more terrain

Author: timtuxworth

common/source/docs/common-terrain-following.rst

@@ -170,12 +170,24 @@ interpolate as necessary to provide the requested grid size.
 
 Terrain Accuracy
 ================
+When the terrain data issue was resolved (see below), the data sources for both 100m and 30m terrain was switched to JAXA's ALOS. There are 2 dataset versions available:
 
-The accuracy of the SRTM database varies over the surface of the earth.
-Typical accuracy is around 10 to 20 meters, although some areas are
-worse. This makes terrain following suitable for aircraft that are
-flying at altitudes of 60 meters or more. Using terrain data for low
-flights is not recommended.
+- SRTM1: Based on JAXA's ALOS dataset and has 30m (1 arc-second) horizontal resolution. It covers all areas between 0 and 84 degrees North/South latitude.
+- SRTM3: Downsampled version of SRTM1 with 100m (3 arc-second) horizontal resolution. It covers all areas between 0 and 84 degrees North/South latitude.
+
+Switching to JAXA's ALOS World 3D (AW3D30) for terrain significantly improves ArduPilot's vertical accuracy, though it does not 
+eliminate the need for safety buffers. While the original NASA SRTM previously used by ArduPilot data had a target vertical accuracy of +/- 16 meters
+ALOS data generally achieves an accuracy of +/- 5 meters.
+
+Impacts of Downsampling to SRTM3
+
+- Vertical Accuracy Degradation: SRTM1 generally achieves a vertical Root Mean Square Error (RMSE) of approximately ±3.56m, whereas downsampled SRTM3 typically degrades to around ±5.89m.
+- Loss of Sharp Features: High-resolution details of valleys, ridges, and hills become "flattened" or smoothed out. SRTM1 is much more reliable at capturing sharp topographic changes.
+- Averaging Errors: Each 100m cell in SRTM3 represents the average height of an area roughly the size of a city block. Small but critical obstacles—such as a single sharp peak or a large building—may be averaged into the surrounding lower terrain, making them "invisible" to the autopilot.
+
+The accuracy of the terrain database varies over the surface of the earth, and can change over time due to mudslides, construction, mining, earthquakes, etc.
+
+Using terrain data for low flights is not recommended.
 
 Setting up for terrain following
 ================================