diff --git a/mapper/src/App.js b/mapper/src/App.js
index e5332b4..63c60e6 100644
--- a/mapper/src/App.js
+++ b/mapper/src/App.js
@@ -112,45 +112,41 @@ function FitBounds({ coords, mapState, end, duration, loading }) {
     const prevEndRef = useRef();
     const prevDurationRef = useRef();
     const refitNeeded = useRef(false);
+    const isInitialLoad = useRef(mapState.center === null);
 
-    // Effect 1: Detects changes in `end` or `duration` and sets a flag.
+    // Effect 1: Flag the need for a refit when end/duration changes.
     useEffect(() => {
-        const prevEnd = prevEndRef.current;
-        const prevDuration = prevDurationRef.current;
-
-        // Run only after initial render where refs are populated.
-        if (prevEnd && prevDuration) {
-            const endChanged = end.unix() !== prevEnd.unix();
-            const durationChanged = duration.id !== prevDuration.id;
+        // Don't run on the very first render, wait until refs are populated.
+        if (prevEndRef.current && prevDurationRef.current) {
+            const endChanged = end.unix() !== prevEndRef.current.unix();
+            const durationChanged = duration.id !== prevDurationRef.current.id;
 
             if (endChanged || durationChanged) {
                 refitNeeded.current = true;
             }
         }
-
-        // Update refs for the next render's comparison.
         prevEndRef.current = end;
         prevDurationRef.current = duration;
     }, [end, duration]);
 
-    // Effect 2: Acts on `coords` changes, but only if the flag is set and data isn't loading.
+    // Effect 2: Perform the fit when conditions are right.
     useEffect(() => {
-        // Do not run this effect if new data is being fetched.
-        if (loading) return;
+        // Don't do anything while new data is being fetched.
+        if (loading) {
+            return;
+        }
 
-        // A refit is needed on initial load (mapState.center is null)
-        // or if the flag has been set by the other effect.
-        if ((mapState.center === null || refitNeeded.current) && coords.length > 0) {
+        // Perform fit on initial load OR if a refit has been flagged.
+        if ((isInitialLoad.current || refitNeeded.current) && coords.length > 0) {
             const bounds = leaflet.latLngBounds(coords);
             if (bounds.isValid()) {
                 map.fitBounds(bounds);
             }
-            // Reset the flag after the fit is performed.
+            // Reset flags after fitting.
             refitNeeded.current = false;
+            isInitialLoad.current = false;
         }
-    }, [coords, mapState.center, map, loading]);
-
-    return null;
+    }, [coords, loading, map]); // Dependencies: run when data is ready.
 }
 
 function Map({end, duration, slider, mapState, setMapState}) {
@@ -159,21 +155,36 @@ function Map({end, duration, slider, mapState, setMapState}) {
 	const range = useMemo(() => parseSlider(end, duration, slider), [end, duration, slider]);
 
 	const coords = useMemo(() => {
-		if (!Array.isArray(data)) return [];
+		// 1. Guard against invalid top-level data
+		if (!Array.isArray(data)) {
+			return [];
+		}
 
 		const result = [];
 		const [startTime, endTime] = range;
 
+		// 2. Loop through the data
 		for (const point of data) {
-			// Ensure the point and its time exist before checking the range
-			if (!point || !point.time) continue;
+			// 3. Guard against malformed points
+			if (!point || typeof point !== 'object') {
+				continue;
+			}
 
-			if (point.time >= startTime && point.time <= endTime) {
-				const { lat, lon } = point;
-				// Strictest possible check for valid, finite, numeric coordinates
-				if (typeof lat === 'number' && typeof lon === 'number' && isFinite(lat) && isFinite(lon)) {
-					result.push([lat, lon]);
-				}
+			const { lat, lon, time } = point;
+
+			// 4. Guard against invalid time
+			if (typeof time !== 'string' || time.length === 0) {
+				continue;
+			}
+
+			// 5. Guard against invalid coordinates
+			if (typeof lat !== 'number' || typeof lon !== 'number' || !isFinite(lat) || !isFinite(lon)) {
+				continue;
+			}
+
+			// 6. Now that all data is known to be valid, filter by time
+			if (time >= startTime && time <= endTime) {
+				result.push([lat, lon]);
 			}
 		}
 		return result;