diff --git a/OpenRA.Game/OpenRA.Game.csproj b/OpenRA.Game/OpenRA.Game.csproj
index 053f561dca..f3bfb5ba67 100644
--- a/OpenRA.Game/OpenRA.Game.csproj
+++ b/OpenRA.Game/OpenRA.Game.csproj
@@ -143,6 +143,7 @@
     <Compile Include="Orders\UnitOrderGenerator.cs" />
     <Compile Include="Player.cs" />
     <Compile Include="Primitives\MergedStream.cs" />
+    <Compile Include="Primitives\SpatiallyPartitioned.cs" />
     <Compile Include="Selection.cs" />
     <Compile Include="Server\Connection.cs" />
     <Compile Include="Server\Exts.cs" />
diff --git a/OpenRA.Game/Primitives/SpatiallyPartitioned.cs b/OpenRA.Game/Primitives/SpatiallyPartitioned.cs
new file mode 100644
index 0000000000..728c711ec4
--- /dev/null
+++ b/OpenRA.Game/Primitives/SpatiallyPartitioned.cs
@@ -0,0 +1,108 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2015 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see COPYING.
+ */
+#endregion
+
+using System;
+using System.Collections.Generic;
+using System.Drawing;
+
+namespace OpenRA.Primitives
+{
+	public sealed class SpatiallyPartitioned<T>
+	{
+		readonly int rows, cols, binSize;
+		readonly Dictionary<T, Rectangle>[] itemBoundsBins;
+		readonly Dictionary<T, Rectangle> itemBounds = new Dictionary<T, Rectangle>();
+		readonly Action<Dictionary<T, Rectangle>, T, Rectangle> addItem = (bin, actor, bounds) => bin.Add(actor, bounds);
+		readonly Action<Dictionary<T, Rectangle>, T, Rectangle> removeItem = (bin, actor, bounds) => bin.Remove(actor);
+
+		public SpatiallyPartitioned(int width, int height, int binSize)
+		{
+			this.binSize = binSize;
+			rows = height / binSize + 1;
+			cols = width / binSize + 1;
+			itemBoundsBins = Exts.MakeArray(rows * cols, _ => new Dictionary<T, Rectangle>());
+		}
+
+		public void Add(T item, Rectangle bounds)
+		{
+			itemBounds.Add(item, bounds);
+			MutateBins(item, bounds, addItem);
+		}
+
+		public void Update(T item, Rectangle bounds)
+		{
+			MutateBins(item, itemBounds[item], removeItem);
+			MutateBins(item, itemBounds[item] = bounds, addItem);
+		}
+
+		public void Remove(T item)
+		{
+			MutateBins(item, itemBounds[item], removeItem);
+			itemBounds.Remove(item);
+		}
+
+		Dictionary<T, Rectangle> BinAt(int row, int col)
+		{
+			return itemBoundsBins[row * cols + col];
+		}
+
+		Rectangle BinBounds(int row, int col)
+		{
+			return new Rectangle(col * binSize, row * binSize, binSize, binSize);
+		}
+
+		void MutateBins(T actor, Rectangle bounds, Action<Dictionary<T, Rectangle>, T, Rectangle> action)
+		{
+			var top = Math.Max(0, bounds.Top / binSize);
+			var left = Math.Max(0, bounds.Left / binSize);
+			var bottom = Math.Min(rows - 1, bounds.Bottom / binSize);
+			var right = Math.Min(cols - 1, bounds.Right / binSize);
+
+			for (var row = top; row <= bottom; row++)
+				for (var col = left; col <= right; col++)
+					action(BinAt(row, col), actor, bounds);
+		}
+
+		public IEnumerable<T> At(int2 location)
+		{
+			var col = (location.X / binSize).Clamp(0, cols - 1);
+			var row = (location.Y / binSize).Clamp(0, rows - 1);
+			foreach (var kvp in BinAt(row, col))
+				if (kvp.Value.Contains(location))
+					yield return kvp.Key;
+		}
+
+		public IEnumerable<T> InBox(Rectangle box)
+		{
+			var left = (box.Left / binSize).Clamp(0, cols - 1);
+			var right = (box.Right / binSize).Clamp(0, cols - 1);
+			var top = (box.Top / binSize).Clamp(0, rows - 1);
+			var bottom = (box.Bottom / binSize).Clamp(0, rows - 1);
+
+			var items = new HashSet<T>();
+			for (var row = top; row <= bottom; row++)
+				for (var col = left; col <= right; col++)
+				{
+					var binBounds = BinBounds(row, col);
+					foreach (var kvp in BinAt(row, col))
+					{
+						var item = kvp.Key;
+						var bounds = kvp.Value;
+
+						// Return items that intersect the box. We also want to avoid returning the same item many times.
+						// If the item is contained wholly within this bin, we're good as we know it won't show up in any others.
+						// Otherwise it may appear in another bin. We use a set of seen items to avoid yielding it again.
+						if (bounds.IntersectsWith(box) && (binBounds.Contains(bounds) || items.Add(item)))
+							yield return item;
+					}
+				}
+		}
+	}
+}
diff --git a/OpenRA.Game/Traits/World/ScreenMap.cs b/OpenRA.Game/Traits/World/ScreenMap.cs
index 0a1cf9af7c..a4415e723e 100644
--- a/OpenRA.Game/Traits/World/ScreenMap.cs
+++ b/OpenRA.Game/Traits/World/ScreenMap.cs
@@ -27,99 +27,71 @@ namespace OpenRA.Traits
 
