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I’d be interested to know how they gathered all that data. Given the very detailed ponds and streams that may seem invisible to our daily lives, did they really

I’d be interested to know how they gathered all that data. Given the very detailed ponds and streams that may seem invisible to our daily lives, did they really send someone out there to survey all of it? Or is this via satellite imagery or something similar?

maxerickson 2y ago
It's derived from LIDAR scans. https://www.usgs.gov/ngp-standards-and-specifications/elevation-derived-hydrography-and-national-hydrography-datasets https://www.usgs.gov/ngp-standards-and-specifications/elevat... I'm not sure it's their best product for elevation anymore, but USGS also publishes 3dep, which is a unified layer of the best available elevation data for the US.
qwertyuiop_ 2y ago
It’s all those “ufos” with clearly marked FAA mandated navigation lights.
driggs 2y ago
From high resolution Digital Elevation Models. They first compute the flow direction from every pixel; from flow direction, the compute flow accumulation at every pixel. They then filter based on some threshold for catchment area, and vectorize the flow paths. The USGS NHD+ Users Guide includes a very detailed description of the technical process: https://pubs.usgs.gov/publication/ofr20191096 https://pubs.usgs.gov/publication/ofr20191096
- SpicyUme 2y ago
  Weird, just spot checking a few places I know relatively well I see a few issues that seem like they would show up from that method. I wonder if there are issue in build up environments? Unfortunately that link is down for me. A collection of streams go uphill in this map where in reality I believe they flow into an underground pipe or something that drains into the lake in a different place. I wouldn't expect the underground section to be perfect, but the hill the streams ascend is ~50 feet. In another place two ponds are not shown, maybe due to dense tree cover? I believe both have water year round, but one was fenced off after a kid drowned and the owners may have filled part of it in.
  - driggs 2y ago
    Hydrological modeling gets tricky when water flows underground, either naturally or unnaturally. A pixel surrounded entirely by higher elevation pixels is a "sink", and if you want to model the overland flow of water a preprocessing step is often to fill sinks and apply a slight gradient to them. I believe that the NHD+ dataset uses an alternate strategy; it burns an artificial channel into a sink, connecting it to a lower-elevation outlet. The dataset includes attributes that identify these "underground flow" segments. I didn't examine ESRI's Living Atlas site closely enough to see if they've used a different symbology to represent these segments, but your comment makes it sound like they did not. A pond often has no inlet and no outlet (as opposed to a lake), so it makes sense that some ponds may not show up in this dataset.
    - SpicyUme 2y ago
      Thanks, that's interesting and makes sense that there are alternate strategies with different tradeoffs. I checked the Lost River and some of the other areas on the Snake river plain. I can see how it would be hard to deal with water flowing underground. I noticed that the lava flows have effectively no streams, which I guess shouldn't be a surprise but it was.
sandworm101 2y ago
While these particular maps may be from scans, the government really does send people into every little field for this type of thing. Between geological surveys and defense planning, I don't there is an acre of land anywhere in the US that hasn't welcomed a government surveyor at some point.
thesuitonym 2y ago
https://www.usgs.gov/special-topics/water-science-school/science/science-topics/how-usgs-measures-water https://www.usgs.gov/special-topics/water-science-school/sci...

maxerickson 2y ago

It's derived from LIDAR scans. https://www.usgs.gov/ngp-standards-and-specifications/elevation-derived-hydrography-and-national-hydrography-datasets https://www.usgs.gov/ngp-standards-and-specifications/elevat... I'm not sure it's their best product for elevation anymore, but USGS also publishes 3dep, which is a unified layer of the best available elevation data for the US.

qwertyuiop_ 2y ago

It’s all those “ufos” with clearly marked FAA mandated navigation lights.

driggs 2y ago

From high resolution Digital Elevation Models. They first compute the flow direction from every pixel; from flow direction, the compute flow accumulation at every pixel. They then filter based on some threshold for catchment area, and vectorize the flow paths. The USGS NHD+ Users Guide includes a very detailed description of the technical process: https://pubs.usgs.gov/publication/ofr20191096 https://pubs.usgs.gov/publication/ofr20191096

SpicyUme 2y ago
Weird, just spot checking a few places I know relatively well I see a few issues that seem like they would show up from that method. I wonder if there are issue in build up environments? Unfortunately that link is down for me. A collection of streams go uphill in this map where in reality I believe they flow into an underground pipe or something that drains into the lake in a different place. I wouldn't expect the underground section to be perfect, but the hill the streams ascend is ~50 feet. In another place two ponds are not shown, maybe due to dense tree cover? I believe both have water year round, but one was fenced off after a kid drowned and the owners may have filled part of it in.
- driggs 2y ago
  Hydrological modeling gets tricky when water flows underground, either naturally or unnaturally. A pixel surrounded entirely by higher elevation pixels is a "sink", and if you want to model the overland flow of water a preprocessing step is often to fill sinks and apply a slight gradient to them. I believe that the NHD+ dataset uses an alternate strategy; it burns an artificial channel into a sink, connecting it to a lower-elevation outlet. The dataset includes attributes that identify these "underground flow" segments. I didn't examine ESRI's Living Atlas site closely enough to see if they've used a different symbology to represent these segments, but your comment makes it sound like they did not. A pond often has no inlet and no outlet (as opposed to a lake), so it makes sense that some ponds may not show up in this dataset.
  - SpicyUme 2y ago
    Thanks, that's interesting and makes sense that there are alternate strategies with different tradeoffs. I checked the Lost River and some of the other areas on the Snake river plain. I can see how it would be hard to deal with water flowing underground. I noticed that the lava flows have effectively no streams, which I guess shouldn't be a surprise but it was.

sandworm101 2y ago

While these particular maps may be from scans, the government really does send people into every little field for this type of thing. Between geological surveys and defense planning, I don't there is an acre of land anywhere in the US that hasn't welcomed a government surveyor at some point.

thesuitonym 2y ago

https://www.usgs.gov/special-topics/water-science-school/science/science-topics/how-usgs-measures-water https://www.usgs.gov/special-topics/water-science-school/sci...