Apr 27, 2021 Generalizing the NHDPlus HR for National Applications

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Topic: Generalizing the NHDPlus HR for National Applications

Presenters: Cindy McKay, Richard Moore, Al Rea

00:02:04 NHDPlus HR National Data Release - Al Rea

00:10:15 EPA Considerations - Shelley Cawley (EPA)

00:13:08 Generalization - Cindy McKay

00:37:24 Disconnected Networks - Al Rea & Rich Moore

EPA has been sponsoring research into how to generalize the NHDPlus HR to a level of detail similar to the older Medium Resolution (1:100,000 scale) NHDPlus Version 2, while retaining the network addresses (ReachCodes and Measures ) of the NHD High Resolution. This will support national modeling efforts, while consolidating network-referencing efforts to a single addressing framework based on NHD High Resolution.

A generalized version of NHDPlus HR is being developed nationally and is nearing completion. The dataset has a stream network similar in density to the NHDPlus V2, and has catchments dissolved to match the network. The new dataset, however, has a lot more disconnected drainage than NHDPlus V2. The presentation will show methods the team is developing to deal with this issue, both in the short term and in the longer term. The presentation also will describe how the numbers of features and feature density variations are being handled. 


Date Taken:

Length: 00:58:15

Location Taken: Reston, VA, US


This is the USGS National Geospatial

Program, hydrography Advisory call.

And today we've got lots to talk about,

so I'm going to try and go

ahead and get started here.

And. The first announcement,

which we just got in here.

This was in the newsletter

that we sent out last week,

I believe, but you know,

we are moving some of our operations between.

Uh. Between our locations at the Angie

talk between Denver in Rolla, Mo and so.

We're going to be shut down from the 23rd,

which was last Friday till May 9th.

So right now you can't do checkouts for

either NHD or WBD and can't do checkins.

During this time we are asking that

you please update your tools if

you're a steward and you're working

with the tools our update tools.

We're asking that you update,

you know to ArcGIS 10.7.1

and update the tools with the

version that you can get on the HDC.

The Hydro

Data community.

So, uh. That's the mean announcement.

I wanted to make.

Was there anything else that I

needed to cover on that Becci? >Nope.

I think that's it. Alright.

OK, and then Vanessa has put some information

in the chat about where you can pick up

the recording of this on our YouTube channel.

It will be showing up there

in the next few days.

So we'll go ahead and get started

with what our main topic today is,

which is basically we're going to

talk about sort of three things,

and they're all pretty closely related.

We're going to talk about a national

data release of the NHD Plus High Res.

Which is going to happen real soon.

We're going to talk about

generalizing the NHD High Res.

And we're also going to talk about.

The problem that we have with the

C plus high res which is directly

inherited from the NHD itself,

and that is a lot of disconnected network.

So those are our three main topics

and I'm going to start here.

With the NHDPlus Hi-Res and

just to kind of reiterate,

this is the map hasn't changed a whole lot,

although we've added a couple of units.

I think in Alaska recently.

But we've had the CONUS the conterminous US.

We've had finished in the beta version

of NHDPlus High Res for a few

months now since late fall last year.

And we are going to be coming

out with a national data release,

which we're going to call national

release one of the NHDPlus HR.

In the in the next few weeks,

probably we are.

We've we've got a candidate data set

that we've had a few people review

and everything is looking pretty good.

We have a few issues to fix on it,

but we're we're getting really close

to being able to release that it's

going to be a national data set.

It will have some updated value added

attributes updated, VAAs, because

as we produced the data, especially,

especially if you think of it.

In terms of the Mississippi River basin,

you know we did like Region 10 and

seven way way back like 4-5 years

ago almost and then you know we

finished Region 8 just last fall.

And so.

There are four,

I think,

4 attributes in the value added

attributes that had that get

updated as we process the data

from upstream to downstream.

And then.

At the very end,

those get updated all the way back upstream,

so it's things like what's the.

The Terminal Path identifier of the last

flow line in the network,

which I'm going to show in a minute.

All of that and like the distance from.

What we call the path link the distance from.

The point of interest downstream

to the end of the network.

So things like that get

updated as we go through,

but we're not going back and re posting.