 	public class ScreenMap : IWorldLoaded
 	{
-		ScreenMapInfo info;
+		static readonly IEnumerable<FrozenActor> NoFrozenActors = new FrozenActor[0];
+		readonly Func<FrozenActor, bool> frozenActorIsValid = fa => fa.IsValid;
+		readonly Func<Actor, bool> actorIsInWorld = a => a.IsInWorld;
+		readonly Cache<Player, SpatiallyPartitioned<FrozenActor>> partitionedFrozenActors;
+		readonly SpatiallyPartitioned<Actor> partitionedActors;
 		WorldRenderer worldRenderer;
-		Cache<Player, Dictionary<FrozenActor, Rectangle>[]> frozen;
-		Dictionary<Actor, Rectangle>[] actors;
-		int rows, cols;
 
 		public ScreenMap(World world, ScreenMapInfo info)
 		{
-			this.info = info;
 			var ts = Game.ModData.Manifest.TileSize;
-			cols = world.Map.MapSize.X * ts.Width / info.BinSize + 1;
-			rows = world.Map.MapSize.Y * ts.Height / info.BinSize + 1;
-
-			frozen = new Cache<Player, Dictionary<FrozenActor, Rectangle>[]>(InitializeFrozenActors);
-			actors = new Dictionary<Actor, Rectangle>[rows * cols];
-			for (var j = 0; j < rows; j++)
-				for (var i = 0; i < cols; i++)
-					actors[j * cols + i] = new Dictionary<Actor, Rectangle>();
+			var width = world.Map.MapSize.X * ts.Width;
+			var height = world.Map.MapSize.Y * ts.Height;
+			partitionedFrozenActors = new Cache<Player, SpatiallyPartitioned<FrozenActor>>(
+				_ => new SpatiallyPartitioned<FrozenActor>(width, height, info.BinSize));
+			partitionedActors = new SpatiallyPartitioned<Actor>(width, height, info.BinSize);
 		}
 
 		public void WorldLoaded(World w, WorldRenderer wr) { worldRenderer = wr; }
 
-		Dictionary<FrozenActor, Rectangle>[] InitializeFrozenActors(Player p)
-		{
-			var f = new Dictionary<FrozenActor, Rectangle>[rows * cols];
-			for (var j = 0; j < rows; j++)
-				for (var i = 0; i < cols; i++)
-					f[j * cols + i] = new Dictionary<FrozenActor, Rectangle>();
-
-			return f;
-		}
-
-		public void Add(Player viewer, FrozenActor fa)
+		Rectangle FrozenActorBounds(FrozenActor fa)
 		{
 			var pos = worldRenderer.ScreenPxPosition(fa.CenterPosition);
 			var bounds = fa.Bounds;
 			bounds.Offset(pos.X, pos.Y);
-
-			var top = Math.Max(0, bounds.Top / info.BinSize);
-			var left = Math.Max(0, bounds.Left / info.BinSize);
-			var bottom = Math.Min(rows - 1, bounds.Bottom / info.BinSize);
-			var right = Math.Min(cols - 1, bounds.Right / info.BinSize);
-
-			for (var j = top; j <= bottom; j++)
-				for (var i = left; i <= right; i++)
-					frozen[viewer][j * cols + i].Add(fa, bounds);
+			return bounds;
 		}
 
-		public void Remove(Player viewer, FrozenActor fa)
-		{
-			foreach (var bin in frozen[viewer])
-				bin.Remove(fa);
-		}
-
-		public void Add(Actor a)
+		Rectangle ActorBounds(Actor a)
 		{
 			var pos = worldRenderer.ScreenPxPosition(a.CenterPosition);
 			var bounds = a.Bounds;
 			bounds.Offset(pos.X, pos.Y);
-
-			var top = Math.Max(0, bounds.Top / info.BinSize);
-			var left = Math.Max(0, bounds.Left / info.BinSize);
-			var bottom = Math.Min(rows - 1, bounds.Bottom / info.BinSize);
-			var right = Math.Min(cols - 1, bounds.Right / info.BinSize);
-
-			for (var j = top; j <= bottom; j++)
-				for (var i = left; i <= right; i++)
-					actors[j * cols + i].Add(a, bounds);
+			return bounds;
 		}
 
-		public void Remove(Actor a)
+		public void Add(Player viewer, FrozenActor fa)
 		{
-			foreach (var bin in actors)
-				bin.Remove(a);
+			partitionedFrozenActors[viewer].Add(fa, FrozenActorBounds(fa));
+		}
+
+		public void Remove(Player viewer, FrozenActor fa)
+		{
+			partitionedFrozenActors[viewer].Remove(fa);
+		}
+
+		public void Add(Actor a)
+		{
+			partitionedActors.Add(a, ActorBounds(a));
 		}
 
 		public void Update(Actor a)
 		{
-			Remove(a);
-			Add(a);
+			partitionedActors.Update(a, ActorBounds(a));
+		}
+
+		public void Remove(Actor a)
+		{
+			partitionedActors.Remove(a);
 		}
 
