Making a map

I have an image that I designed based on some google maps and some other maps. It is more or less a combination of a few different places. It appears to be roughly a square mile. What would be the best way to go about designing a terrain for use in Unity with this image? I dont really want to just put the image on my second monitor then eyeball it. I would rather superimpose the image on a terrain then work on the height like that. Would that work? Would there be a better method or program to use?

There are some assets for this on store. Examples:

And there are others I’ve seen in the past.
They should at least give you some ideas on how to approach this.
Basically you would use a height map data to generate the terrain mesh and then impose pictures on top of it. This way you can automate the entire process.