SRTM and NED elevation data

Posted by asciiphil on 15 June 2010 in English (English)

In the comments on my previous diary entry, people pointed out that the CGIAR data has a no-commercial-use restriction on it, which would be incompatible with OSM's database license if I wanted to derive anything from it. I was only planning on using it for display purposes, but I decided to look into other sources of elevation data anyway. I found the National Elevation Dataset from the US Geological Survey.

What's nice is that there are actually three NED datasets: NED1, NED3, and NED9. NED1 has a resolution of one arc-second, the same as SRTM1. NED3, however, is 1/3 of an arc-second, and NED9 is 1/9 of an arc-second, which ends up being about 3 meters. NED9 isn't available everywhere, so I decided to use NED3 and NED9 together.

A very illustrative test rendering is the Marriottsville Quarry in Howard County. For reference, here's what it looks like with the SRTM data:

Marriottsville Quarry with SRTM1 hillshading

It's not bad, but also not very precise. In particular, the dam in the upper lefthand corner extends well beyond its actual location.

Let's compare it to the NED3/NED9 rendering. (One of the reasons this makes a good test region is that it's right at the edge of one area of NED9 coverage.)

Marriottsville Quarry with NED9 hillshading

It's obviously more precise, but it seems almost too precise. I really don't like the look of the sharp edges, and those sharp edges are everywhere. (There's also some unusual stuff that Liberty Lake is hiding; bodies of water are not always flat in NED9 and introduce some really weird hillshading artifacts.) Furthermore, there are obvious edges where the NED9 data ends.

I tried rendering just the NED3 data (which is still about nine times more precise than SRTM1), but it seemed less accurate: it didn't show the quarry pit at all, though it showed a little of the hill next to it. (I've been by there, and the pit really does exist.) Given that, I really wanted to use the NED9 data where I could.

I then tried using an interesting denoising algorithm I read about online, but it used up all 4 GB of my RAM on a 0.5 square degree chunk of NED3+NED9 data, so I gave up on it. I tried using ImageMagick to apply a blur filter, but the NED data is made up of 32-bit floating-point numbers, and ImageMagick only seems to like TIFFs if each pixel is eight bits wide (or is a 24-bit triplet of 8-bit integers). I ended up just using gdalwarp to reduce the resolution on the NED9 data down to 1/3 of an arc-second. That effectively blurred the elevations, and I'm reasonably pleased with the result, even if the edges of the NED9 data are still visible:

Marriottsville Quarry with smoothed NED9 hillshading

Note that the contour lines in all rendering are SRTM1-based. It's a lot harder for me to swap out contour lines than it is color relief and hillshading data.

Comment from Gnonthgol on 16 June 2010 at 06:34

This looks great. I am stunned of the NED9 quality. You can even map buildings and roads from this.
Witch made me think; I am not an expert on US informational justice but isn't this data public domain? That means that we might use it for mapping.
After reading about NED I found out that it is actualy an ortogram. It is derrived from areal photographs for the use of rectification. Then where is the original ortphoto? And isn't that something we might trace?

Comment from asciiphil on 16 June 2010 at 12:01

According to the USGS, the NED data comes from a variety of sources, including SRTM (where there's nothing better, presumably).

However, the USGS also has aerial photography for much of the US that's often higher resolution than Yahoo's. They have WMS capability URLs at . Note that while any data produced by the USGS is public domain (since they're part of the US federal government), some of the aerial photography in their WMS layers has been produced by others and is licensed to the USGS. In the WMS capabilities, the licensed layers are marked as "VIEW ONLY"; we can't use those in OSM.

Comment from SamatJain on 17 June 2010 at 22:49

What does NED9 look with a different type of shading, perhaps hypsometric tinting?

AFAIK this is what Google uses for their terrain maps, which also seem high resolution and look quite good.

Comment from asciiphil on 18 June 2010 at 00:45

I'm more or less doing hypsometric coloring, too. The lowest layer of my rendering is a relief-colored image based on the same data as the hillshading. The colors I'm using are pretty subtle, though. See my previous diary entry for some examples of the different colors that can show up.

Basically, since I'm doing very gradual color changes, there's practically no difference between NED and SRTM colors for my current rendering.

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