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interpolate_gpx.py

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+#!/usr/bin/env python3
+import sys
+import xml.etree.ElementTree as ET
+from datetime import datetime, timedelta, timezone
+from copy import deepcopy
+
+NS = {"gpx": "http://www.topografix.com/GPX/1/1"}
+ET.register_namespace("", NS["gpx"])
+
+def parse_time(t: str) -> datetime:
+    # GPX ISO8601 times end with Z (UTC)
+    return datetime.fromisoformat(t.replace("Z", "+00:00")).astimezone(timezone.utc)
+
+def fmt_time(dt: datetime) -> str:
+    return dt.astimezone(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")
+
+def get_ele(tp):
+    el = tp.find("gpx:ele", NS)
+    if el is not None and el.text not in (None, ""):
+        try:
+            return float(el.text)
+        except ValueError:
+            return None
+    return None
+
+def set_ele(tp, value):
+    if value is None:
+        return
+    el = tp.find("gpx:ele", NS)
+    if el is None:
+        el = ET.SubElement(tp, f"{{{NS['gpx']}}}ele")
+    el.text = f"{value:.2f}"
+
+def interpolate_segment(seg):
+    """Return a NEW <trkseg> with gaps >1s filled at 1s steps (linear lat/lon/ele)."""
+    pts = seg.findall("gpx:trkpt", NS)
+    if len(pts) < 2:
+        return deepcopy(seg)
+
+    new_seg = ET.Element(f"{{{NS['gpx']}}}trkseg")
+    inserted_total = 0
+    gaps = 0
+
+    for i in range(len(pts) - 1):
+        cur, nxt = pts[i], pts[i + 1]
+        new_seg.append(deepcopy(cur))
+
+        t1_el = cur.find("gpx:time", NS)
+        t2_el = nxt.find("gpx:time", NS)
+        if t1_el is None or t2_el is None or not t1_el.text or not t2_el.text:
+            continue
+
+        t1, t2 = parse_time(t1_el.text), parse_time(t2_el.text)
+        dt = int((t2 - t1).total_seconds())
+        if dt <= 1:
+            continue
+
+        gaps += 1
+        lat1, lon1 = float(cur.get("lat")), float(cur.get("lon"))
+        lat2, lon2 = float(nxt.get("lat")), float(nxt.get("lon"))
+        ele1, ele2 = get_ele(cur), get_ele(nxt)
+
+        # Fill each missing second with linear interpolation
+        for s in range(1, dt):
+            alpha = s / dt
+            lat = lat1 + (lat2 - lat1) * alpha
+            lon = lon1 + (lon2 - lon1) * alpha
+            ele = None if (ele1 is None or ele2 is None) else (ele1 + (ele2 - ele1) * alpha)
+
+            new_tp = ET.Element(f"{{{NS['gpx']}}}trkpt", attrib={
+                "lat": f"{lat:.7f}",
+                "lon": f"{lon:.7f}",
+            })
+            if ele is not None:
+                ele_el = ET.SubElement(new_tp, f"{{{NS['gpx']}}}ele")
+                ele_el.text = f"{ele:.2f}"
+            time_el = ET.SubElement(new_tp, f"{{{NS['gpx']}}}time")
+            time_el.text = fmt_time(t1 + timedelta(seconds=s))
+
+            new_seg.append(new_tp)
+            inserted_total += 1
+
+    # append last original point
+    new_seg.append(deepcopy(pts[-1]))
+    return new_seg
+
+def main(in_path, out_path):
+    tree = ET.parse(in_path)
+    root = tree.getroot()
+
+    # keep original structure, only replace each trkseg content with interpolated version
+    changed = False
+    for trk in root.findall(".//gpx:trk", NS):
+        for seg in trk.findall("gpx:trkseg", NS):
+            new_seg = interpolate_segment(seg)
+            if new_seg is not None:
+                trk.remove(seg)
+                trk.append(new_seg)
+                changed = True
+
+    if not changed:
+        print("No <trkseg> found or no changes made.", file=sys.stderr)
+
+    tree.write(out_path, encoding="utf-8", xml_declaration=True)
+    print(f"Interpolated GPX saved to: {out_path}")
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        print("Usage: python interpolate_gpx.py input.gpx output.gpx", file=sys.stderr)
+        sys.exit(1)
+    main(sys.argv[1], sys.argv[2])