-		public static readonly IEnumerable<FrozenActor> NoFrozenActors = new FrozenActor[0].AsEnumerable();
 		public IEnumerable<FrozenActor> FrozenActorsAt(Player viewer, int2 worldPx)
 		{
 			if (viewer == null)
 				return NoFrozenActors;
-
-			var i = (worldPx.X / info.BinSize).Clamp(0, cols - 1);
-			var j = (worldPx.Y / info.BinSize).Clamp(0, rows - 1);
-			return frozen[viewer][j * cols + i]
-				.Where(kv => kv.Key.IsValid && kv.Value.Contains(worldPx))
-				.Select(kv => kv.Key);
+			return partitionedFrozenActors[viewer].At(worldPx).Where(frozenActorIsValid);
 		}
 
 		public IEnumerable<FrozenActor> FrozenActorsAt(Player viewer, MouseInput mi)
@@ -129,11 +101,7 @@ namespace OpenRA.Traits
 
 		public IEnumerable<Actor> ActorsAt(int2 worldPx)
 		{
-			var i = (worldPx.X / info.BinSize).Clamp(0, cols - 1);
-			var j = (worldPx.Y / info.BinSize).Clamp(0, rows - 1);
-			return actors[j * cols + i]
-				.Where(kv => kv.Key.IsInWorld && kv.Value.Contains(worldPx))
-				.Select(kv => kv.Key);
+			return partitionedActors.At(worldPx).Where(actorIsInWorld);
 		}
 
 		public IEnumerable<Actor> ActorsAt(MouseInput mi)
@@ -141,51 +109,31 @@ namespace OpenRA.Traits
 			return ActorsAt(worldRenderer.Viewport.ViewToWorldPx(mi.Location));
 		}
 
+		static Rectangle RectWithCorners(int2 a, int2 b)
+		{
+			return Rectangle.FromLTRB(Math.Min(a.X, b.X), Math.Min(a.Y, b.Y), Math.Max(a.X, b.X), Math.Max(a.Y, b.Y));
+		}
+
 		public IEnumerable<Actor> ActorsInBox(int2 a, int2 b)
 		{
-			return ActorsInBox(Rectangle.FromLTRB(Math.Min(a.X, b.X), Math.Min(a.Y, b.Y), Math.Max(a.X, b.X), Math.Max(a.Y, b.Y)));
+			return ActorsInBox(RectWithCorners(a, b));
 		}
 
 		public IEnumerable<Actor> ActorsInBox(Rectangle r)
 		{
-			var left = (r.Left / info.BinSize).Clamp(0, cols - 1);
-			var right = (r.Right / info.BinSize).Clamp(0, cols - 1);
-			var top = (r.Top / info.BinSize).Clamp(0, rows - 1);
-			var bottom = (r.Bottom / info.BinSize).Clamp(0, rows - 1);
-
-			var actorsChecked = new HashSet<Actor>();
-			for (var j = top; j <= bottom; j++)
-				for (var i = left; i <= right; i++)
-					foreach (var kvp in actors[j * cols + i])
-					{
-						var actor = kvp.Key;
-						if (actor.IsInWorld && kvp.Value.IntersectsWith(r) && actorsChecked.Add(actor))
-							yield return actor;
-					}
+			return partitionedActors.InBox(r).Where(actorIsInWorld);
 		}
 
 		public IEnumerable<FrozenActor> FrozenActorsInBox(Player p, int2 a, int2 b)
 		{
-			return FrozenActorsInBox(p, Rectangle.FromLTRB(Math.Min(a.X, b.X), Math.Min(a.Y, b.Y), Math.Max(a.X, b.X), Math.Max(a.Y, b.Y)));
+			return FrozenActorsInBox(p, RectWithCorners(a, b));
 		}
 
 		public IEnumerable<FrozenActor> FrozenActorsInBox(Player p, Rectangle r)
 		{
-			var left = (r.Left / info.BinSize).Clamp(0, cols - 1);
-			var right = (r.Right / info.BinSize).Clamp(0, cols - 1);
-			var top = (r.Top / info.BinSize).Clamp(0, rows - 1);
-			var bottom = (r.Bottom / info.BinSize).Clamp(0, rows - 1);
-			var bins = frozen[p];
-
-			var actorsChecked = new HashSet<FrozenActor>();
-			for (var j = top; j <= bottom; j++)
-				for (var i = left; i <= right; i++)
-					foreach (var kvp in bins[j * cols + i])
-					{
-						var actor = kvp.Key;
-						if (actor.IsValid && kvp.Value.IntersectsWith(r) && actorsChecked.Add(actor))
-							yield return actor;
-					}
+			if (p == null)
+				return NoFrozenActors;
+			return partitionedFrozenActors[p].InBox(r).Where(frozenActorIsValid);
 		}
 	}
 }