AP_Terrain: check for old DAT files in pre-arm

Add a version_minor to the terrain data structure to allow checking
for old DAT files. This pushes users towards downloading new terrain
data to avoid issues with old terrain data that have been recently
fixed.

If the user has not set the TERRAIN_OPTIONS bit to accept old terrain
data then they will get a pre-arm error if any old terrain data is
found during the normal pre-flight check of terrain data.

Note that this doesn't check every block in all DAT files as that will
take too long, but it is likely to push users to clear their old data
or download new data from the server.

Author: tridge

libraries/AP_Terrain/AP_Terrain.cpp

@@ -57,8 +57,8 @@ const AP_Param::GroupInfo AP_Terrain::var_info[] = {
 
     // @Param: OPTIONS
     // @DisplayName: Terrain options
-    // @Description: Options to change behaviour of terrain system
-    // @Bitmask: 0:Disable Download,1:Disable Disk
+    // @Description: Options to change behavior of terrain system. The Accept Old Terrain Data option can be used to accept terrain data generated from before the terrain database bugs were fixed. The bugs in the data were all fixed from 24th February 2026. If you really want to risk using the old terrain data then you can set this option, otherwise remove the old terrain data by formatting your microSD card or renaming the /APM/TERRAIN folder. Then downloaded updated data from https://terrain.ardupilot.org, or let the automatic terrain download repopulate your terrain data.
+    // @Bitmask: 0:Disable Download,1:Disable Disk,2:Accept Old Terrain Data
     // @User: Advanced
     AP_GROUPINFO("OPTIONS",   2, AP_Terrain, options, 0),
 
@@ -449,6 +449,16 @@ bool AP_Terrain::pre_arm_checks(char *failure_msg, uint8_t failure_msg_len) cons
         return false;
     }
 
+    if (!option_set(Options::AcceptOldData) && found_old_data) {
+        /*
+          we have found old terrain data on the microSD card, warn the
+          user that they have out of date terrain data that may
+          contain errors.
+         */
+        hal.util->snprintf(failure_msg, failure_msg_len, "terrain data expired, possible errors");
+        return false;
+    }
+
     return true;
 }
 

libraries/AP_Terrain/AP_Terrain.h

@@ -53,6 +53,10 @@
 // format of grid on disk
 #define TERRAIN_GRID_FORMAT_VERSION 1
 
+// min minor version for data read from microSD
+// raise this to force a refresh of data from the terrain servers
+#define TERRAIN_VERSION_MINOR_MIN 1
+
 // we allow for a 2cm discrepancy in the grid corners. This is to
 // account for different rounding in terrain DAT file generators using
 // different programming languages
@@ -242,6 +246,10 @@ class AP_Terrain {
         // rounded latitude/longitude in degrees. 
         int16_t lon_degrees;
         int8_t lat_degrees;
+
+        // minor version. Note! this and any bytes after this are
+        // excluded from the CRC for backwards compatibility
+        uint8_t version_minor;
     };
 
     /*
@@ -383,10 +391,15 @@ class AP_Terrain {
     enum class Options {
         DisableDownload = (1U<<0),
         DisableDisk = (1U<<1),
+        AcceptOldData = (1U<<2),
     };
 
+    inline bool option_set(enum Options option) const {
+        return (options.get() & uint16_t(option)) != 0;
+    }
+
     inline bool diskless() const {
-        return (options.get() & uint16_t(Options::DisableDisk)) != 0;
+        return option_set(Options::DisableDisk);
     }
 
     // cache of grids in memory, LRU
@@ -482,6 +495,9 @@ class AP_Terrain {
     bool memory_alloc_failed;
 
     static AP_Terrain *singleton;
+
+    // true if we have found old disk blocks when loading
+    bool found_old_data;
 };
 
 namespace AP {

libraries/AP_Terrain/TerrainGCS.cpp

@@ -44,7 +44,7 @@ bool AP_Terrain::request_missing(GCS_MAVLINK &link, struct grid_cache &gcache)
 
     struct grid_block &grid = gcache.grid;
 
-    if (options.get() & uint16_t(Options::DisableDownload)) {
+    if (option_set(Options::DisableDownload)) {
         return false;
     }
 

libraries/AP_Terrain/TerrainIO.cpp

@@ -25,6 +25,7 @@
 #include <AP_Common/AP_Common.h>
 #include <AP_Math/AP_Math.h>
 #include <stdio.h>
+#include <GCS_MAVLink/GCS.h>
 
 extern const AP_HAL::HAL& hal;
 
@@ -333,6 +334,19 @@ void AP_Terrain::read_block(void)
                (int)ret,
                (unsigned long long)disk_block.block.bitmap);
 #endif
+        if (disk_block.block.version_minor < TERRAIN_VERSION_MINOR_MIN) {
+            /*
+              this triggers a pre-arm warning to prompt the user to
+              download new terrain data
+             */
+#if HAL_GCS_ENABLED
+            if (!found_old_data && hal.util->get_soft_armed()) {
+                // in-flight warning for GCS operator
+                gcs().send_text(MAV_SEVERITY_WARNING, "terrain data expired, possible errors");
+            }
+#endif
+            found_old_data = true;
+        }
     }
     disk_io_state = DiskIoDoneRead;
 }

libraries/AP_Terrain/TerrainUtil.cpp

@@ -135,6 +135,7 @@ AP_Terrain::grid_cache &AP_Terrain::find_grid_cache(const struct grid_info &info
     grid.grid.lat_degrees = info.lat_degrees;
     grid.grid.lon_degrees = info.lon_degrees;
     grid.grid.version = TERRAIN_GRID_FORMAT_VERSION;
+    grid.grid.version_minor = TERRAIN_VERSION_MINOR_MIN;
     grid.last_access_ms = now_ms;
 
     // mark as waiting for disk read
@@ -173,7 +174,12 @@ uint16_t AP_Terrain::get_block_crc(struct grid_block &block)
 {
     uint16_t saved_crc = block.crc;
     block.crc = 0;
-    uint16_t ret = crc16_ccitt((const uint8_t *)&block, sizeof(block), 0);
+    /*
+      note that we exclude version_minor and any later bytes from the
+      CRC to maintain backwards compatibility so old versions of
+      ArduPilot accept new terrain blocks
+     */
+    uint16_t ret = crc16_ccitt((const uint8_t *)&block, offsetof(struct grid_block, version_minor), 0);
     block.crc = saved_crc;
     return ret;
 }