All of these,

I don't know how many there are

hundred close to 100 maybe.

VPU Level 4 digit HUC datasets for

all of the Mississippi Basin.

For example, we're not going back

to update all of that later,

but it is going to be all those updated

VAAs in this national release will be

releasing those soon, so I want to just.

Give you a little 

preview of that and of course,

as always, you know,

we invite people to review and

provide us feedback through the.

The markup application,

if you find issues in any of

this beta datasets in NHD plus

or in NHD or WBD for that matter.

So again just a reminder about.

The NHDPlus or I'm

sorry the markup application.

Anyone can go in and provide a

suggested edit to us and we will

get it in the queue to get fixed.

So I'm going to do just a very

brief overview of some of the value

added attributes that are changing

in this new national data set.

Just kind of give you kind of a preview

of what to expect in that, so just.

A reminder about the value added

attribute stream level stream level.

Here we we show we basically start

at the coastline of River network.

We work our way upstream.

A stream that flows into the

coast is a Level 1 stream.

We work our way upstream and each

time we come to a confluence,

we decide which one which of the

two paths coming together there is

the major path and we follow that

major path upstream and we call

that all the level one path right.

Then anything that's a tributary to

a level one path is is a level 2.

And then anything that's tributary

to a Level 2 path is a Level 3,

so we're we're showing some of those here.

So the level path.

That's that's what a level path is.

And then we have a level path.

We have two two attributes.

The Level path identifier is the

hydro sequence number of that.

The last flowline in the network.

So what I'm showing here is the

last full line in the Mississippi

River down at the crows foot,

or where where it goes

into the chicken's foot.

I don't forget what that's called

down at the Delta in where it

flows into the Gulf of Mexico.

The last line in that network.

And you know,

we get the hydro sequence number

from that and you can see if you

select the level path ID you're

going to get the Mississippi River

basin all the way to its head water.

Uhm? If, on the other hand,

you select the Terminal path

identifier having that same number,

the hydro sequence number

of this very last flowline,

you're going to get every flowline

the flows into the Mississippi River.

So here's what that looks like.

This is the level path identifier.

We did a query on that.

And this is that national data

set that I was just referring

to that national data release.

Is the preview version of it,

and this selection took like a

minute or or a minute and a half.

Maybe took quite a lot longer

than that to draw all the lines.

But it's.

It's pretty handy to have it

all in one big data set.

This is showing the terminal path ID again,

so it's every flowline or

highlighting every flow line.

Here that flows into the Mississippi

River and out through the birdsfoot

someone added that in the chat. Thank you.

Uh, it down into the Mississippi Delta so.

So that's just a preview really of

the kind of analysis that could be

done with this national data set.

The national data set is really big.

It's about 25 gigabytes if I recall

so it's a, you know it's a big download,

but it's really useful because you

can do this sort of analysis spanning.

The entire data set. Just in.

Quick reminder of what some of the

other value added attributes are.

Here's a list and linked to the web

page on value. Added attributes.

Lots more information there on

that web page about the VAA's.

So just wanted to.

Give you sort of a taste of

that national data set.

Next, what I've got is

I've got a a quick slide.

This is from Shelly Cawley at EPA Shelly's.

Not able to join us today,

but she did provide a slide with narration,

so if this is going to,

hopefully this is going to work right

and you'll be able to hear Shelly's voice

describing some of what some of the

rationale and why this generalization

effort for the NHD High Res is being.


So here is Shelly.

>Hi, my name is Shirley Cawley and

I work for the EPA office water in

a water data integration branch.

I'm going to set the stage for

this discussion today by giving

you a little bit of background

on EPA considerations.

EPA is a very active community of users

using the NHDPlus, the original NHDPlus

medium res or 1:100K scale product.

In fact it seems as if this

community of users is still growing.

At the national level, EPA Office of Water,

we do a lot of activities such

as tracking and assessing and

monitoring and modeling that are

very well suited to this resolution.

Data set. For national activities,

having a data set that is nationally

consistent having a network that's

traversable for upstream and

downstream search search is necessary.

We also have quite a few users within

the EPA community that have begun

moving towards the high resolution

product at the EPA regional level

and with our state and local.

