We use satellite (or aerial) imagery for some of the wall art that we make. In this post, I'll explain how we make these images by walking through the creation of a single image: the Big Island of Hawaii.
The first step in creating one of our Natural Color images is to find the perfect source image to start from. Depending on the size of the area, we might use aerial or satellite imagery.
Aerial imagery generally has a higher resolution than satellite imagery, so for smaller areas we use aerial imagery. (We always want the data resolution to exceed our printed resolution. See this post for more on that.)
Since the Big Island is around 80 miles wide, satellite imagery will do the trick for us.
Our ideal image will be free of clouds and include the whole island in a single pass. A quick look at the available Sentinel-2 imagery reveals that we won't get so lucky!
We won't be able to get the entire big island in a single pass and it turns out that a nearly 14,000 foot volcano in the middle of the ocean is a pretty cloudy place!
After looking through the entire Sentinel-2 record of the big island, I found 4 images (2 for each side of the island) with clouds in different areas that we'll cobble together to get a single, coherent image.
Once we've decided on the images we'll use, we need to download them and then export them from our GIS application. I tend to prefer doing the atmospheric correction myself, so I download the uncorrected versions, which gives me more control over how the colors look in the final version.
Pulling the satellite imagery together involves merging the red, green, and blue bands into a single raster and ensuring we export the resulting raster without losing any data. Typically this will result in a dark image (so we don't lose any of the highlights) with a bluish tint because it hasn't been corrected for atmosphere yet.
Now we're ready for Photoshop!
Now we've got 4 satellite images that we want to make look like a single image. What I like to do is choose my favorite single image and use that as the base. I'll then go and try and make the others look exactly like this one.
To do this I'll have each of the images in the same photoshop file as separate layers. I then create a curve adjustment layer and push the colors around until I can't see the line between the layers.
Often this isn't enough and I need to go in and add a gradient or touch up some areas by hand.
Now we've got 4 layers that look great, but we've still got clouds covering land that we want to show. Fortunately, since we've got at least 2 layers for every pixel in our image, we can go ahead and simply erase the clouds!
At this point I'll pull my favorite of the layers on top since they will get most of the real estate in our final image. The other layers are just behind them though, so I'll just go over the image with my eraser and erase all of the clouds in those top layers, revealing the cloudless image below.
Now, most of the clouds are gone except for where they exist in multiple layers. Unfortunately, now we're out of data, so it's time for a little of what we like to call virtual world building!
To replace the remaining clouds with what Photoshop thinks the area under the clouds would look like we use a few tools, namely the content-aware fill, clone stamp, or magic healing brush.
And there we have it: a single, cohesive, cloud free image of the big island of Hawaii. But we're not done yet!
An important but subtle part of this whole process is cleaning up the hillshade. We're going to remove the natural hillshade in the satellite passes and add our own.
We do this to our Natural Color maps for a couple of reasons. The first and main reason is for perceptual accuracy. We're mostly making maps in the northern hemisphere, where the sun comes from the south, but terrain tends to look inverted when lit from below (you can read about this quirk of human perception here). To combat this, we remove the hillshade from the satellite image and add our own.
When we're working with multiple passes such as for our Big Island map, each pass will also have different hillshade angles, so removing them and adding our own also has the benefit of making the overall image more coherent.
Most of our Natural Color maps don't include bathymetry (underwater terrain) but the Hawaiian islands are an exception. The way they rise out of the seafloor is striking and it feels like a not-to-be-missed part of the story of that landscape.
I want to preserve some of the mystery of the ocean, so I don't give the underwater terrain the same treatment as that above sea level. For this particular map, I've given the hillshade 50% opacity and used the Multiply blending mode so that the brighter areas are more transparent than the darker shadows.
I then add a brightness adjustment-layer using the raw bathymetry data to make the deeper water darker and less transparent. This is an attempt to mimic how water looks in shallower areas, but extended to the sea floor.
Now that we've got a single coherent image with some nice, consistent, perceptually accurate hillshade we can fine tune the colors in our image to make it brighter and more dynamic. We'll use the curve adjustment layer to accomplish this.
This part is as much an art as any of the steps and little changes can take your image from natural looking to cartoonish in a hurry. But, with a subtle hand and a good eye, you'll end up with a map that is clean, dynamic, and looks great on the wall!
If you've read this far you should check out our available maps. I'm really proud of how our images look on our website but I promise, they are even more incredible on the wall!