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Mapping under the trees

Posted by Black_Diamond on 8 September 2023 in English.

The USGS 3D Elevation Program (3DEP) is awesome. When mapping roads, creeks and trails that are covered by trees, it’s nearly impossible to use any satellite imagery in any meaningful way. Even if someone spends the time to use GPS to track a road or hiking trail, it is even harder to find someone to walk down the middle of a lot of creeks to track the path of the water flow. It’s also not uncommon for the GPS trace to be off by several feet or even have gaps in the data near tall dense trees, which reduces the accuracy of the path being traced on a map. 3DEP can help in such situations.

three dimensional elevation program (3DEP) Logo

I believe that this data is only available for most of the US territories. So if you are mapping outside outside of the US, this may not be beneficial to use.

Unfortunately, diary entries don’t allow attaching images of examples of when 3DEP had superior visibility of what was under the trees over the satellite imagery. I can say that it has been a beneficial mapping background layer when mapping features hidden under the trees.

I’ve seen examples where streams go through a thick patch of trees in the satellite imagery. Based on the shadows of the trees, it can seem like a creek is following the right path. When you use the 3DEP imagery, you can see some of those creeks are misplaced. Some go through small hills. Isolated houses in satellite imagery in the middle of a forest can have visible roads in the 3DEP imagery when they have shoulders, embankments or cuttings.

How is this done? It’s done with Lidar. A plan is flown over the terrain with Lidar scanning the surface below. While a lot of the Lidar is reflected back from the leaves and other foliage, some of the laser pulses will go between the leaves and take longer to be reflected back to the Lidar sensor.

Airplane using lasers to scan under the foliage

So if you can’t find something under the trees, I recommend turning on the USGS 3D Elevation Program background layer in the OpenStreetMap iD editor that you can use in your web browser. You might be surprised what you can find. Even old landslides that are overgrown with trees are much easier to see with this background layer.

Happy mapping! 🌎🗺️

Discussion

Comment from n76 on 9 September 2023 at 04:39

Do you know of a way to use this USGS 3D Elevation Program as a background layer in JOSM?

Comment from n76 on 9 September 2023 at 18:00

Turns out that JOSM has that layer too. I just never noticed it in the list of available imagery. Thank you for the tip on using it!

Comment from Yury Yatsynovich on 12 September 2023 at 15:58

Wow, this one covers almost the entire US! I’ve been using LIDAR from MassGIS – it was extremely helpful for mapping smaller rivers and streams.

Comment from rtnf on 13 September 2023 at 09:52

Really cool. Thanks for the info. 👍

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