National
Hydrography Dataset and Networks in ArcGIS
Prepared
by Victoria Samuels and David R. Maidment
Center
for Research in Water Resources
University
of Texas at Austin
September
2001
Contents
·
Computer and Data
Requirements
Part 1. The
National Hydrography Dataset
·
Obtaining
National Hydrography Dataset Data
·
Viewing
and Inspecting NHD Feature Classes
·
Building
a Geometric Network in ArcCatalog
·
Using the Utility
Network Analyst Toolbar in ArcMap
Goals of
the Exercise
This exercise has two parts. Part
1 introduces the user to map hydrography data depicting water features of
the landscape, and specifically hydrography data from the National Hydrography
Dataset. The user learns to symbolize and differentiate between the
feature and reach data layers. The attributes accompanying the hydrography
data are also described.. Part 2 uses the the network
capabilities in ArcGIS. The user:
·
Builds a Geometric Network of river and coastline features
·
Assigns Sinks and Calculates Flow Direction on the Network
·
Performs Traces traversing the Network upstream and
downstream
The study area selected for this
exercise is HUC #12040204, West Galveston Bay. This area is located on
the Southeast coast of Texas near Houston.
Computer
and Data Requirements
To carry out this exercise, you need to
have a computer which runs ArcInfo version of ArcGIS. The first
part of this exercise can be carried out using ArcView, but the construction of
a geometric network in the second part of this exercise cannot be done with
ArcView. In order to download the National Hydrography
Dataset data, you need internet access. However, the data are also
provided in the accompanying zip file, Networks.zip, . For UT
Austin students, the files are located on the LRC NT network in the directory class\maidment\giswr\networks\.
The data files used in the exercise
consist of ArcInfo coverages. All of the data being used is in the
Geographic projection, NAD 83 datum. The following files are required for
this exercise and are stored in the file http://www.ce.utexas.edu/prof/maidment/giswr2001ex4/networks.zip
·
12040204 zip file:
the NHD data downloaded from the USGS National Hydrography website, http://nhd.usgs.gov.
·
Sinks.shp, routeline.shp: shapefile
containing the network lines and sinks for the study area.
Obtaining
National Hydrography Dataset Data
The National Hydrography Dataset (NHD)
is an substantial set of digital data and contains information about the
surface water drainage network of the United States. The data consists of
naturally occurring and constructed bodies of water, natural and artificial
paths which water flows through, and related hydrographic entities. The
NHD is distributed by the United States Geological Survey (USGS) and is
available to the public for download. An optional part of this exercise
is manually downloading the data from the website, which is useful for future
reference. If you choose to not download your NHD data, skip
to the section headed Structure of the National Hydrography Dataset.
The NHD is available at the website http://nhd.usgs.gov. At this web site,
click on the Data tab on the left side of the screen and then click on
the first bullet, Obtaining NHD Data. The NHD is organized by
Hydrologic Cataloging Unit (HUC). You will see a map of the United States
in which you can zoom in and navigate to the HUC of interest. Another
option is using the FTP site to obtain the data. The data you will be
using is for HUC #12040204. The first method described is downloading the
NHD using the map. Skip the
next section if you get a “Server is busy” message.
Zoom in several times on Eastern Gulf
Coast of Texas near the Louisiana border (the divide between green and light
green HUCs). Eventually zoom in to where you can differentiate between
HUCs and their number.
When you can distinguish HUC #12040204
(the third green HUC to the left after the border between green and light green
HUCs), change the radial button at the top of the map to CU Download and
click on HUC #12040204. Fill out the NHD Download screen
information and Continue. Click Yes to the Security Warning
and click Download on the Download page. Navigate to the
location you want to place the 12040204 file, and click OK to the Successful
Download window. You should now have the 12040204.tgz file, a
compressed folder with the NHD data.
It can occur that you get the response:
If so, you can go to the Trouble
Downloading? link, and choose Alternative
Download Methods and go to Medium
Resolution Data. Scroll down the
list to the 12040204.tgz link, click on it, then Save this file to a
disk. Navigate to the directory you want to place the data.
Now you have the NHD data for HUC
#12040204. If you chose not to download the data from the website, the
zip file 12040204.tgz can be found in the Networks.zip file accompanying
this exercise.
