On Tue, Sep 1, 2020 at 2:23 PM Richard Welty <[email protected]> wrote:
> On 9/1/20 1:32 PM, Russell Nelson wrote: > > On 9/1/20 1:05 PM, Clifford Snow wrote: > >> I discovered what appears to be an ongoing import of > >> National Hydrography Dataset (NHD). > > I've seen NHD data in places. It's pretty awful. It contains rivulets > > you can barely see on the ground. The rivulets aren't connected to the > > creeks, the creeks not to the streams, and the streams not to the > > rivers. Fortunately, I added nearly every stream and every river in NY > > State, so nobody has felt the need for NHD imports in NY. > > > > NHD: Just Don't Do It > > it's fairly old data, and if you import it in developed areas you're > going to run into a lot of things that are outdated due to said > development. > > not great import data. i started to do some in upstate NY and > backed off. > I do indeed want the minor watercourse. I tend to map any place where someone might be able to filter a litre of water, or might get their feet wet on a bushwhack, even if collecting the water might need a cup and some patience, or the wet feet would happen only seasonally. And then I don't like doing just where I hike, so I tend to fill in at least a little of the surrounding area to a similar level of detail - and usually try to make sure that whatever I've mapped connects downstream. For that reason, my home county (Schenectady) has pretty thorough tracing of streams from 'leaves off', 'high water' aerials and has rather too much water on it for Russ's taste. (It's a friendly disagreement, since we both agree that putting something on the map that's in the field can be wrong only in extraordinary circumstances.) I do have a few disconnected streams, but that's usually because they run into storm drains or culverts or sinkholes and I have no idea where they emerge. The NHD data are of highly variable quality. In the Adirondack counties, the USGS never did its own hydrographic survey. In fact, it appears not to have surveyed the area at all until the 1980's. It produced 1:62500 maps from the state surveys of 1903 and 1954. It surveyed the area in 1989, and produced a series of large-scale maps at the unusual scale (in the US) of 1:25000 with contour lines in metres. This was right before the first Bush administration defunded it; since then, USGS has done field surveys only for special projects. (The US has much better maps of Iraq than of the United States!) In the Adirondacks, NY State's hydrographic data are even worse in terms of topological consistency. Wetlands, streams, and open water are all tossed in the same bucket, and nothing connects. The files were produced in 1994-95 by a team of students at SUNY Plattsburgh; I don't recall the name of their supervisor. The data are packaged in a set of shapefiles called 'Wetland Cover Types' https://gis.ny.gov/gisdata/inventories/member.cfm?organizationID=508. I use it for rendering my own map, because it's the only wetland data source I have for the Adirondacks. The USFWS data don't cover the area. It's dodgy enough that I'd never contemplate a wholesale import. In the Capital Region, NHD's topology is pretty good, but, as Richard points out, it's 1954 data, and development has affected a lot of stuff. In the Schoharie Valley and the Catskills, development, particularly if affects hydrology, is pretty strictly controlled to protect New York City's water supply. Given that stability, NHD was quite reliable indeed - right up to 2011, when the storms moved even rivers like the Schoharie Creek. Now, it's something for OHM - but most published maps still show where the streams _used_ to run! If I'm doing hydrography, I'll often bring up NHD as a layer in JOSM and copy-and-paste something if it looks to align with current aerials, rather than do the laborious click-click-click of drawing it. NHD is public domain, so it at least has that going for it. A lot of the hydrography that I map is patchy, because it's for particular projects that I have in mind. -- 73 de ke9tv/2, Kevin
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