Sorry if I'm barging in here, just though all of this information would be best in the same place for anyone looking for resources.
This isn't as hard as I thought, but still not an exact science. This method would be extremely useful for cities with boundaries that are more at an angle than in a square, Washington DC and Dubai come to mind, there are probably many others. So instead of being limited to a 90 degree import you can import at any degree using this method basically.
For me personally since I believe 40x40km is the most accurate to real scale, that is the scale I would like to have in the end. If that is not what you want then you will just have to adjust your math accordingly.
So to get that we will actually set the dimensions in terrain.party to 60x60km (the maximum). Make sure to include any area that you will want included in the final heightmap.
Then you will need extract and locate the heightmap so you can edit it. This is covered in other tutorials, so I won't be covering that. I personally almost always use the merged heightmap, so for this tutorial that is what we will be using.
You will need photoshop or equivalent, for this we will be using photoshop. Open the file.
You will notice that regardless of the dimensions you import the heightmap in, the size (in pixels) will always be 1081x1081. Now to get the map into our proper scale from 60x60km to 40x40km we will need to multiply the image size by 1.5 (60 is exactly 50% more than 40) which comes to 1621.5, but we will round up to 1622 since PS will only accept whole integers.
I have found bicubic resampling to be the best in the past with other image scaling, so it is what I picked to start. You could play around with the settings to see if any of them yield better results. Next we will need to rotate the image.
For this tutorial we will be doing a 45 degree right rotation, though PS can do it at any angle. After you click custom it will bring up another dialogue box where you can select right or left rotation, and any custom degree.
Then we will need to make a rectangular selection, with a fixed size of 1081x1081 pixels.
You can see now why we initially imported at 60x60km and have now converted it to a 40x40km scale. If we did the import process at the same scale as our desired output, you would lose all of the corners.
This is the part where your initial selection (in the terrain.party website), and the current selection really matters. Its also not an exact science, as you may want to use the zoom features to make sure you haven't clipped the edge or left out any area you want included. As long as you have the selection at the correct pixel size it will still work, but this part is up to your own eyes to get correct.
Though if you wanted to get really fancy you could import the street view and mask that layer with some opacity to be able to get it perfect.
Next we will copy that selection and create a new image from the clipboard.
For me, PS gave me some messages about the document containing 16 bit layers and PS not supporting that. I suppose this may have something to do with the final quality, though when I looked at it in the game it was still pretty good. If anyone knows of another program (or plugins) that supports 16 bit layers please post it.
The only real option from there is to convert the depth to an 8 bit layer which works.
Then you simply need to save the file as a PNG, and place it into your heightmaps folder so CS can find it. The latter being covered in other tutorials.
Then when you start up a new map in the terrain editor, import the heightmap, add some water, you have yourself a custom angled height map!
Possibly because of the resampling, rescaling, and the 8 bit layer conversion, the terrain does seem to be slightly rougher than it was in a 90 degree import. If you have a better version of PS you can probably do it in 16 bit, my copy is very old. In a way it seems more detailed as you can see a lot of the old dried up river beds. Most of those are just very tiny ridges that are not really steep at all, but I would understand if you would not want this in your city.
This can be fairly quickly alleviated by smoothing the terrain with a soft brush, like I did very quickly in the center of the screen just as an example. The slight increase in roughness actually helps with relatively flat maps in defining a solid river bed without needing to dig it out or make many adjustments.
Also if I were doing this to publish it or just to play, I probably would have readjusted the 1081x1081 selection a little lower and to the right to have the river enter exactly in the corner of the map, and include some of the area across the river in the playable grid, but this is just for an example.
This does have its limited use, but for cities like Washington DC or Dubai using this method can be crucial to fitting in all the areas you want. For example every map of Dubai on the workshop includes the Palm Jumeirrah or the Burj Khalifa downtown area, but not both (or if there is one, it is horribly out of scale). Using this method you can include both.
Hope somebody finds this useful.
I remember a few, one in particular with lots of tips that I would love to see again!