Structure of the National Hydrography Dataset
Unzip the 12040204.tgz file
using the Windows utility Winzip. Extract the file to the 12040204
folder. Click Yes to the Winzip window asking if Winzip should
decompress 12040204.arc.tar to a temporary folder. In Windows Explorer,
navigate to the second 12040204 folder. Please note that using Winzip for
uncompressing the NHD files has some limitations not important to this
exercise. These limitations are important if you want to append or
join NHD files for several adjacent HUC units. In that event, use the
uncompression software provided on the NHD website.
The NHD is organized as three ARC/INFO
coverages, many related INFO tables, and text files containing metadata.
The nhd coverage contains the line and polygon features. This
coverage has line, polygon and node topology, which together is network
topology. The nhdpt coverage contains point features related to
the hydrography. The third coverage, nhdduu, contains metadata and
information about sources and updates of the hydrographical information.
The spatial elements of the surface water network are found in the nhd
and nhdpt coverages.
Viewing
and Inspecting NHD Feature Classes
Open a new empty map in ArcMap
and Add Data. Browse to the second 12040204 folder, then to the nhd
folder. Note the different elements in the nhd coverage.
Arcs, nodes, and polygons are the typical spatial elements originally in
ArcInfo. The NHD forms groups of arcs or polygons as single entities and
labels them as routes or regions, respectively. Add the region.rch,
region.wb, route.drain, and route.rch data layers.
Waterbody Features
Now turn off (remove the check in the
box) for all layers except region.wb. This theme contains the
areal hydrographic features representing waterbodies. They are organized
in "regions" (a group of polygons) because one waterbody may be
composed of many polygons.
The region.wb layer depicts waterbody
features. These can be of the following types (not all of which are
present in this particular HUC unit): Area of Complex Channels, 2-D
Canal/Ditch, Estuary (in the next release of NHD), Ice Mass, Lake/Pond,
Reservoir, Sea/Ocean, Swamp/Marsh, 2-D Stream/River, Playa and Wash.
To classify the feature types uniquely within the region.wb layer, doubleclick
on the nhd region.wb data layer name, and go to the Symbology tab.
To symbolize each type of waterbody uniquely, click on Categories and
highlight Unique values. From the Value Field dropdown
menu, select FTYPE. Then click on Add All Values.
This adds all the different values of Ftype present in this data layer to the
legend and symbolizes them differently. For each feature class, the Ftype
attribute describes what type of feature an element is. The Fcode
attribute is a coded value for that type.
Choose each type of waterbody to look
differently. To change all of the Ftype legends at once, change the color
ramp from the Color Scheme dropdown menu. You can change the
individual types by clicking on the color box next to the type name.
ArcMap has preset legend styles contained in the symbology options. These
preset styles store different industry's standards of representing certain
spatial data. These include waterbodies, transportation ways, signage,
etc. Double click on the color box of Swamp/Marsh. Scroll
down on the Symbol Selector window, and select Swamp.
To make the background color of the
Swamp filled in and easier to see on the map, click on Properties.
On the Picture Fill tab, change the background color to be a color as
opposed to white. Leave the foreground color as blue, or change it as you
would like. Click OK and click OK to close the Symbol
Selector window. Change any additional Waterbody types to look as you
would like. Click OK and close the Layer Properties
window.
Save your ArcMap project
using File/Save As in the Main ArcMap Menu bar.
Zoom in on various areas on the map to
see the boundaries between lakes, swamps, oceans and streams. In addition to the Ftype attribute, the
region.wb data layer contains other descriptive information. Right
Click on the nhd region.wb data layer and Open the Attribute Table.
The fields of interest for the region.wb data layer are:
·
FTYPE - the type of waterbody feature, in text form.
·
FCODE - a numeric value coding the type and values of the
characteristics of the waterbody feature. The first three digits describe
the feature type, the last two digits describe the characteristics associated
with that feature type.
·
ELEV - the elevation of the waterbody, in meters above the
vertical datum. In the initial release, most of the elevations are not
known, and therefore contain the value -9998 to indicate it is
unspecified. A value of -9999 indicates the elevation attribute is not
appropriate for this feature and is therefore not applicable.
·
STAGE - the height of the water surface which is the basis
for the elevation. The possible values of stage are: Average Water
Elevation, Date of Photography, High Water Elevation, Normal Pool, or Spillway
Elevation.
·
SQ_KM - the area of the feature in square kilometers.
