Fixes #2907.

raster-testkit/src/main/scala/geotrellis/raster/testkit/TileBuilders.scala

@@ -163,6 +163,25 @@ trait TileBuilders {
     CompositeTile(tiles.flatten.map(a => createTile(a, pixelCols, pixelRows)), layout)
   }
 
+  private
+  lazy val rng = new scala.util.Random(42)
+
+  /** Return a copy of the given tile with `num` cells, at random indexes, replaced by the NoData value. */
+  def injectNoData(num: Int)(t: Tile): Tile = {
+    val indexes = List.tabulate(t.size)(identity)
+    val targeted = rng.shuffle(indexes).take(num)
+    def filter(c: Int, r: Int) = targeted.contains(r * t.cols + c)
+
+    val injected = if(t.cellType.isFloatingPoint) {
+      t.mapDouble((c, r, v) ⇒ (if(filter(c,r)) doubleNODATA else v): Double)
+    }
+    else {
+      t.map((c, r, v) ⇒ if(filter(c, r)) NODATA else v)
+    }
+
+    injected
+  }
+
   /**
    * 9x10 raster of 90 numbers between 1 - 100 in random order.
    */

raster/src/main/scala/geotrellis/raster/ArrayTile.scala

@@ -232,7 +232,14 @@ trait ArrayTile extends Tile with Serializable {
       case ct: CroppedTile =>
         ct.combine(this)((z1, z2) => f(z2, z1))
       case t =>
-        this.map((col, row, z) => f(z, t.get(col, row)))
+        this.map((col, row, z) => {
+          if (isNoData(z)) z
+          else {
+            val v = t.get(col, row)
+            if (isNoData(v)) v
+            else f(z, v)
+          }
+        })
   }
 
   /**
@@ -277,7 +284,7 @@ trait ArrayTile extends Tile with Serializable {
       case ct: CompositeTile =>
         ct.combineDouble(this)((z1, z2) => f(z2, z1))
       case t =>
-        this.mapDouble((col, row, z) => f(z, t.get(col, row)))
+        this.mapDouble((col, row, z) => f(z, t.getDouble(col, row)))
     }
   }
 

raster/src/test/scala/geotrellis/raster/ArrayTileSpec.scala

@@ -166,7 +166,7 @@ class ArrayTileSpec extends FunSpec
     }
 
     it("should combine with non-standard tiles") {
-      withClue("IntArrayTile") {
+      withClue("constant IntArrayTile") {
         val at = IntArrayTile(Array.ofDim[Int](100*100).fill(50), 100, 100)
         val fauxTile = new DelegatingTile {
           override protected def delegate: Tile = IntConstantTile(0, 100, 100)
@@ -175,14 +175,37 @@ class ArrayTileSpec extends FunSpec
         assertEqual(at.combine(fauxTile)((z1, z2) => z1 + z2), at)
       }
 
-      withClue("DoubleArrayTile") {
+      withClue("constant DoubleArrayTile") {
         val at = DoubleArrayTile(Array.ofDim[Double](100*100).fill(50), 100, 100)
         val fauxTile = new DelegatingTile {
           override protected def delegate: Tile = DoubleConstantTile(0.0, 100, 100)
         }
 
         assertEqual(at.combineDouble(fauxTile)((z1, z2) => z1 + z2), at)
       }
+
+      withClue("IntArrayTile with NoData") {
+        val at = injectNoData(4)(createConsecutiveTile(10))
+          .convert(DoubleUserDefinedNoDataCellType(98))
+        val twice = at * 2
+        val fauxTile = new DelegatingTile {
+          override protected def delegate: Tile = at
+        }
+
+        assertEqual(at.combine(fauxTile)((z1, z2) => z1 + z2), twice)
+      }
+
+      withClue("DoubleArrayTile with NoData") {
+        val at = injectNoData(4)(createValueTile(10, 10.2))
+          .convert(DoubleUserDefinedNoDataCellType(33.2))
+        val twice = at * 2
+        val fauxTile = new DelegatingTile {
+          override protected def delegate: Tile = at
+        }
+
+        assertEqual(at.combineDouble(fauxTile)((z1, z2) => z1 + z2), twice)
+        assertEqual(fauxTile.combineDouble(at)((z1, z2) => z1 + z2), twice)
+      }
     }
   }
 }