Hi guys, so I´ve been playing around with L3dt but I dont get good results, the landscapes aren´t detailed enough. And the materials are also very bad 
is there another FREE programm with which I can generate detailed Landscapes for my open world game?
Pimney,
Try using a tool called World Machine. They have a paid version but the free version works pretty good for starting out and they provide a large amount of customization for different maps. Exporting out to UE4 is super easy and I was able to generate some interesting worlds.
hope this helps.
but I won´t be able to release a full game with this terrain will I ? because isn´t the free version non-commercial?
Correct, it’s just another tool you can use and experiment with, I didn’t know you wanted this to be production quality. In that case you have to spend money to make money 
Best of luck.
Well thanks for the answer I´ll see how I will do further
For free options - the USGS has a number of maps you can use without limitation. They used to offer all of them for any purpose free of charge, but it looks like they’ve started charging for some of it.
Natural Earth has some as well.
And you can get heightmaps of a number of places with no limit on usage from various other sources.
If all else fails, there are tutorials on how to make them by hand using Photoshop (and you can download CS2 for free from Adobe) -
…
Making Heightmaps with Photoshop