·
GNIS_ID - the Geographic Names Information System (the
Federal Government primary source for identifying official names) eight-digit
identifier for the name of the entity.
·
NAME - the text waterbody name according to the Georgraphic
Names Information System.
Additional attributes are
identifiers. One identifier common to all of the NHD region and route
data layers is the COM_ID. This is a unique identifier given to each NHD
feature or reach. In this data layer it is called the WB_COM_ID.
The COM_ID, or common identifier is a 10 digit number which is distinct for
each feature within all of the NHD. It is used as a reference to relate
the various data layers as will be seen later in the exercise. An
additional attribute, RCH_COM_ID, will be discussed later as
well. Close the Attribute Table.
Click on the "Options"
bar at the bottom of the Attribute Table, and choose Select by Attribute:
in the resulting box select all the
Lake/Pond Features:
Make a note of the number of selected
features, and the Feature Code (FCODE) associated with Lake/Pond
features. Use Options "Clear Selection" to clear the
selected features. Repeat this query for each of the other four
feature types.
To be turned in:
A table showing the Feature Type, Feature Code, and number of features in the
region.wb feature class. What % of the total number of features are
contained in each Feature Type?
Drainage Network Features
Zoom out to the entire extent and turn
on (place a check in the box next to) the route.drain theme. This
layer encompasses the entire linear surface water drainage network. The
feature types which can be represented in this layer are: Stream/Rivers,
Canal/Ditches, Pipelines, Artificial Paths that run through the waterbodies
described earlier, and Connectors. As described above, these
linear features are grouped together as routes rather than simple lines because
several lines may comprise one route. Symbolize all values uniquely based
on Ftype, as you did in the previous section.
Observe the artificial paths that run
through the waterbodies of the region.wb layer. Zoom in on the lake in
the upper right corner of the basin.
Notice the artificial paths running
through the lake/pond and 2-D stream/river features. Coastlines bordering
the sea/ocean are also considered artificial paths. The artificial path
immediately changes to a stream/river when it exits the waterbody. These
artificial paths represent flow paths where there is realistically no actual
channel. Using these artificial paths aids in performing network tasks
which you do later in this exercise. Without the artificial paths and
connectors, the network would have breaks at waterbody features.
Also notice the large number of canals
and ditches compared with the number of natural streams in the area.
Because this area is so flat and near the coast, the natural streams are
insufficient to carry away storm water flow and a constructed drainage ditch
and canal system exists to supplement the natural streams.
In addition to the Ftype attribute, the
route.drain data layer also contains other descriptive information.
Rightclick on the nhd route.drain data layer and Open Attribute Table. The
fields of interest for the route.drain data layer are:
·
COM_ID - the unique common identifier for each
element.
·
FTYPE - the type of waterbody feature, in text form.
·
FCODE - the numeric value coding the type and values of the
characteristics of the waterbody feature.
·
METERS - the length of the feature in meters.
·
WB_COM_ID - the unique identifier of the waterbody from the region.wb
theme through which the artificial paths run. In the initial release of
the NHD, this field is populated with -9998 for applicable routes and -9999 for
routes that do not have a corresponding waterbody.
Again, the additional attribute of
RCH_COM_ID will be discussed later.
To be turned in:
A table showing the Feature Types, Feature Codes, and number of features for
the route.drain layer. What % of the total number of features are
contained in each Feature Type?
Reach Network Features
Now, zoom back out and turn on the route.rch
layer. This is the linear drainage network as well, broken up into
different pieces called reaches. A reach is a collection of
surface water features with similar hydrologic characteristics. Reaches
can be either pieces of stream/rivers, or portions of lake/ponds. There
are three types of reaches: transport, coastline, and waterbody.
The transport and coastline reaches are found in the route.rch data
layer, while the waterbody reaches are found in region.rch data layer which
will be studied next. A fourth type of reach, shoreline reach, has not be
developed yet. Reaches are used as tools to geocode information about a
linear or areal surface water feature because of their identifying
attributes.
Transport reaches represent the path of
water moving across the drainage network. Coastline reaches represent the
coastline of the Atlantic, Pacific, or Artic Ocean, the Great Lakes, the Gulf
of Mexico, or the Caribbean Sea. They are used to reference the location
of the ocean in respect to the drainage network. Coastline reaches are
only composed of artificial paths.