Stakeholders there are many activities such

as permitting more recently Clean Water Act.

Jurisdictional determinations

have been much talked about.

Clean up efforts and Superfund and

criminal enforcement activities.

These are all things that are well

served by high resolution product.

So as we pause and take stock

and helped to best serve this

diverse community of users,

the final consideration that

really helped us with our path

forward has to do with the push,

especially within the EPA,

to adopt the principles of the

Internet of water and one of the

main pillars of the Internet.

Water is to use a common hydrography.

To adopt the common hydrography

would really be a motivation for

us to try to usher all of our

users into a shared data set.

All of these considerations taken

together really drove us to the

concept of generalization and the

methodology you're about to see.

Next up we're going to have Cindy McKay talk

about this generalization  concept

with the NHDPlus high res. > I'm going to give a few

abbreviated notes right here at the

beginning to complement what Shelly said.

EPA most states report EPA

using high res NHD.

And so for the past.

Oh many years,

EPA has spent a great deal of

time moving all of the things that

they have linked to NHDPlus V2,

which is medium res.

I'm sorry like all the things

that states send to NHDPlus

medium res so that they can do

their national applications and tracking.

During the past several years,

there's been a strongly advocated need

for updating NHDPlus V2.

Lots of people have updated it on their own.

These have not been particularly useful

to the larger NHDPlus community

they've been for specific applications.

But now that we have High Res


EPA decided now is the time that we

need to move there so that everybody

is on the same playing field.

In terms of linking information

and sharing information.

The high res NHD has some very strong

Aspects to it. First of all,

it has stewardship on.

I'm sure many of you on

this call are the stewards.

And the stewardship

comes from states and federal agencies

and also in any specific location.

It is generally the most detailed

scale of hydrography available.

And finally, the data tends to

stay current with on the ground

conditions through photo revision and

updates from Lidar and IfSAR sources.

But it has two major disadvantages,

which sort of stem from its advantages.

The number of features in high res.

NHD is 10 times the number of features

in medium res, and NHDPlus.

and High res

NHD and initially plus have extremely

varied densities across the lower 48 states.

Many places are 1:24K and a lot

of places are becoming 1:2K

or maybe even denser,

so the idea of generalizing the initially

NHDPlus started to take shape,

and that's what we're here to talk about.

I saw some I wasn't hearing what was

going on, but I saw some slides that

Al was showing about the NHD

Plus this is just a snapshot of what's in

initially plus and it doesn't matter whether

it's med res or high res. This is it.

You have the snapshots of the NHD.

The 3DEP elevation

watershed boundary data set.

You have all the wonderful

attributes that have been added.

Flow volume or velocity,

The USGS gauges and stream flow measurements

all sorts of value added attributes,

and you have the catchments,

which is really what NHDPlus is

all about because that links the

stream network to the landscape.

The objective of NHD

high res generalization.

The first objective was to enable users

of NHDPlus V2 medium res

to become users of NHD plus high res.

NHD high res as I said,

has nearly ten times the feature count,

so the stated requirement for

generalization was created less than

stream network to support models

and processes that have software

that has feature limitations and

or high performance requirements.

Here's a picture of the feature

volume difference.

On the left is NHD

plus version 2.

On the right is NHD plus high res.

For the same area you can see

the counts down in the bottom

we've 2.7 million features in.

In the medium RES network,

and we have 26 million.

And actually this slide

is probably out of date.

It's probably more than that

now in the high res NHD plus.

It's interesting to look at the lakes.

Uhm? There are about 10 times more lakes.

In I'm sorry 17 times more likes

in high res NHDPlus but if

you look at the Lake area it's 241,000

versus 442,000

which is only about double what's

happened in high res is many

many many very tiny lakes are now

in the NHDPlus and NHD.

The second objective for the generalization.

Was to take the best available

data that's in high res.

An address the extreme 

differences in stream density. Uhm?

And use the generalization process.

To do an NHD high res derivative

that was more consistent across the

country and hydrographically based

rather than cartographically based,

which is what it is now.

Here's an example.

This very, very dense area in the

southern side of this HUC4.

That that's from Lidar.

