USGS Multimedia Gallery: (Video)--Navigating the NHD Network

Good day everyone. My name is Bill Smith. I am

currently the Regional Point of Contact (POC) for

Region 2 which includes, Louisiana, Texas, New Mexico,

Arizona, Oklahoma, Colorado, and Utah. Today I would

like to talk about the network in the National

Hydro Dataset. We will discuss the Utility Network

Analyst tool, and finally we will discuss the NHD

Flow Table which may be ordered with any NHD dataset

you wish. First some basic concepts concerning the

flow network in the NHD. Only NHD Flowline feature

class feature will participate in any NHD network.

Other NHD feature class features, NHD Points, NHD

Lines, NHD Areas, NHD Waterbodies, or any of the event

features will not participate in the NHD network.

Flow within the NHD is based on the Hydro Net feature

node-to-node or junction-to-junction navigation

system. Connectivity is key. For NHD Flowline features

to participate in the network they must be connected.

There can be no gaps, no breaks or missing features.

Connectivity is the key. NHD Flowline features should

all have the correct flow direction. There is an

attribute for each NHD Flowline feature which may be

set to “WithDigitized” or “Uninitialized”. All

features attributed as “WithDigitized” will

participate in the network. If a user is unsure of

the flow direction or other information concerning

any NHD Flowline feature, those features may be

attributed with “Uninitialized” and they will not

participate in the network. Flow direction in the NHD

is initially determined by the direction of vector

collection. In other words, flow direction is from the

start junction to the end junction. With that said,

any NHD Flowline feature may be flipped to correct the

flow direction. In this example I have opened up an

ArcMap session and an ArcCatalog session and I am

showing the Hydro Net Junction feature class. Again,

the Hydro Net Junction feature class must be present

for the flow network to work within the NHD. There are

many issues that affect features from participating in

the flow. Incorrect flow direction on any NHD Flowline

feature, gaps or breaks between NHD Flowline features,

missing NHD Flowline features that create a gap or a

break, isolated networks, and finally the

“WithDigitized” versus “Uninitialized” attribute. In

this example I am showing incorrect flow direction and

how that might affect the network. You will notice

most features flow from West to East in this example,

except for the features highlighted in yellow. I

have purposefully flipped that arc incorrectly for

this example. What would happen if I was to attempt

to trace downstream on any feature to the West of the

yellow highlighted Flowline feature, flow would move

downstream until it hit the incorrect direction and

then it would not move any further. That would prevent

this small group of features from participating in the

larger network. In this example I am showing a gap

between the Stream/River to the West of the large

lake. This gap between the Stream/River and the

Artificial Path going through that Lake/Pond would

prevent the network. To the North of the large lake

you will see a missing NHD Flowline feature between

the lake and the Stream/River. Of course flow may not

flow through a feature that is missing. Here we have

an example of isolated networks. These networks are

all over the NHD and are not correct to be there. If

we were 100% certain that this small isolated network

should connect to the larger network, we have the

ability to add a connector that would allow the

isolated network to connect to the larger network.

Here we have an example of “Uninitialized” versus

“WithDigitized”. You will notice many features as

attributed with “Uninitialized”; they will NOT

participate in the network. You will also notice many

features attributed with “WithDigitized”; they WILL.

The Utility Network Analyst tool was developed by ESRI

and is included in every copy of ArcGIS. To open the

Utility Network Analyst tool there are various

methods. In this example I am highlighting the “View”

button found in the main menu area. I am then

selecting “Tools” from the dropdown menu and then

placing a checkmark to the left of the Utility Analyst

tool. I may also simply hover my cursor over the gray

portion of the main menu area, right-mouse click which

drops down the dropdown menu, and again I am placing a

checkmark next to the left of the Utility Network

Analyst. Both methods should display the Utility

Network Analyst tool. I am highlighting the Hydro Net

Junction feature class that must be present for the

Network Analyst tool to operate with the NHD. Going

through the Utility Network Analyst tool, I am

dropping the dropdown menu from the flow button. You

will notice my first option is to display arrows for

the Flowline feature class. If I select the second

display arrows button, it will place an arrow,

indicating directional flow on all features

participating in the network. Finally, if I select the

properties button it will open the flow display

properties window which allows a user to set symbology

for determinant flow, indeterminate flow, or

uninitialized flow. We will now look at the flag

barrier tool. You will notice there is an add junction

flag and an add edge flag tool. The difference between

the junction flag and edge flag would be the location

of the point where the action will start. To use the

junction flag I must place the point at the junction

of two NHD Flowline features. I personally prefer to

use the edge flag simply because I do not have to zoom

as far and I can place that edge flag tool at any

point on an NHD Flowline. You will also notice the

junction barrier and the edge barrier tools. These

tools will prevent from going through any feature

that they are placed upon. In this example I have set

an edge flag on an NHD Flowline feature. I have set my

“Trace Path” to trace upstream and I have selected the

solve button. All features that participate in the

network upstream from my edge flag placement will now

be highlighted in red. I have cleared my results. I

have left my “Trace Task” to trace upstream and I have

set a barrier tool just below the dam on the large

lake. Once I hit the solve button you will notice that

the network flows up to the barrier tool flows up to

the barrier tool and does not flow through that

feature. This is very handy for modeling flow, real

world flow, through the NHD. Moving on to the Analyst

button, I have several options. I can disable various

layers: the NHD Flowline or the NHD Hydro Net Junction

feature classes. I can clear any flags that I have

set, clear my barriers, clear my results. I also have

the options button which allows me to select various

options; I can set weights. I would like to highlight

the drawing versus selection option in the window on

the lower right. In the example on the left you will

notice I have left the drawing button highlighted. I

have set two edge flags on a particular set of

Flowline features. I have set my “Trace Task” to “Find

Path”. When I hit the solve button it finds all

features between the two edge flags. On the right side

you will notice I have changed “Drawing” to

“Selection”. As opposed to drawing a red bold line

indicating flow, it now selects the features that are

participating in this particular query. In this case

I have opened the Flowline table, I have placed my

cursor over the “LengthKM” column in the Flowline

table, right-mouse clicked and I have selected

“Statistics” in the dropdown menu. This allows me to

determine the total length between the two edge flag

tools that I have placed. In this example, 82

kilometers. Again, very helpful in modeling with the

NHD. There are various trace tasks available for a

user in the Utility Network Analyst tool. I usually

use the trace upstream, trace downstream, or find

path. And finally the solve button; very critical to

complete any query I have set up. I would like to

point that working with the ESRI Utility Network

Analyst tool is completely user interactive and can

not be programmed. It is user interactive only. Here

is a screen-print of the NHD Flow Table which may be

ordered with any NHD order you place. The NHD Flow

Table is created for users to program outside of a

Geographic Information System. It represents the same

connectivity as the Hydro Net Junction but may be used

outside of a GIS. To order the NHD Flow Table when

placing an order, I simply have to place a checkmark

to the right of the include flow. If I am curious what

is the Flow Table, I may select the “What is flow”

button to the right. Here is an example of the Flow

Table able showing the “FromComID” to the “ToComID”.

Again, the Flow Table is very useful to analyse NHD

flow outside of a GIS. That concludes this portion

discussing NHD flow. If anyone has any questions, feel

free to contact me, Bill Smith at(303) 202-4493.

Thank you.