Comment by trs8080 | Hacker News Reader

Comment by trs8080 | Hacker News Reader

I think there's a misunderstanding of the data here. It appears that the underlying data used for this map is DEM data (https://en.wikipedia.org/wiki/Digital_elevation_model), which encodes values in the red, green, and blue channels of an image representation of a geographic area. You're not supposed to visualize this data directly -- instead, you combine those values in a simple formula to obtain a height value, then you use that value to create visual maps. This map just restyles what is geographically analogous to the binary representation of "hello, world" instead of the output itself.

You can search google for "terrain map" or "elevation map" and find many beautiful examples of accurate elevation data.

Edit: This is a good tutorial on how to derive the correct values: https://github.com/syncpoint/terrain-rgb

Edit 2: Looks like I was wrong about this. Apologies to the author and thanks to everyone who helped me understand GIS a bit better! I'll keep this up as a reminder that just because my voice is loud doesn't mean I'm right.

Oilslick was made with elevation data. In the the middle of the text in the left-hand panel, the author wrote:

> The cycling and purity of colors looks a lot like a thin-film interference pattern, hence the name: oilslick. This could be a point of confusion for my more GIS-educated viewers: the pattern is easily mistaken for the raw result of radar interferometry elevation mapping [0] (which was the method used to produce most of the data in this dataset). But this is not the case; what you see here is final, post-processed elevation data.

[0] https://www.esa.int/var/esa/storage/images/esa_multimedia/im...

Which part of the text on the site gave you the impression they made this misinterpretation? It clearly explains what its visualizing and why it looks like that, and has nothing to do with what you claim is their mistake.

But of course you said something negative about the content, so clearly it has to be the top comment on HN, can't have that otherwise...

>You're not supposed to visualize this data directly -- instead, you combine those values in a simple formula to obtain a height value, then you use that value to create visual maps.

Isn't this exactly what has been done here? You can even see the legend at the right side of the screen.

This still shows the uncombined components. What does the color blue represent in this map? Is it a height? -500m? 5000m? 6000m? What does white represent? This map is a fundamental misrepresentation of the actual data.

This is the formula:

height = -10000 + ((R * 256 * 256 + G * 256 + B) * 0.1)

No it doesn't. I think you're misunderstanding the site. It deliberately uses a weird banded colour scale so you can see where height changes on a fine scale.

They didn't interpret the data wrong.

I think there’s a misunderstanding of the data _here_. The underlying data are indeed a DEM, distributed as a single band geotiff file. There’s no R, G, or B, just elevation. The linked tutorial is simply how to take those data and convert them to a format that can be visualized on screen _using_ R, G, and B values derived from the formula you posted in a child comment.

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