The rest of it was probably

mostly from 1 to 24,000,

but you can see there are a lot

of areas that have been densified,

and the there's an extreme

difference across this HUC4.

The third objective is to encourage users

to adopt A single hydrography data set.

As Shelly said, that will be the

standard reference layer for linking

all external information to the Surface

water network into water bodies,

and that was a stated objective of the

generalization to establish a derivative

from the standard reference layer.

That still links back to the

standard reference layer,

so if you adopt a generalized high-res

you haven't left high res,

you still have the identifiers that

you can go back and forth between

high res and the generalized version.

The main way of linking data to

the NHD and the NHD plus is through

the reach code and this is a picture

of where we stand on reach codes

when high res was built and she

high res was built the first time.

The medium res reach codes were

conflated onto the high res.

Line work and there was pretty close to

100% agreement between medium res reach

codes and their representation in high res.

On the left hand side you see medium res,

the red lines are the ones that have

reached codes that are still in high res.

The blue lines have reached codes that

are no longer in high res and that that

same color scheme is used on the right

for high resolution you can see that

that there are still a lot of lot of

flow lines that have the same reachcodes

but there's a whole lot

of blue lines that don't.

There is about when we did it.

Counted this probably a year ago.

There was a 70 to 75% correspondence between

medium and high res using reach codes.

So our generalization

technique is pretty simple.

It's three steps. The first is.

Select the desired stream

network that you want.

And we left this as as sort of a user.

Activity. And then we develop the rest

of generalization technique to take a

users selected stream network and turn it

into a generalized Hi-res NHDPlus.

You can select the network using

the high res visibility filter.

This is an attribute that's in

an NHD high res. It's very good,

but it does have some limitations.

You can use high res NHD

plus attributes like stream flow.

You can use points of interest.

You can take permitted discharges

and just keep the network that's

downstream of permitted dischargers.

Or you can just pick a random

area of interest network.

Out of the middle of NHD and that

can be your desired stream network.

Then we have a tool that will

prune the stream network.

Of all unwanted surface water segments.

In other words,

everything you didn't pick it will remove.

And then there's a generalization

tool that takes NHDPlus

high-res components.

And generalizes those to match

the prune network.

I have to say that in the generalized

version that we are currently creating,

we wanted to preserve the NHD plus Value added

network attributes as much as we could.

So we added a small number of pruned

flowlines once that have been

discarded back into the network.

The selected Network for LevelPaths

and for NamedPaths we wanted.

The whole Mississippi.

We wanted the whole level path for a stream.

Then we generalize the catchments.

We navigate downstream from a pruned

or remove flowline until we get to

the network that has been selected

and we dissolve all the catchments

that have been navigated into

that selected flowlines catchment.

We had just some of the NHD

plus for attributes.

Most of them survive the

generalization without adjustment.

But a few do not an we we adjust those.

One set of attributes we have

not adjusted yet,

which we could is stream order.

Stream order doesn't seem to make a

lot of sense in the generalized network,

but we're sort of waiting to hear

from end users to find out if

that's a true statement or not, and.

If they want stream order 

There is software to generate

it and we can do that.

The other thing that's in the

generalized network is a cross-walk.

Every single flow line that is removed.

Is cross-walked to the flow

line that receives its flow?

So everything as you navigate

downstream and you gather up all the

catchments and dissolve them into

the the flow line in the network.

You also crosswalk all those flow lines

that you've navigated to that flow

line that's in the selected network.

We use the left hand side of this to do

our generalization for the most part.

We used the visibility filter.

But there's nothing to say you

can't just pluck a partial watershed

out of the network and use the

NHD plus generalized catchments as

the upstream boundary conditions.

There was another discussion about

whether we wanted to combine

features and thin the coordinates.

We have opted not to do that.

Mainly because if you combine features,

you start losing the reach codes.

And you start losing F codes.

Of course, you could always limit

the combination on F code, but.

We just decided it was better to

keep all the features, then do that.

Thinning the coordinates was another thought.

That will change the measures along

the reaches so it will change

how things are linked.

So we have also left that

as a maybe someday to do.

If somebody really needs that

doesn't care about reaches.

Here is a HUC4

We picked a network dark blue lines

here just out of the middle of this.

