import geotrellis.vector._
 import geotrellis.raster._
+import geotrellis.raster.buffer.BufferedTile
 import geotrellis.raster.mapalgebra.focal._
-import geotrellis.layer._
-import geotrellis.util.MethodExtensions
+import geotrellis.util._
+
 
 object CollectionFocalOperation {
   private def mapOverBufferedTiles[K: SpatialComponent](
@@ -31,7 +32,7 @@ object CollectionFocalOperation {
     calc: (Tile, Option[GridBounds[Int]]) => Tile
   ): Seq[(K, Tile)] =
     bufferedTiles
-      .map { case (key, BufferedTile(tile, gridBounds)) => key -> calc(tile, Some(gridBounds)) }
+      .mapValues { case BufferedTile(tile, gridBounds) => calc(tile, Some(gridBounds)) }
 
   def apply[K: SpatialComponent](
     seq: Seq[(K, Tile)],
@@ -44,22 +45,65 @@ object CollectionFocalOperation {
     mapOverBufferedTiles(seq.bufferTiles(neighborhood.extent), neighborhood)(calc)
 
   def apply[K: SpatialComponent](
-    rdd: Seq[(K, Tile)],
+    collection: Seq[(K, Tile)],
     neighborhood: Neighborhood,
     layerBounds: TileBounds
   )(
     calc: (Tile, Option[GridBounds[Int]]) => Tile
   ): Seq[(K, Tile)] =
-    mapOverBufferedTiles(rdd.bufferTiles(neighborhood.extent, layerBounds), neighborhood)(calc)
+    mapOverBufferedTiles(collection.bufferTiles(neighborhood.extent, layerBounds), neighborhood)(calc)
 
   def apply[K: SpatialComponent](
     rasterCollection: TileLayerCollection[K],
     neighborhood: Neighborhood
   )(
     calc: (Tile, Option[GridBounds[Int]]) => Tile
   ): TileLayerCollection[K] =
-    rasterCollection.withContext { rdd =>
-      apply(rdd, neighborhood, rasterCollection.metadata.tileBounds)(calc)
+    rasterCollection.withContext { collection =>
+      apply(collection, neighborhood, rasterCollection.metadata.tileBounds)(calc)
+    }
+
+  private def applyOnRaster[K: SpatialComponent: GetComponent[?, SpatialKey]](
+    bufferedTiles: Seq[(K, BufferedTile[Tile])],
+    neighborhood: Neighborhood,
+    keyToExtent: SpatialKey => Extent
+  )(
+    calc: (Raster[Tile], Option[GridBounds[Int]]) => Tile
+  ): Seq[(K, Tile)] =
+    bufferedTiles
+      .map { case (key, BufferedTile(tile, gridBounds)) =>
+        val spatialKey = key.getComponent[SpatialKey]
+
+        key -> calc(Raster(tile, keyToExtent(spatialKey)), Some(gridBounds))
+      }
+
+  def applyOnRaster[K: SpatialComponent: GetComponent[?, SpatialKey]](
+    collection: Seq[(K, Tile)],
+    neighborhood: Neighborhood,
+    layerBounds: TileBounds,
+    keyToExtent: SpatialKey => Extent
+  )(
+    calc: (Raster[Tile], Option[GridBounds[Int]]) => Tile
+  ): Seq[(K, Tile)] =
+    applyOnRaster(
+      collection.bufferTiles(neighborhood.extent, layerBounds),
+      neighborhood,
+      keyToExtent
+    )(calc)
+
+  def applyOnRaster[K: SpatialComponent: GetComponent[?, SpatialKey]](
+    rasterCollection: TileLayerCollection[K],
+    neighborhood: Neighborhood
+  )(
+    calc: (Raster[Tile], Option[GridBounds[Int]]) => Tile
+  ): TileLayerCollection[K] =
+    rasterCollection.withContext { collection =>
+      applyOnRaster(
+        collection,
+        neighborhood,
+        rasterCollection.metadata.tileBounds,
+        (key: SpatialKey) => rasterCollection.metadata.mapTransform.keyToExtent(key)
+      )(calc)
     }
 }
 
