If you make your overlay at the same time as you download your heightmap from terrain.party, you'll get a quality overlay that you never have to move around, rescale nor adjust in an overlay mod ever again! This helps you ensure your city or map is of uniform and accurate scale.

Some examples of using an overlay to recreate real-life infrastructure and natural features

Walt Disney World Magic Kingdom, parking and transportation center, and monorail system

-------------------
Beaches, breakwaters and jettys of Tel Aviv


-------------------

Although you can use these ideas for making an overlay for any mod, this guide is made with the Overlayer v2 Mod in mind, which allows large size overlays and drapes the overlays over the terrain instead of remaining a flat 2D image that you have to raise and lower. Overlayer v2 mod's description has some complex instructions and details that will help you use more advanced techniques than in this guide here, but if you don't want to get too complicated you don't need to! Just make your overlay.png, put it in the right folder, and click Overlayer v2 mod's button in game. Done! Specific instructions here:

OVERLAYER V2 MOD: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=662933818

1. Type in terrain.party into your browser, find your real world location. Set the box to your ideal position over the map. Leave it at the default 18 km x 18 km setting on terrain.party. Get it exactly where you want it, as it will be difficult to re-center later.

2. Take a screenshot of your browser showing terrain.party's square. This will be your stencil. Don't close that browser!

3. Now hit the download button on terrain.party. This downloads a zip of 1081 x 1081 pixel .png heightmaps

4. Unzip the heightmaps into the folder C:\Users\YOURUSERNAME\AppData\Local\ColossalOrder\Cities_Skylines\Addons\MapEditor\Heightmaps
USGS is the best (only available for US locations), SRMT 3 is the better of the rest

5. Open your screenshot in GIMP or Photoshop. Crop the image to only the blue grid, the blue border of the terrain.party grid being the outer edge. Be sure the image is a perfect square down to the pixel.


6. Scale image to 1081 x 1081 pixels




1. Overlay image: scale the layer 104.16% (select "expand from center").


(CHECK: If you see the guide lines expand outside of the image, you did it correctly!)


2. Do the same to the heightmap image: scale the layer 104.16% (select "expand from center").

Export both as PNG files. Now overlay matches heightmap perfectly, and your city will be 1:1 real life scale.


1. Keep an original copy of the overlay you just made.
2. In GIMP set the opacity of the layer where the yellow box is. I use 55%
3. Export this as overlay.png to C:\...\steamapps\common\Cities_Skylines\Files

(this section by AmiPolizeiFunk Nov.2018)

You're not the first person to want a stitched large-scale map image from the OSM database. Others have paved the way before you, creating cool websites and scripts to automate the process. It may be a good idea to quickly familiarize yourself with GPS, as well as with the various OSM zoom levels. Just go to openstreetmap.org and zoom in and out. See how much more detail there is when you zoom in? Notice the zoom level in the url. Here is an example of Berlin:

https://www.openstreetmap.org/search?query=berlin#map=13/52.5103/13.3941

The three numbers at the end of the url are the zoom level (13), and two GPS coordinates. This view shows the entire Berlin Ringbahn, but you can't see much detail. Now let's zoom in a bit...

https://www.openstreetmap.org/search?query=berlin#map=17/52.51542/13.37706

At zoom level 17, you can see the footprint of every single building, as well as small paths through the park, and underground tunnels. You could try screenshotting at this zoom level, but for an entire C:S map at 1:1, you would probably need a hundred screenshots. But fear not, there are scripts!


BigMap is a website that acts as a GUI for the OSM database. The BigMap 2 portal can be found here:

http://bigmap.osmz.ru/

and more information about how to use it: https://wiki.openstreetmap.org/wiki/Bigmap

Let's start by pasting the first OSM url from Berlin (the first link given in this section) into BigMap 2 and pressing "Submit." This focuses the map on Berlin, and now we can use the custom BigMap 2 GUI to control what kind of image we want. Where should it be centered? How big should it be? How zoomed in do we want it? The interface is a big tricky at first, but play around with it and you'll find that it's easy to use.

After hitting Submit, I see a map of Berlin, and the BigMap overlay tells me:

"Map is 4x3 tiles (1024x768 px) at zoom 13, aspect 1.33:1"

Use "EXPAND" or "SHRINK" to make a square image. I did one "EXPAND bottom" and now we're here:

"Map is 4x4 tiles (1024x1024 px) at zoom 13, aspect 1:1"

Okay cool, but I want a bigger image that is more zoomed in! So let's press "ZOOM in/double size". The framing stays the same, but now our image is twice as big with twice as much detail:

"Map is 8x8 tiles (2048x2048 px) at zoom 14, aspect 1:1"

BigMap 2 has a function where it can automatically download and stitch an image for you, but only up to 10x10 tiles. We're within that range (at 8x8), so let's use the automated function. Press "Enqueue". You may have to wait a few minutes, but eventually, you will be able to simply download and save a 2048x2048 .png file, which will make a nice overlay.

If you want more zoom and an even bigger image, you'll have to learn how to use a script. I found it easy enough to install Perl and download the "Perl" script from Bigmap2, execute it on my box, and make a whopping massive image. The largest image that I've made so far is 10,240x10,240 px (40x40 tiles) at zoom level 16. Check the wiki for more info on how to use the scripts (it can generate a Python script too, as well as a few other languages).

Go back to the "Finishing up" text in the last section to properly crop your image and make it semi-transparent.

Open your stencil overlay image (the screenshot you resized to 1081x1081 pixels) and resize it to 2048x2048 pixels (for large) 4096x4096 pixels for (really large). You could do 8192x8192 I guess, but... there are limits. I like a larger overlay, so I usually have the simulation on pause while the overlay is enabled.

Now take the image you downloaded from Bigmap and paste it as a new layer. Now move it around and resize it until you get it to be the same size & dimensions. Use layer activation and transparency to help you get this done.

You're ready to use your overlay in game!! See the workshop page or Overlayer v2 mod (or whatever mod comes in the future) if you need further instructions on how to use the mod. With this guide, all you need to know for Overlayer v2 is contained here. Just put overlay.png in the right folder as indicated, activate the mod and click its button in game. Done!

OVERLAYER V2 MOD: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=662933818

If you've made an overlay after a map was made (example - you're making this overlay for a map made by someone else), then you'll have to scale and position the overlay as most users of the mod have to.

If you made a map from the beginning with this ^^ overlay process, you will not have to adjust the position nor the size of your overlay, ever! Just click it on and off, no hassle! Yay!

Thanks to jchar96, we have some alternatives for the method I've used so far:
http://www.chengfolio.com/google_map_customizer


-----------------------------------------------------------------------
Shared by jchar96: Alternative methods for saving images (and stitching them together)

http://dylanbrowndesigns.com/tutorials/how-to-save-or-print-high-resolution-images-from-google-earth/

-----------------------------------------------------------------------