HUC4 and then we generalize the rest

of the HUC4 and we end up with this.

So now you have this little

generalized network and you

have these big catchments here.

The drain into the upper end of this

network and they can provide all the

boundary conditions from upstream.

Here is a picture of the visibility filter.

I'm sure you've seen this

probably many times on this call.

This is, it has nine values there 8

listed here and also as a value of 0 where

visibility filter hasn't been assigned,

and that's generally lines that

have been added from Lidar, IfSAR

and are less than 1:24k scale.

Basically the visibility filter is

trying to tell you what lines are

added to the map at that map scale.

So here is 1:5,000,000,

1:2,000,000, 1:1,000,000.

500,000, 250,000, 100,000,

50,000 and 24,000.

so that's what the visibility

filter does for you.

And it's a very handy way to

do a first

Level of pruning on the network.

But we decided it was not necessarily the.

beyond end all of the prune network.

So the goals of the ongoing project

are design and develop first version of

the tools to generalize NHDPlus High res.

Then we want to build a user tool

that will create a generalized

initially plus data set from users

pruned NHDPlus high res flowlines.

We have the tool is not.

Ready for prime time in in the sense

that has very little documentation

and it hasn't really been turned

into something that you would want

to give to a user and hope that they

could understand it and use it.

We want to test the tools on users

pruned NHD data set so people want

to send this data sets will try the

pruning tool and send them back to prune


and let them see how it worked out.

And most importantly,

we want to create a national generalized

NHDPlus High Res that has the

network density and usability of the

medium resolution HD plus version 2.

But it still links back to

the High Res


Here is. The visibility

filter applied to 0602,

which is the middle of the Tennessee.

And you can see that it really,

you know, smooths out this huge

density area dense area over here.

Another dense areas.

We ended up using 1:250K

We tried a lot of HUC4s

across the country. And.

1:250K was a little too dense,

but 1:100K was way too dense,

so we stuck with 1:250K and

We have actually run.

The generalization process on the lower 48,

and if you look at the stream miles

that are in the generalized data set,

it is really,

really close to NHDPlus version 2.

Zooming in here,

so here's a closer look at what we did.

These are the flow lines.

When we prune the green lines went away

the blue ones remained.

Here is the cross-walk to show you that.

You know this helps you figure out

where approved Flowline ended up in

the the in the network relative to

its catchment area and its attributes.

Here's an example closer than

specifically of the crosswalk.

These little yellow lines.

Are in this table and you

can see that all of them.

Discharge to 26115,

which is this guy down here.

I hope you can see my cursor.

If the crosswalk table also tells you,

tells you how much Network had to

be traversed in order to get from

the flow line that was pruned to

the flow line that was selected.

Here's the catchments.

You can see each flow line has a catchment,

even the green flow lines,

but because they were removed we want

to get rid of there. Catchments.

And here's what what it looks like.

Once those catchments have been removed.

But you look at that just for a second.

OK, its easier to see when you

take away the green lines,

you can see that each flowline

still has a catchment,

but the catchments have gotten bigger.

And they represent all the

tributaries that were pruned out.

So the benefits of generalization.

These alternative versions of NHD & NHD

plus can trade information by all having

a crosswalk to the High Resolution NHD

and NHDPlus and thereby to each other.

We've enabled,

we hope we've enabled national,

regional and local modeling with less

dense networks and results of these

modeling efforts can be combined.

We created versions of NHD at desired

drainage densities that still have

the spatial precision and 

attribute richness of High Res NHDPlus.

And we provided a migration path for

NHD plus version 2 users. Shelly

mentioned that that community is still growing.

In fact, the other day we got an email going.

What happened to version one?

EPA had taken that off their website

and people still want it again.

So we provided this migration

path for NHDPlus V2 users

by supplying a similar Surface

water network based on high res.

And we've allowed,

we hope NHD plus to move forward.

As NHD high res, 3DEP

Elevation and WBD are updated.

Sticking with NHDPlus V2,

that was a very heavy lift to do that,

but now that all we have to do is run

an automated generalization process.

I think generalization can keep up with the.

Hi-res maintenance.

