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
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.
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.
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:
Area to Be Submerged
Lock Chamber (2-D)
Special Use Zone
Lock Chamber (1-D)
Sounding Datum Line
Special Use Zone Limit
Lock Chamber (0-D)
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.
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:
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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