diff --git a/tar1090.sh b/tar1090.sh
index b30aa65..75c090c 100755
--- a/tar1090.sh
+++ b/tar1090.sh
@@ -108,7 +108,7 @@ prune() {
     jq -c <"$1" >"$2" '
     .aircraft |= map(select(has("seen") and .seen < '$INTERVAL' + 2))
     | .aircraft[] |= [.hex,
-    (if .alt_baro != null then .alt_baro elif .altitude != null then .altitude else .alt_geom end),
+    (if .ground == true then "ground" elif .alt_baro != null then .alt_baro elif .altitude != null then .altitude else .alt_geom end),
     (if .gs != null then .gs else .tas end),
     .track, .lat, .lon, .seen_pos,
     (if .mlat != null and (.mlat | contains(["lat"])) then "mlat"