There are at. If you haven't heard

a lot of isolated networks and in

NHD High-Res and we would like to.

resolve those in NHD generalized

and also in NHD high-res and Rich

is going to has developed some.

Techniques and procedures.

And he's going to talk to you about those.

And so that's our next step in this work.

>Pete McCarthy is asking will events

and data that have been attached

to NHDPlus V2 be conflated

to the generalized NHDPlus HR?

We certainly hope so.

EPA is going to have to do that.

Well, I I'm saying that the EPA

events that are on NHD

plus V2 that EPA uses for their

national analysis and tracking will

get moved to the generalized data set.

There are a lot of things out there

that have been put on medium res.

By others, and.

We're hoping that folks will take the

initiative to move it to generalized.

Keep in mind,

we have we still have.

Are each code correspondence that

will probably do a lot of those events?

We have not figured out the percent

of correlation of reach codes.

Between medium res V2 and

Hi-res NHD but we will do that

that's been on my list here for

a few weeks and we'll figure out

whether it's still just 70 or 75%,

or whether it's gone up.

Because what we've illuminated

is is a lot of the new dense


>Great, OK, so we'll move on a little bit now.

And I want to just talk

about disconnected networks.

'cause Cindy talked she briefly.

Mentioned the idea of isolated networks and

disconnected networks an I want to just.

That's going to be the topic that

we're going to spend the rest

of the hour talking about here.

And you know, just want to point out

that there's there's quite a bit of this,

and I think most of you have seen

cases like this where we have

lines that just don't connect to

the network to the main network,

and this was, you know,

this is common in the NHD and it's.

Really kind of going back to the source,

which was the the USGS topo Maps.

Generally these were these were

cartographic product and generally if the

analysts couldn't see the cartographer,

couldn't see a channel in aerial photography,

they didn't draw it.

That was the that was sort of the rule,

but from a hydrologic standpoint,

particularly from a hydrologic

modeling standpoint,

we need that network to be connected.

We don't want it to be completely isolated.

As if it as if any water that falls on

the catchment for these lines just will

never make it to the stream network,

so we need some way to to handle that.

So how big a problem do we have?

It's a pretty big problem,

so using the NHDPlus HR value added

attributes and that national data set,

the national data release

that I showed earlier.

This is what we have in the data set.

250,000 of these.

Network ends these are places where the

network stops that are not at a coastline.

Now a lot of those whoops, sorry.

A lot of those.

Nearly half of them actually are

single isolated for the lines like

the ones that I just showed up,

and so we can remove those pretty

easily just by querying them out.

And in fact,

in NHD medium res those single isolated

flow lines were treated that way.

They were just ignored in NHD medium res,

and that's what we plan to do with the.

NHD High-res when we update we're going

to just ignore those single isolated

flow lines because there's so many of

them and they don't really contribute

that much to the whole picture.

So we're going to just ignore those. Uh.

So, but that still leaves us with 140,000

 you know,

isolated networks,

and we need some way to deal with those.

This is a histogram looking at

how much contributed you know,

total drainage area.

Those networks account for that.

You know that 140,000 + flow lines.

The vast majority are pretty small

in terms of their drainage area.

The cumulative draining area

being you know at the end of

that network that's isolated,

so you know the vast majority are under

2 kilometers under 2 square kilometres.

And then there's this long tail of

lots and lots of you know larger ones,

and if we go out to here and look

at everything that's larger than

50 square kilometres,

we do have a sizable number of those.

So what we've been doing is we've

been looking at larger drainage

area networks that are disconnected,

and we've been trying to figure

out how to connect those up.

But Rich is going to talk next.

About a process that we're

gonna try to use to identify

how we can connect up these really

large numbers of of isolated networks.

So Rich I'm going to stop sharing and.

Let you share your screen and take it.

It's good Rich and we can hear you fine.

so great. >>Yeah, what it is.

It's this tool that's been created.

It's called fill and spill and it

it starts with the downstream end

of each isolated network and

Tries to create a path from that point

down to where it will hit a hit a,

flowline, another flowline basically.

So it's the purpose is to connect up

these isolated networks downstream.

High high res flow lines and it

was the tool as it exists now,

was created by Justin Myers and it was

basically done following the work

of Curtis Price you created created

the prototype for it and it it's

written in Python And it uses Arc Pro.

