diff --git a/README.md b/README.md
index ff1b429..68276fd 100644
--- a/README.md
+++ b/README.md
@@ -57,18 +57,19 @@ For best performance you should use [TimescaleDB](https://www.timescale.com), wh
     ```
     
 8. Optional: Import world elevation data (needed for AGL calculation)
-    For Europe we can get the DEM as GeoTIFF files from the
-    [European Environment Agency](https://land.copernicus.eu/imagery-in-situ/eu-dem/eu-dem-v1.1).
-    Because the spatial reference system (SRID) of these files is 3035 and we want 4326 we have to convert them:
+8.1 Sources: There are many sources for DEM data. It is important that the spatial reference system (SRID) is the same as the database.
+8.1.1 The [GMTED2010 Viewer](https://topotools.cr.usgs.gov/gmted_viewer/viewer.htm) provides data for the world. Just download the data you need.
+8.1.2 For Europe we can get the DEM as GeoTIFF files from the [European Environment Agency](https://land.copernicus.eu/imagery-in-situ/eu-dem/eu-dem-v1.1).
+    Because the SRID of these files is 3035 and we want 4326 we have to convert them:
     
     ```
     gdalwarp -s_srs "EPSG:3035" -t_srs "EPSG:4326" source.tif target.tif
     ```
     
-    Then we can import the GeoTIFF into the elevation table:
+8.2 Then we can import the GeoTIFF into the elevation table:
     
     ```
-    raster2pgsql -c -C -I -M -t 100x100 elevation_data.tif public.elevation | psql -d ogn
+    raster2pgsql -s 4326 -c -C -I -M -t 100x100 elevation_data.tif public.elevation | psql -d ogn
     ```
     
 There is also a [Vagrant](https://www.vagrantup.com/) environment for the development of ogn-python.
diff --git a/ogn_python/collect/takeoff_landings.py b/ogn_python/collect/takeoff_landings.py
index d42f849..e5c0d38 100644
--- a/ogn_python/collect/takeoff_landings.py
+++ b/ogn_python/collect/takeoff_landings.py
@@ -21,6 +21,7 @@ def update_entries(session, start, end, logger=None):
     # considered time interval should not exceed a complete day
     if end - start > timedelta(days=1):
         logger.warn("timeinterval start='{}' and end='{}' is too big.".format(start, end))
+        return
 
     # check if we have any airport
     airports_query = session.query(Airport).limit(1)
@@ -133,6 +134,6 @@ def update_entries(session, start, end, logger=None):
     result = session.execute(ins)
     session.commit()
     insert_counter = result.rowcount
-    logger.warn("Inserted {} TakeoffLandings".format(insert_counter))
+    logger.info("Inserted {} TakeoffLandings".format(insert_counter))
 
     return "Inserted {} TakeoffLandings".format(insert_counter)
diff --git a/ogn_python/gateway/process_tools.py b/ogn_python/gateway/process_tools.py
index 2b178b8..35f0367 100644
--- a/ogn_python/gateway/process_tools.py
+++ b/ogn_python/gateway/process_tools.py
@@ -130,10 +130,11 @@ def update_aircraft_beacons(postfix):
             quality = CASE WHEN ab.location IS NOT NULL AND r.location IS NOT NULL AND ST_DistanceSphere(ab.location, r.location) > 0 AND ab.signal_quality IS NOT NULL
                         THEN CAST(signal_quality + 20*log(ST_DistanceSphere(ab.location, r.location)/10000) AS REAL)
                         ELSE NULL
-            END
+            END,
+            agl = CAST(ab.altitude - ST_Value(e.rast, ab.location) AS REAL)
 
-        FROM devices AS d, receivers AS r
-        WHERE ab.device_id IS NULL and ab.receiver_id IS NULL AND ab.address = d.address AND ab.receiver_name = r.name;
+        FROM devices AS d, receivers AS r, elevation AS e
+        WHERE ab.device_id IS NULL and ab.receiver_id IS NULL AND ab.address = d.address AND ab.receiver_name = r.name AND ST_Intersects(e.rast, ab.location);
     """.format(postfix))
     db.session.commit()