Now, let's look at how several features
in the route.drain layer can comprise one element in the route.rch
layer. Each reach element in route.rch is given a unique identifier,
called the Reach Code. The Reach Code is a 14 digit number with two
parts: the first eight digits are the Hydrologic Cataloging Unit code for
the Unit in which the reach is located, and the second six digits are a unique
number assigned to each reach arbitrarily. You will symbolize two of
these reaches uniquely to look at their composition. Double-click
on the route.rch layer, click on Categories and highlight Unique
values. Change the Value Field dropdown menu to RCH_CODE. Click
on Add Values... and click Yes to the warning about
exceeding 50 unique values. Highlight reaches 12040204000174 and 12040204000948
by holding down the Control key to select the second choice. Click
OK. Change these colors to something bright and noticeable.
Zoom in to the area containing both these reaches (the bottom left of the
screen).
Both of these reaches are composed of
multiple drain feature elements. To see the grouping of network elements
in one reach, go to the Selection menu and drag down to Set
Selectable Layers, check nhd route.drain and uncheck the other
layers. Using the Select Features button, click on one of the
reaches. Notice how only a portion of the reach becomes highlighted.
This is only one of the network elements that make up the one reach.
Reach 12040204000174 is made up of three distinct features and reach
12040204000948 is made up of 2 different feature elements. The drain
features composing each reach and that corresponding reach are linked together
through the RCH_COM_ID in the route.drain layer. In the
route.drain layer, the RCH_COM_ID is the COM_ID identifier of the reach which
the network element is part. Using the Identify tool, click on the
selected portion of the reach. Switch back and forth on the left side
menu in the Identify Results box between nhd route.rch (the name
of the reach) and route.drain (the type of feature). Note that the
COM_ID for Reach 12040201000174 is 1568586 and the RCH_COM_ID for all three
drain features that comprise it is also 1568586.
|
|
Another important attribute in route.rch
is Level. The Level attribute characterizes the stream level of
each reach. The level is determined by first identifying the endpoint or
sink of the surface water drainage network, and working backwards by the flow
relationships. The lowest level (one) is assigned to reaches which flow
into the endpoint and to upstream transport reaches which trace the main flow
of water back to the head of the stream. The level is then increased by
one for reaches which terminate at the main flow path, i.e. reaches which are
tributaries to the main flow path. This procedure continues to assign a
level attribute to all reaches. If a reach has a level of -9998, the
level for that reach is currently undefined. Either the reach is isolated
and not connected to the network, or the level is not yet determined.
Most of the canals in this area of of level -9998 since their complex nature
does not allow a flow direction to be defined. Additionally, the
coastline reaches are also -9998 level because they do not have a specified
flow direction.
Go to the Selection menu and Clear
Selected Features. Go to the Symbology tab of the nhd
route.rch Properties and change the Value Field to LEVEL.
Add all the values and choose a Color Scheme that displays the levels
clearly and vibrantly. Zoom to Full Extent (the globe button on
the Tools toolbar) and then again zoom into the area by the lake.
Notice all four level values as well as the level value -9998. These
Levels are not the same classification scheme for rivers that we examined in
Exercise 4, where level 1 was most upstream, and the numbers increased going
downstream. In the NHD classification scheme the numbers start low near
the coast and increase going upstream.
Go to the Selection Menu and go
to Set Selectable Layers. Change the selectable layer from nhd
route.drain to nhd route.rch. Using the Select Features
tool on the Tools toolbar, select the lines in the lake. Notice
how all the line features within the lake are selected as a single reach.
Because all these internal drain elements contain the same hydrologic
characteristics, they are considered one reach. Select a few of the
upstream level one segments flowing into the lake. Open the attribute
table of nhd route.rch and click Selected for Show ... Records.
Each of the selected streams has a
level of one, which is the main flow path from the bordering bay. The nhd
route.rch data layer also has the attributes of GNIS_ID and NAME
as in the region.wb data layer. All of these reaches make up a part of
Clear Creek, which is referenced by the GNIS as 01332928. An additional
attribute of note of the route.rch layer is the RCH_DATE. This the
date that the Reach Code (RCH_CODE) was first assigned. The additional
attributes will be discussed later. Clear the selected
features.
To be Turned In:
What is the total number of route.drain features? What is the
total number of route.rch reaches? What is the ratio of the number
of drainage network features to the number of drainage network reaches?
