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What is it ?
This is a google map to grayscale heightmap.
It's the same thing as 3D map generator - Terrain but it's free and you can zoom in more than in the 3D Map.

How it works ?
Go on google map and search your desired location.
Go in "Map" mode and copy the link and paste it here.
Next, go on the link generated just under and click on Export on the top right.
Save the file as .png and open it in picture editing software.
Go back on your google maps and go in Satellite mode.
Press Alt + Print Screen and paste it in your software and you just have to adjust screen place.

Good job, you have heightmap with an "overlay".

You just have now to do a square selection, crop it, resize it in your terrain and extract it as terrain.png and do terrain.hf
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Link of the google maps location
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https://www.google.com/maps/@51.8516402,-0.5409124,15.25z?entry=ttu
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Please remove your link once you finish
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Link of the grayscale heightmap
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https://tangrams.github.io/heightmapper/#15.25/51.8516402/-0.5409124
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Video tutorial :
https://www.youtube.com/watch?v=3yQM3-gaz7c