@@ -74,4 +118,12 @@ abstract class CollectionFocalOperation[K: SpatialComponent] extends MethodExten
       calc(tile, bounds, cellSize)
     }
   }
+
+  def focalWithExtents(n: Neighborhood)(calc: (Raster[Tile], Option[GridBounds[Int]], CellSize) => Tile): TileLayerCollection[K] = {
+    val cellSize = self.metadata.layout.cellSize
+
+    CollectionFocalOperation.applyOnRaster(self, n){ (raster, bounds) =>
+      calc(raster, bounds, cellSize)
+    }
+  }
 }

layer/src/main/scala/geotrellis/layer/mapalgebra/focal/FocalTileLayerCollectionMethods.scala

@@ -18,7 +18,7 @@ package geotrellis.layer.mapalgebra.focal
 
 import geotrellis.raster._
 import geotrellis.raster.Neighborhoods.Square
-import geotrellis.raster.mapalgebra.focal.{Kernel, Aspect, Sum, Max, Min, Mean, Median, Mode, StandardDeviation, Convolve, Slope}
+import geotrellis.raster.mapalgebra.focal.{Kernel, Aspect, Sum, Max, Min, Mean, Median, Mode, StandardDeviation, Convolve, Slope, ZFactor}
 
 trait FocalTileLayerCollectionMethods[K] extends CollectionFocalOperation[K] {
 
@@ -47,10 +47,12 @@ trait FocalTileLayerCollectionMethods[K] extends CollectionFocalOperation[K] {
    *
    * @see [[geotrellis.raster.mapalgebra.focal.Slope]]
    */
-  def slope(zFactor: Double = 1.0, target: TargetCell = TargetCell.All) = {
+  def slope(zFactor: ZFactor, target: TargetCell = TargetCell.All) = {
     val n = Square(1)
-    focalWithCellSize(n) { (tile, bounds, cellSize) =>
-      Slope(tile, n, bounds, cellSize, zFactor, target)
+    focalWithExtents(n) { (raster, bounds, cellSize) =>
+      val zValue = zFactor.fromExtent(raster.extent)
+
+      Slope(raster.tile, n, bounds, cellSize, zValue, target)
     }.mapContext(_.copy(cellType = DoubleConstantNoDataCellType))
   }
 }

project/Dependencies.scala

@@ -113,4 +113,6 @@ object Dependencies {
   val scalaj              = "org.scalaj"                  %% "scalaj-http"             % "2.4.1"
 
   val scalapbRuntime      = "com.thesamet.scalapb"        %% "scalapb-runtime"         % scalapb.compiler.Version.scalapbVersion
+
+  val squants             = "org.typelevel"               %% "squants"                 % "1.4.0"
 }

project/Settings.scala

@@ -413,6 +413,7 @@ object Settings {
       jts,
       catsCore,
       spire,
+      squants,
       monocleCore,
       monocleMacro,
       scalaXml,