Waterbody Reach Features
The final layer you will look at is the
waterbody reach data layer. Turn on the nhd region.rch layer, and
highlight its name. Click and drag the data layer to above the region.wb
data layer. These polygons are regions which represent waterbody
reaches. These regions are composed of one or more regions found in
region.wb. Just as transport and coastline reaches allow for information
to be linked to the linear network features, waterbody reaches allow for
information to be attached to areal features. In this first release of
the NHD, waterbody reaches are only defined for lake/pond features in
region.wb. For these lake/pond areas, it is possible for both a transport
and waterbody reach to be defined; the transport reach represents the
artificial path of flow through the lake while the waterbody reach describes
the area.
Go to the Symbology tab of the Properties
of region.rch and symbolize the data layer by RCH_CODE. Click on the
minus sign next to the region.rch layer name in the Display Table of Contents
to shorten the legend list. The attributes of nhd region.rch are:
·
COM_ID - the unique common identifier for each element.
·
RCH_CODE - the 14-digit code which identifies each reach.
·
RCH_DATE - the date the Reach Code was assigned.
·
SQ_KM - the area of the waterbody reach region in square
kilometers.
·
GNIS_ID - the GNIS identifier for the waterbody, if
appropriate.
·
NAME - the GNIS name of the waterbody, if appropriate.
It is possible to view those
waterbodies that have names assigned by the Geographic Naming Information
System. Right-click on the region.rch layer name. Go Properties,
then go to the Labels tab. Change the dropdown Label Field:
menu to NAME. Make sure the "Label Features" box is
checked. Click OK.
Right-click again on the region.rch
data layer. Go to the Label Features option. Now all the waterbody
reach regions that have names assigned by the Geographic Naming Information
System are shown. Zoom in on Carancahua Lake and Cedar Lake. You can
change the appearance of the labels from the Labels tab of the Properties,
and make the text larger and darker. The relationship between the
route.drain and route.rch data layers also exist between the region.wb and
region.rch data layers through the RCH_COM_ID. Check the different
attributes of COM_ID and RCH_COM_ID using the Identify tool for Caranchahua and
Cedar Lakes.
To be Turned In:
What is the total number of region.wb features? What is the total
number of region.rch reaches? What is the ratio of the number of
water body features to the number of water body reaches?
What is the Reach Code of Cedar Lake?
Other National Hydrography Dataset Layers
In addition to the layers described
above, the National Hydrography Dataset contains hydrographic features which do
not necessarily play a role in the network. These features are known as Landmarks
and are found in the NHD as route.lm and region.lm in the nhd coverage folder
for lines and areas and as the nhdpt coverage for points.
The attributes of these landmark layers
are shown in the table below. The attributes are all found in other data
layers and their description can be found earlier in this exercise.
Region.lm |
Route.lm |
Nhdpt |
COM_ID FTYPE FCODE ELEV STAGE SQ_KM GNIS_ID NAME |
COM_ID FTYPE FCODE METERS GNIS_ID NAME |
COM_ID FTYPE FCODE GNIS_ID NAME |
For the FTYPE for each data layer, the
options are diverse and encompass many types of hydrologic landmark
feature. The types of feature for each data layer are listed below:
Region.lm |
Route.lm |
Nhdpt |
Area to Be Submerged Bay/Inlet Bridge (2-D) Dam/Weir (2-D) Foreshore Hazard Zone Inundation Area Lock Chamber (2-D) Special Use Zone Submerged Stream Spillway Rapids (2-D) |
Bridge (1-D) Dam/Weir (1-D) Gate (1-D) Lock Chamber (1-D) Nonearthen Shore Rapids (1-D) Reef Sounding Datum Line Special Use Zone Limit Tunnel Wall Waterfall (1-D) |
Fumarole Gaging Station Gate (0-D) Geyser Lock Chamber (0-D) Mudpot Rapids (0-D) Rock Spring/Seep Waterfall (0-D) Well |
Different features may be represented
in multiple dimensions, such as Rapids, which can be present in all three data
layers. Many of these types of features have multiple subtypes which is
characterized in the FCODE attribute. The first three digits of the FCODE
indicate the feature type and the last two digits describe the distinct
characteristics. To look at the different types of characteristics
associated with each FTYPE and to determine the meaning of the FCODE, you will
load a table that comes with the NHD data. Click on the Add Data button
and navigate up to the root 12040204 folder and add nhd.fcode.