You know? Tools to to do that.

Processing tools.

This and it's it's mainly a a

raster process that it does 'cause

it's working off of either the

hydro DEM or the elevation model.

The DEMS on what I've done

here is shown in black.

The locations of the existing

flow lines in the NHD.

An actually it's a.

It's a product that comes with the NHD

called the Surface Water Network.

And it's a raster data set and

what it what we're doing is

following the down the sink at the

downstream end of these networks and

Following it, following it down to the.

To the next flowline an.

In this case,

it's one of those isolated

networks that AL just spoke of.

Then that that will be not used

to the next version of the.

High res NHD.

It is is in the existing version,

the beta version.

Anyways it goes down.

It identifies the path

down to this flow line,

and then from the end of that isolated

network you can see this secondary path.

The second path going down

to the next network,

and so this geometry is is available

and this these green lines are

created from the existing.


DEM we call its ElevCM is the is the file name.

Your other examples,

you can see two flow lines

coming together and ending.

It creates a connection all the way

down to this next network account

to flow lines coming together and

the missing link is is put in there.

Uhm? As Al said, there are.

There are a huge number of of

flowlines they're disconnected.

This is the for CONUS. Alright.

It's it's a huge number.

And what?


What I've done here is shown how how

many disconnected networks there are

by hydro region in the first column.

And in the second column looking at.

At by HUC4s looking at the worst,

this is. Extreme cases in Hydro

Region 16 and it's the four digit.

1606 we picked that one is sort

of the worst case scenario test,

and I've also looked.

At looking further down the list, you can.

I picked 1401 in Colorado to be the sort of

one more typical of what you might find.

Average count is 1081 sinks or isolated

networks in that one particular.

HUC4s there's pretty close

to the same number, 1040.

So these are the two areas

that were selected for testing,

one at the one in Nevada is huge and

actually it looks a little bigger than

it is 'cause I 'cause it's got the

width of the circles of the selected circles,

the sinks at the end of

these isolated networks,

but the one in Nevada and the

one in Colorado.

We're done for testing and I'm just

going to talk about the Colorado one

because of the time constraint and and.

Nevada has a lot more complexity to it

that has that we are able to address as well.

But in this particular

HUC that was picked there,

there are five different 8 digit

HUC's in the program works more

efficiently by doing it by 8 digit HUC.

So it cycles through.

These these different HUCs and.

You can see this has account by a digit HUC.

This shows you. The.

The geographic distribution of

those those the sinks at the ends

of these isolated network networks,

all 1040.

This particular HUC is you're

you're familiar with Denver and

going up over Loveland Pass,

it's just over soon as you

pass the pasture in that HUC.

And so these are all the what we're trying

to connect up an running the program.

Justin Myers program I.

I've highlighted in this illustration

the fill and spill lines are

typically pretty small,

and I've.

But I made him a similar color as

the background so you can see it's

the ones that didn't come up with

a solution or highlighted in red.

So most of them it generates

a solution for 92% of them.

We get an answer for directly that way

using the elevation model itself.

This is just repeating what was said.

Zooming in. I'm looking to show

you a few examples of this.

Connections again to flow

lines coming together and.

Fill and Spill line going down

to the next isolated flowline

and then from there, down.

Come. Here's one again.

It's to flow lines coming together.

And then fill and Spill line

following the elevation model down

until it hits the other network.

So we have this geometry being generated.

You know the editors may want to.

Make it look cleaner,

but but at least we've got a basis to go by.

Additionally we.

Can use the hydro the hydro DEM

It's a hydrologically modified DEM and 

The program actually runs through

that as well automatically.

And we find 12 of those,

those missing ones.

We can get a solution for that we

didn't get before and that brings

it up to 93% that we have answers

for in terms of in the geometry.

After connect them up.


This is an example of one that was connected

up with the Hydro DEM and a number of.

These are where.

The NHD has it ending as the

the network ending in a pond.

And sometimes it has trouble.

Determining the the fill and spill

path in that case using the elevation,

but the hydro DEM

Works works fine to find a connection.

Another issue is that a bunch

of them the ones that are are not.

