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I had to teach a class on Friday and it involved NHD Data. <br>
<br>
NHD data is supposed to be an ever evolving dataset. The beginning's
of it are 1:24k USGS Topographic Maps. As time goes on and <a
href="http://en.wikipedia.org/wiki/LIDAR">Lidar </a>becomes more
prevalent the dataset will improve. Tennessee is slated to get a
high resolution dataset of NHD data collected form
photogrametrically acquired data in the next few years. <br>
<br>
Because NHD data is based of 1:24k quad sheets (and in come cases
USFWS Wetland Maps) it's dated - in Chattanooga it's probably 40 to
50 years old. Streams change. Ponds disappear. Things become
channelized. If you compare it to the NAIP or Bing Aerial Imagery in
some cases it's remarkably close and in some it so far off you
wonder what happened. <br>
<br>
There is also a second glitch with the data - since NHD is based off
the 1:24k topo maps it's not entirely accurate. The USGS changed
their definitions of what consisted of a blue line stream from "it's
a drain" to "it's got water in it". It didn't affect Lakes/Rivers so
much but the blue line streams are questionable unless they are
viewed with Aerial Photography (and in my opinion need to be viewed
in Stereo or site visit to see what is occurring with it). <br>
<br>
I say all of that - it's better than having nothing. At least here
we can improve it. <br>
<br>
Randy<br>
<pre class="moz-signature" cols="72">Randal Hale, GISP
North River Geographic Systems, Inc
<a class="moz-txt-link-freetext" href="http://www.northrivergeographic.com">http://www.northrivergeographic.com</a>
423.653.3611 <a class="moz-txt-link-abbreviated" href="mailto:rjhale@northrivergeographic.com">rjhale@northrivergeographic.com</a>
twitter:rjhale
<a class="moz-txt-link-freetext" href="http://about.me/rjhale">http://about.me/rjhale</a></pre>
<br>
On 4/9/2012 2:11 AM, Martijn van Exel wrote:
<blockquote cite="mid:4F827DAB.5090908@gmail.com" type="cite">On
4/9/2012 12:00 AM, Clifford Snow wrote:
<br>
<blockquote type="cite">
<br>
<br>
On Sun, Apr 8, 2012 at 10:39 PM, Martijn van Exel
<<a class="moz-txt-link-abbreviated" href="mailto:mvexel@gmail.com">mvexel@gmail.com</a>
<br>
<a class="moz-txt-link-rfc2396E" href="mailto:mvexel@gmail.com"><mailto:mvexel@gmail.com></a>> wrote:
<br>
<br>
Hi,
<br>
<br>
Remapping in CA, I come across some weird stuff.
<br>
Here's some NHD 'data':
<br>
<a class="moz-txt-link-freetext" href="http://www.openstreetmap.org/?__lat=35.17764&lon=-119.12641&__zoom=16">http://www.openstreetmap.org/?__lat=35.17764&lon=-119.12641&__zoom=16</a>
<a class="moz-txt-link-rfc2396E" href="http://www.openstreetmap.org/?lat=35.17764&lon=-119.12641&zoom=16"><http://www.openstreetmap.org/?lat=35.17764&lon=-119.12641&zoom=16></a>
<br>
Either the aerial imagery is way off here, or this is just
bad data.
<br>
If it is, I presume that there has been a review of this
data before
<br>
import, this is the exception, and the vast majority of
imported NHD
<br>
objects actually do represent reality. I hope.
<br>
--
<br>
Martijn van Exel
<br>
<br>
<br>
Are you talking about the water - lakes and ponds? From reading
<br>
nmixter's diary, he/she has posted comments about mapping
farms. One
<br>
comment suggested taking the import to the talk-us mailing list.
BTW - I
<br>
did just drive through some farm land in Western Washington.
Farmers
<br>
had dug temporary canals to help drain (or so I assumed) the
water from
<br>
the field so they could plant. I probably wouldn't map it
unless they
<br>
were permanent.
<br>
<br>
</blockquote>
<br>
Yes, I see a lot of water features that are just not corroborated
by the aerial imagery, which could mean one of at least three
things:
<br>
1) The aerial imagery is out of date
<br>
2) The NHD data is out of date
<br>
3) The NHD data represents something I don't understand (the
future, a temporary situation (which should not be in OSM),
something underground?)
<br>
<br>
</blockquote>
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