The Table of Contents should have
switched to the Source tab with the table nhd.fcode at the
bottom. Right-click on nhd.fcode and go to Open. The table
lists the various FCODE values, the FTYPE text that accompanies that FCODE, and
the description of the FCODE which highlights the differences between
them. Notice how many different subtypes of each one feature type that
exists.
There are no linear or areal landmark
features for the area studied in this exercise. However, there are
landmark points. Add the data layer nhdpt from the folder
12040204. Right-click on the nhdpt point data layer and Zoom to
Layer. Symbolize the nhdpt layer uniquely based on FCODE. Look
up what each FCODE means in the nhd.fcode table to determine what each type of
point is.
To be Turned in:
There are three types of NHD point features in this cataloging unit. Use
the feature code to determine what kind of feature each of these is. How
many of each of these three feature types are in this cataloging unit?
You have now thoroughly inspected the
feature data from the National Hydrography Dataset. Save this map and exit
ArcMap. Additional information about the NHD can be found at http://nhd.usgs.gov/techref.html in
the documents:
·
NHDinARC Quickstart
·
Concepts and Contents
·
Introducing the NHDinARC
You will now use the route.rch data
layer to perform network analysis functions available in ArcGIS 8.0.
Building
a Geometric Network in ArcCatalog
Within the new functionality of ArcGIS,
the ability to trace along networks based on flow directions and relationships
is central. In order to use these functions, a geometric network must be
built. Open ArcCatalog and navigate to the 12040204 folder.
Right-click on the 12040204 folder in
the Table of Contents on the left. Drag down to New, then drag
over to Personal Geodatabase.
Name the geodatabase GalvestonBay.
Right-click on GalvestonBay.mdb and drag down to New and then to Feature
Dataset....
Enter the Name: for the Feature
Dataset as West_Galveston_Bay. Click Edit.. for the Spatial
Reference and click on Import... Navigate to the 12040204
folder/nhd/route.rch. This sets the extent for the feature dataset to the
same as the extent for the route.rch data layer. Click OK.
At this point, you want to enter data
layers directly into the feature dataset West_Galveston_Bay. Right-click
on the feature dataset West_Galveston_Bay and drag down to Import and Shapefile
to Geodatabase. For the Input shapefile:, browse to the routeline.shp
data layer. Click OK. The software then converts the
shapefile into data within the feature dataset.
Similarly, add the Sinks.shp
shapefile from the Networks.zip file to the West_Galveston_Bay feature
dataset by importing from Shapefile to Geodatabase in
ArcCatalog. Ok, now for the cool part, we are going to create a
geometric network!!!
From the West_Galveston_Bay feature
dataset, right-click and drag down to New and to Geometric Network...
This launches a Build Geometric Network
Wizard to help you create a network from a feature class in a feature
dataset. Click Next.
Because you have the route.rch
data layer as the arcs which you want to use as your network and Sinks
to determine flow direction, you want to Build a geometric network from
existing features. Make sure this radial button is highlighted and
click Next.
The next screen determines which
existing features should be incorporated in the network. Because you have
entered two feature classes into the feature dataset, they are listed as
defaults. Check both nhd_route_rch and Sinks and enter the
name West_Galveston_Bay_Net. Click Next. Click OK
to the ArcCatalog Warning about the "Enabled" field.
For Do you want to preserve existing
enabled values?, say yes. Enabled
edges and junctions permit flow.
For Do you want complex edges in
your network?, the nature of the data model that we are working with only
supports relationships with simple edges. Therefore, make sure the radial
button for No is highlighted and click Next.
The sinks accompanying the network were
selected directly from junctions on the nhd_route_rch data layer; therefore
they fall exactly on the network and do not need to be snapped. Highlight
the radial button No for Do your features need to be snapped? and
click Next.
The file Sinks contains the
sinks for this coastal area of West Galveston Bay. Click Yes for
the network to contain sinks and make sure that Sinks is checked. Click Next.
If there is a Warning, click OK.
The network you are working with has no
weights assigned to it, so make sure the radial button has No
highlighted and click Next. A summary is then displayed of the
input information. Check over this information to ensure it is correct,
and click Finish. The elements of the feature classes are then
converted to have network topology. If you get an error message saying
that some features cannot be built into a network, don’t worry about this, just
continue on. Voila!!
