If the FSX or P3D terrain.cfg file becomes corrupted you may encounter strange textures across parts of the landscape that typically change rapidly when approaching. For example, see screenshots here: http://www.orbxsystems.com/forum/topic/75071-nzni-strange-ground-textures/?p=689048
The issue stems from FSX/P3D not being able to find specific landclass textures and thus randomly displaying others; it doesn't mean that the textures are actually missing just that the terrain.cfg reference to their location is wrong or missing. The easiest fix -- rather than hunting for specific incorrect or missing entries -- is to "refresh" the terrain.cfg file. To do so you can use a backup of the original FSX/P3D version that we include in the \Orbx\Scripts\FTXCentral\Work folder.
Note: if you don't have a copy of terrain.cfg in your \Orbx\Scripts\FTXCentral\Work folder please download the one attached to the bottom of this post.
Note: this procedure also works if you receive the error message "the terrain.cfg file doesn't contain a [landclasses] section".
1. Navigate to your main FSX/P3D folder, locate your terrain.cfg file, and make a safety backup copy
UPDATE: please note that with P3D v2.5 and P3D v3.x the active terrain.cfg resides in C:\ProgramData\Lockheed Martin\Prepar3D v2 or C:\ProgramData\Lockheed Martin\Prepar3D v3, respectively
2. Copy the backup of the original terrain.cfg file and paste it into your main FSX/P3D folder, clicking on yes when asked to overwrite (for P3D v2.5/v3.x use the above folders instead!)
3. Start FTX Central, which will trigger silent background checks and automatically re-insert all required FTX custom terrain.cfg entries.
UPDATE: Start Orbx Central and open the Settings > Help page. Run [Sync Simulator]
4. Start FSX and check whether the issue has disappeared. If not please post in our respective support forums and let us know where the issue occurs and what steps you've taken thus far
5. If the issue has been resolved, and you own other add-ons that require custom entries, like the Ultimate Terrain X series or FSAddon products (Vancouver+, Tongass Fjords X), you'll need to restore those entries too. For Ultimate Terrain all you need to do is to start each product's Setup.exe tools once (Windows Start Menu > All Programs > Flight One Software > Ultimate ... > Setup Tool). For other add-ons you need to use their respective repair options, e.g. Windows Start Menu > All Programs > FSAddon > Tongass Fjords FSX > Repair