diff --git a/cmd/mtwebmapper/forwardupdates.go b/cmd/mtwebmapper/forwardupdates.go
index c2b0dea..d492d9d 100644
--- a/cmd/mtwebmapper/forwardupdates.go
+++ b/cmd/mtwebmapper/forwardupdates.go
@@ -27,7 +27,7 @@ type connection struct {
 }
 
 type msg struct {
-	tiles map[xz]bool
+	tiles []xz
 	pls   []*player
 }
 
@@ -57,12 +57,8 @@ func (wsf *websocketForwarder) run() {
 			}
 			encMsg := map[string]interface{}{}
 
-			if tiles := message.tiles; tiles != nil {
-				xzs := make([]xz, 0, len(tiles))
-				for xz := range tiles {
-					xzs = append(xzs, xz)
-				}
-				encMsg["tiles"] = xzs
+			if message.tiles != nil {
+				encMsg["tiles"] = message.tiles
 			}
 
 			if message.pls != nil {
@@ -101,7 +97,7 @@ func (wsf *websocketForwarder) ServeHTTP(rw http.ResponseWriter, r *http.Request
 	c.reader()
 }
 
-func (wsf *websocketForwarder) BaseTilesUpdated(changes map[xz]bool) {
+func (wsf *websocketForwarder) BaseTilesUpdated(changes []xz) {
 	wsf.broadcast <- msg{tiles: changes}
 }
 
diff --git a/cmd/mtwebmapper/tilesupdater.go b/cmd/mtwebmapper/tilesupdater.go
index 4d360e0..8b09a0e 100644
--- a/cmd/mtwebmapper/tilesupdater.go
+++ b/cmd/mtwebmapper/tilesupdater.go
@@ -25,7 +25,7 @@ import (
 )
 
 type baseTilesUpdates interface {
-	BaseTilesUpdated(map[xz]bool)
+	BaseTilesUpdated([]xz)
 }
 
 type tileUpdater struct {
@@ -165,11 +165,11 @@ func extractChanges(changes map[xz]bool) []xzc {
 	return chs
 }
 
-func activeChanges(changes []xzc) map[xz]bool {
-	chs := make(map[xz]bool)
+func activeChanges(changes []xzc) []xz {
+	chs := make([]xz, 0, len(changes))
 	for i := range changes {
 		if !changes[i].canceled {
-			chs[changes[i].xz] = true
+			chs = append(chs, changes[i].xz)
 		}
 	}
 	return chs
@@ -222,8 +222,8 @@ func (tu *tileUpdater) doUpdates() {
 		}
 
 		parentJobs := make(map[xz]uint16)
-		for c := range actChs {
-			pxz := c.parent()
+		for i := range actChs {
+			pxz := actChs[i].parent()
 			parentJobs[pxz.P] |= pxz.Mask
 		}