In this video I show you and where to create height maps for Cities Skylines and then import them into the ingame Map Editor so you can edit and then use them for your gameplay.
Hope this was helpful! :) If you have questions, please leave them down in the Comments section.

Link to Height Map site: http://terrain.party/
Twitter: twitter.com/Xterminator5
Patreon:   / xterminator5