diff --git a/html/script.js b/html/script.js
index 3b9fe9a..cc40b7d 100644
--- a/html/script.js
+++ b/html/script.js
@@ -7446,6 +7446,8 @@ function drawHeatmap() {
                     alt |= -65536;
                 if (alt == -123)
                     alt = 'ground';
+                else if (alt == -124)
+                    alt = null;
                 else
                     alt *= 25;
 
@@ -7930,6 +7932,8 @@ function replayStep(arg) {
             alt |= -65536;
         if (alt == -123)
             alt = 'ground';
+        else if (alt == -124)
+            alt = null;
         else
             alt *= 25;