raster/src/main/scala/geotrellis/raster/mapalgebra/focal/ZFactor.scala

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2019 Azavea
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package geotrellis.raster.mapalgebra.focal
+
+import geotrellis.vector.{Extent, Point}
+
+import org.apache.commons.math3.analysis.interpolation._
+
+import squants.space.{LengthUnit, Feet, Meters}
+
+
+/** Produces a ZFactor for a given point using the prescribed
+ *  conversion method.
+ *
+ *  @param produceZFactor A function that takes a latitude point and
+ *    converts it to its corresponding ZFactor.
+ */
+class ZFactor(produceZFactor: Double => Double) extends Serializable {
+  /** Produces the associated ZFactor for the given location.
+   *
+   *  @param extent The center point of this [[Extent]] will be used to
+     *  determine the ZFactor.
+   */
+  def fromExtent(extent: Extent): Double =
+    fromPoint(extent.center)
+
+  /** Produces the associated ZFactor for the given `Point`'s Y coordinate. */
+  def fromPoint(point: Point): Double =
+    fromLatitude(point.y)
+
+  def fromLatitude(lat: Double): Double =
+    produceZFactor(lat)
+}
+
+/** When creating a ZFactor instance, the projection of the target
+ *  Tile(s) needs to be taken into account. If the Tiles are in
+ *  LatLng, then the conversion between Latitude and ZFactor can
+ *  already be calculated. Otherwise, one will need to supply the
+ *  transformation required to produce the ZFactor.
+ */
+object ZFactor {
+  final val LAT_LNG_FEET_AT_EQUATOR = 365217.6
+  final val LAT_LNG_METERS_AT_EQUATOR = 11320
+
+  /** Creates a [[ZFactor]] instance. specifically for Tiles
+   *  that are in LatLng.
+   *
+   *  @param unit The unit of measure that the Tile(s) are in. Only two
+   *    `LengthUnit`s are supported, `Feet` and `Meters`.
+   */
+  def forLatLng(unit: LengthUnit): ZFactor =
+    unit match {
+      case Feet =>
+        ZFactor((lat: Double) => 1 / (LAT_LNG_FEET_AT_EQUATOR * math.cos(math.toRadians(lat))))
+      case Meters =>
+        ZFactor((lat: Double) => 1 / (LAT_LNG_METERS_AT_EQUATOR * math.cos(math.toRadians(lat))))
+      case _ =>
+        throw new Exception(s"Could not create a ZFactor instance from the following unit: ${unit}. Please use either Feet or Meters")
+    }
+
+  /** Creates a [[ZFactor]] instance which uses linear interplation to calculate
+   *  ZFactors. The linear interploation itself is derived from the values given.
+   *
+   *  @param mappedLatitudes A Map that maps latitudes to ZFactors. It is not required
+   *    to supply a ZFactor for every latitude intersected by the Tile(s). Rather,
+   *    based on the values given, a linear interpolation will be created and
+   *    any latitude not mapped will have its associated ZFactor derived from
+   *    that interpolation.
+   */
+  def interpolateFromTable(mappedLatitudes: Map[Double, Double]): ZFactor = {
+    val interp = new LinearInterpolator()
+    val spline = interp.interpolate(mappedLatitudes.keys.toArray, mappedLatitudes.values.toArray)
+
+    ZFactor((lat: Double) => spline.value(lat))
+  }
+
+  def apply(produceZFactor: Double => Double): ZFactor =
+    new ZFactor(produceZFactor)
+}

raster/src/main/scala/geotrellis/raster/package.scala

@@ -99,6 +99,8 @@ package object raster extends Implicits {
 
   val Stitcher = stitch.Stitcher
 
+  val ZFactor = mapalgebra.focal.ZFactor
+
   // Keep constant values in sync with macro functions
   @inline final val byteNODATA = Byte.MinValue
   @inline final val ubyteNODATA = 0.toByte

spark/src/main/scala/geotrellis/spark/mapalgebra/focal/FocalOperation.scala

@@ -16,13 +16,14 @@
 
 package geotrellis.spark.mapalgebra.focal
 
-import geotrellis.layer.{SpatialComponent, TileBounds}
+import geotrellis.layer.{SpatialComponent, TileBounds, SpatialKey}
+import geotrellis.vector.Extent
 import geotrellis.raster._
 import geotrellis.raster.buffer.BufferedTile
 import geotrellis.raster.mapalgebra.focal._
 import geotrellis.spark._
 import geotrellis.spark.buffer._
-import geotrellis.util.MethodExtensions
+import geotrellis.util._
 