Don't we don't find solutions for

come really close and for some

reason the program doesn't work as.

In cases where it's often

very close sometimes,

but by simply in terms of just knowing

where they are and where they connect.

About 40 of these downstream

ends things are within 50

meters of a targeted flowline.

And so using that tabularly we can identify

what the next flowline downstream is,

really bringing the number of matches

up to 97% in this particular HUC. Come.

In which. So I should point out that

that we have results not only the the

physical line that's being created,

but we're creating a table that says.

Gives the down the flow line ID of the

downstream end of the isolated network

and also the ID of the flowline.

The NHD plus ID.

I call it target ID here of

the receiving flow line.

So this is this is another product

that's being produced and.

Uh. We're we're. You know,

gearing up to do this nationally?

These are, you know, again, how many.

Isolated networks were working with

per HUC were actually starting,

mainly because of the fact that.

Hydro Region One is the next one

in the in the production process.

But in terms of the NHD processing,

so we're actually starting

with the easiest one,

which is probably a good thing.

In the process is to

Create these.

We run a preprocess procedure to

which helps to avoid certain errors

and then we run the fill and spill

program which is already programmed

and then we do a post process to to

identify the target flow lines and

basically we're gearing up to to do

this starting with hydro Region 1.

And that's that's it for my slides.

>Uh, OK rich. Could you back up a slide?

The yeah this one I wanted to

mention on this that fill area,

fill volume and time to fill those

fields are also computed as part

of this process so you know the the

generally what you would see if.

Let's say that the end of the flow

line the sink is in a depression.

We wanted to compute to see you know how

much volume does that depression have,

and that's the fill volume.

An the fill area would be that just

the area of those cells that are

covered in that depression when

you fill it up and then make it in,

spill it over to where connects up.

And so we've got that information,

and we've also got the time to fill

based off of the mean annual flow.

So I believe this is in.

Days I can't remember.

I think it's in days yeah yeah so so

we've got more information about these

connections and we are there's a lot of us,

a lot of questions to

still answer things about.

Like you know how do we decide

which connections to make?

Which ones not to make?

Then you know we can make these

connections both in this tabular form

the way that is shown with this table.

Let's see. I guess it was the

previous slide or the next slide.

I can't remember the one that

shows the table connections, Rich.

>>Well, this is the table

connections the..>>Oh yeah yeah,

this is it so so we can make these

connections just in terms in a tabular form.

And we can do that,

sort of as an interim step.

And later on we can go in and add those.

The actual connections themselves , lots

to be answered about that whether,

like what kind of feature we should

make those connections, you know,

do we use connector feature?

Do we need a special type of feature code?

You know an F code for

this sort of connection?

Bunch of questions that we

still have to answer on this,

but I wanted to show everybody this so

that you kind of know what we're doing.

You may see if you're a steward you

may see some of these connections

showing up in the mark up.

Some of the initial ones that

are that we mentioned earlier.

The really large drainage areas

that need to be connected up.

Some of those have been showing

up in in markups,

and some of the stewards on the

call may have seen some of those.

So, uh.

Again, we this is all sort of evolving,

but we're trying to.

Develop ways to make this network much

better connected so that it can be

used for hydrologic modeling purposes,


We are a little bit over the end of the hour.

I will stick on around here for questions.

I'm going to see if I can answer

one question from Terra Mano about

populous areas in region one.

Yeah, I mean,

we know that a lot of times in built up

areas or where where we lose streams

they they end because they go into

a built up area and they couldn't

see the channel anymore on the.

In aerial photography,

maybe goes underground or something so.

Those are all kind of questions that

we're still trying to sort out, but.

I this process hopefully will

give us an idea of where those

things ought to connect to.

You know, at least based off of the DEM and.

Work. We're still there's.

There's a lot of questions to answer,

I think still.

Yeah, I think. I think that's

probably where we're 5 minutes over,

so we'll try to answer some of the

questions may be separately the

ones that show up here in the chat.

I want to thank Cindy and

Rich for presenting and.

Hope that this is been informative

for everybody and we will see

you all in a month. Be the 4th.

Tuesday of next month will

be our next advisory call,

so thanks for coming and we'll

see you next time.