Congratulations. You have just created a geometric network! What
this means is that lines are transformed to network edges, edges
meet at points called junctions, and the connectivity of lines and
junctions is defined by an internal data structure called a logical model
(as distinct from the geometric model of the lines and edges which
defines where they are located in geographic space). Sinks are
special kinds of junctions where flow terminates or drains out of the
network.
In ArcCatalog, look in the feature
dataset West_Galveston_Bay. A new icon and new feature classes are
added, the West_Galveston_Bay_Net network and its accompanying West_Galveston_Bay_Net_Junctions
which are automatically created when the feature classes are converted to have
topology. Close ArcCatalog. Now you want to look at the network
within ArcMap to use the network functionality.
Lets Preview the Network that you've
just created. Click on the Network Icon and select Preview in the
adjacent right window tabs. Click individually on the 4 feature classes
nhd_route_rch, sinks, West_Galveston_Bay_Net, and West_Galveston_Bay_Net_Junctions
to see the individual elements which make up the network. Pretty
cool! Now we're going to assign flow direction on the network edges
and do some trace tasks on the network.
Using the
Utility Network Analyst Toolbar in ArcMap
Open the ArcMap project you worked on
earlier. In the ArcMap View window, switch from the Data View to
the Layout View.
From the Insert Menu, choose Data
Frame. This is somewhat like creating a new View in ArcView 3. You'll
see a dashed box appear on your layout window.
Use the View menu to switch back
to the Data View, as before.
Add to the new data frame, the
geometric network, West_Galveston_Bay_Net, that you
created.
The data layers Sinks, and
West_Galveston_Bay_Net_Junctions are added to the nhd_route_rch
feature dataset you already have in the display.
To generate the flow direction along
the network, the sinks must have an Ancillary Role value of 2,
indicating a sink. The other available Ancillary Role values are 0 for
None and 1 for Source. You will now assign the Ancillary Role value of
Sink to the points in the Sinks feature class. Add the Editor
toolbar by going to the View menu, Toolbars, and make sure there
is a check next to Editor. On the Editor Toolbar, click on Editor
and drag to Start Editing. Select GalvestonBay.mdb as the
dataset to edit. Set the Edit Target box to Sinks.
Right-click on Sinks and Open Attribute Table. Notice that
the points currently have a AncillaryRole of 0. Right click
on the Ancillary role field and use the Calculate Values to set
AncillaryRole = 2.
Close the attribute table and click
again on Editor. Drag to Save Edits. Now the flow
direction for the network can be determined. From the View menu,
go to Toolbars and check the Utility Network Analyst
toolbar.
Between Flow and Analysis
is an icon with sets flow direction. Click this icon, then click on the Flow
dropdown menu and go to Display Arrows. The arrows may not all
appear and only some dark circles may appear. The software is still not
foolproof when displaying the arrows. If you have a problem, try turning
on off display arrows, and that may work. If not, the picture below shows
how the arrows should appear.
Each reach in the network has an arrow
or a circular "blob" on it to indicate the flow direction along that
reach. A circular "blob" indicates that the flow direction is
indeterminate along that reach. This is caused by looping in the network,
in which the path that the water would flow cannot be chosen. Zoom into
an area to see the arrows and how the flow progresses down the network.
The area studied in this exercise has many indeterminate flow paths because it
is along the coast. Coastal areas generally have more looping in the
network, leading to undetermined flow direction.
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Because it takes longer for the screen
to redraw with the arrows, turn them off by going back to the Flow dropdown
menu and click again on Display Arrows. Now that the flow
direction has been determined, it is possible to perform traces on the network
to determine topology relationships between reaches. The Trace Task
menu lists the options that are available:
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We first want to set a few general
parameters before tracing. Click on the Analysis dropdown menu and
go to Options... On the General tab, click Directed
trace tasks include edges with indeterminate and uninitialized flow.
This means that the tracing tasks will go through those reaches with
indeterminate flow direction and keep tracing instead of stopping when the trace
meets a reach with undefined flow. With this option, tracing along loops
and the coastline is possible. Click Apply. On the Results
tab, change the Results format to Selection. Now the trace
results will be shown immediately as a selection which can then be exported to
a feature class if desired. Click OK.