 import org.apache.spark.rdd.RDD
 import org.apache.spark.Partitioner
@@ -55,6 +56,41 @@ object FocalOperation {
     rasterRDD.withContext { rdd =>
       apply(rdd, neighborhood, rasterRDD.metadata.tileBounds, partitioner)(calc)
     }
+
+  private def applyOnRaster[K: SpatialComponent: ClassTag: GetComponent[?, SpatialKey]]
+    (bufferedTiles: RDD[(K, BufferedTile[Tile])], neighborhood: Neighborhood, keyToExtent: SpatialKey => Extent)
+    (calc: (Raster[Tile], Option[GridBounds[Int]]) => Tile): RDD[(K, Tile)] =
+      bufferedTiles
+        .mapPartitions({ case partition =>
+          partition.map { case (k, BufferedTile(tile, gridBounds)) =>
+            val spatialKey = k.getComponent[SpatialKey]
+
+            k -> calc(Raster(tile, keyToExtent(spatialKey)), Some(gridBounds))
+          }
+        }, preservesPartitioning = true)
+
+  def applyOnRaster[K: SpatialComponent: ClassTag: GetComponent[?, SpatialKey]](
+    rdd: RDD[(K, Tile)],
+    neighborhood: Neighborhood,
+    layerBounds: TileBounds,
+    partitioner: Option[Partitioner],
+    keyToExtent: SpatialKey => Extent
+  )(calc: (Raster[Tile], Option[GridBounds[Int]]) => Tile): RDD[(K, Tile)] =
+      applyOnRaster(rdd.bufferTiles(neighborhood.extent, layerBounds, partitioner), neighborhood, keyToExtent)(calc)
+
+  def applyOnRaster[
+    K: SpatialComponent: ClassTag: GetComponent[?, SpatialKey]
+  ](rasterRDD: TileLayerRDD[K], neighborhood: Neighborhood, partitioner: Option[Partitioner])
+  (calc: (Raster[Tile], Option[GridBounds[Int]]) => Tile): TileLayerRDD[K] =
+    rasterRDD.withContext { rdd =>
+      applyOnRaster(
+        rdd,
+        neighborhood,
+        rasterRDD.metadata.tileBounds,
+        partitioner,
+        (key: SpatialKey) => rasterRDD.metadata.mapTransform.keyToExtent(key)
+      )(calc)
+    }
 }
 
 abstract class FocalOperation[K: SpatialComponent: ClassTag] extends MethodExtensions[TileLayerRDD[K]] {
@@ -68,4 +104,11 @@ abstract class FocalOperation[K: SpatialComponent: ClassTag] extends MethodExten
     val cellSize = self.metadata.layout.cellSize
     FocalOperation(self, n, partitioner){ (tile, bounds) => calc(tile, bounds, cellSize) }
   }
+
+  def focalWithExtents(n: Neighborhood, partitioner: Option[Partitioner])
+      (calc: (Raster[Tile], Option[GridBounds[Int]], CellSize) => Tile): TileLayerRDD[K] = {
+    val cellSize = self.metadata.layout.cellSize
+
+    FocalOperation.applyOnRaster(self, n, partitioner){ (raster, bounds) => calc(raster, bounds, cellSize) }
+  }
 }

spark/src/main/scala/geotrellis/spark/mapalgebra/focal/FocalTileLayerRDDMethods.scala

@@ -18,7 +18,7 @@ package geotrellis.spark.mapalgebra.focal
 
 import geotrellis.raster._
 import geotrellis.raster.Neighborhoods.Square
-import geotrellis.raster.mapalgebra.focal.{Kernel, Aspect, Sum, Max, Min, Mean, Median, Mode, StandardDeviation, Convolve, Slope}
+import geotrellis.raster.mapalgebra.focal.{Kernel, Aspect, Sum, Max, Min, Mean, Median, Mode, StandardDeviation, Convolve, Slope, ZFactor}
 import geotrellis.spark._
 
 import org.apache.spark.Partitioner
@@ -106,13 +106,15 @@ trait FocalTileLayerRDDMethods[K] extends FocalOperation[K] {
    * @see [[geotrellis.raster.mapalgebra.focal.Slope]]
    */
   def slope(
-    zFactor: Double = 1.0,
+    zFactor: ZFactor,
     target: TargetCell = TargetCell.All,
     partitioner: Option[Partitioner] = None
   ) = {
     val n = Square(1)
-    focalWithCellSize(n, partitioner) { (tile, bounds, cellSize) =>
-      Slope(tile, n, bounds, cellSize, zFactor, target)
+    focalWithExtents(n, partitioner) { (raster, bounds, cellSize) =>
+      val zValue = zFactor.fromExtent(raster.extent)
+
+      Slope(raster.tile, n, bounds, cellSize, zValue, target)
     }.mapContext(_.copy(cellType = DoubleConstantNoDataCellType))
   }
 }