Tracing tasks are defined by first
placing flags and barriers along the network to indicate where the trace should
start and stop respectively. The icon after the Analysis menu places the
flags and barriers. There are four options:
Junction flags and barriers are placed
at junctions, while edge flags and barriers are placed along edges. When
using an edge flag or barrier, the trace either begins or ends and includes
that entire edge, as opposed to only a portion of the edge.
The first trace you will perform is
Tracing Upstream. Place a Junction Flag at the sink with ObjectID
(FID) 74, at the upper right side of the area, the 5th sink in. Use
the Identify tool to find Sink 74, or select it using the Attribute Table of
the Sinks feature class and then use Selection/Zoom to Selected Features to
find this sink). Place Edge Barriers on the reaches on either side
of the Junction Flag. Be sure to place the edge barriers a significant
distance from the junction. The purpose of these barriers is to stop flow
along these reaches because we do not want them selected. Because they
have indeterminate flow and we turned on the option of tracing along undefined
flow paths, a trace would include these reaches; however, the barriers should
stop this.
To perform the trace function, you must
set the Trace Task from the drop down menu as Trace Upstream and
then click the Solve icon next to the menu. The solution appears
as a selection. From the Selection menu, drag down to Zoom to
Selected Features to see the reaches upstream from Sink 74.
To be turned in:
A map of the features selected by the Trace Upstream task. How many
features are selected by this trace?
You can see that most of the reaches were
selected as the solution to the trace upstream. This does not appear
correct; however, it is for the task defined. Again, because we chose to
trace along reaches with undefined flow direction, the vast looping in the area
wound up connecting most of the reaches to the junction in question. We
can see the loops in the area by using the Find Loops trace tasks.
We will take off the barriers from the network to find all the loops.
From the Analysis menu, go to Clear Barriers. From the Selection
menu, chose to Clear Selected Features. Now change the trace
task to Find Loops and click the Solve icon. This selection
shows the loops from that one junction. Now extrapolate this results to
the whole study area, and you can realize how many of the reaches are connected
by some sort of loop. This explains the Trace Upstream results.
A trace that will yield cleaner results
is the Find Path task. You will find the path between Sink 74 and
the apparent headwater junction, at the top left of the area (see the below
picture for the location). Clear the selected features. Place
another Junction Flag at headwater junction. Change the Trace Task
to Find Path and click the Solve icon.
To be turned in:
A map of the features selected by the Trace Downstream task. How many
features are selected by this trace?
The final trace task you will perform
will be a Trace Downstream from that apparent headwater junction. Clear
the flags and the selected features. Place a new flag at headwater
junction. You will also utilize the other method of returning results in
tracing. From the Analysis dropdown menu, go to Options..
and the Results tab. Change the radial button for Results
format to Drawings. By returning the results as a drawing, you
cannot use the results for anything other than viewing. With the results
as a selection, it is possible to convert that selection to a new feature
class, or look at the selected features in the attribute table. Change
the Trace Task to Trace Downstream and click the Solve
icon. The results are similar to the Trace Upstream results because of
the looping.
To clear the results as a drawing, go
to the Analysis menu and go to Clear Results. Save your
ArcMap map. Be creative and try other traces from other junctions.
Experiment using barriers on both edges and junctions. When you are
finished, close the Edit Session by going to the Editor menu and Stop
Editing. Say Yes to saving the edits, and save and close the
map document.
You have now finished this
exercise. Congratulations! Because this network is near the coast
it is particularly complicated. Drainage networks for inland regions are
typically more dendritic and simpler to navigate than this one.
(1) A table showing the Feature Type,
Feature Code, and number of features in the region.wb feature
class. What % of the total number of features are contained in each
Feature Type?
(2) A table showing the Feature
Types, Feature Codes, and number of features for the route.drain layer.
What % of the total number of features are contained in each Feature Type?
(3) What is the total number of route.drain
features? What is the total number of route.rch reaches?
What is the ratio of the number of drainage network features to the number of
drainage network reaches?
(4) What is the total number of region.wb
features? What is the total number of region.rch reaches?
What is the ratio of the number of water body features to the number of water
body reaches? What is the Reach Code of Cedar Lake?
(5) There are three types of NHD
point features in this cataloging unit. Use the feature code to determine
what kind of feature each of these is. How many of each of these three
feature types are in this cataloging unit?
(6) A map of the features
selected by the Trace Upstream task. How many features are selected by
this trace?
(7) A map of the features
selected by the Trace Downstream task. How many features are selected by
this trace?
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