User:Jcreer/SMS:Extract Features Tool

The Extract Features tool allows using a raster to generate features arcs on a map coverage. Currently, the extract feature feature arcs can be centerlines of channels, centerlines of ridges, cross sections of channels, and banklines of channels.

Preprocessor

The extract features tool requires the use of a preprocessor: TOPAZ or TauDEM. These pre-processors will analyze a raster and determine flow directions and flow accumulation. This data is used to determine the location of channels and ridges. The preprocessor writes the data to files used for the run are kept kept for extracting features.

If there is a change to the extraction setup, the preprocessor may need to be run again. These changes include changing the preprocessor, changing the feature type between embankment or channel centerline, or changing depression points.

The user may run the preprocessors using the right-click menu on the rasters in the " GIS Data" folder of the Project Explorer o from the Extract Features dialog. If the preprocessors are launched from a raster in the Project Explorer, it will create the files necessary for embankment and channel centerlines. If it is launched in the Extract Features dialog, it will only generate the files needed for the current configuration.

The preprocessor will run in the background and allow the user to work elsewhere until it is finished.

Extracting All Centerlines

You can extract all of the centerlines from a raster depending on a threshold area. This means if you extract all stream centerlines with a threshold area of 900 square feet, The Extract Features tool will determine when the flow from 900 square feet of raster would combine and start drawing an arc from that point downstream until it ‘flows’ off the raster. The principle is if you need longer and more centerlines, you make the threshold number smaller and if you need them shorter and fewer, you make the threshold number larger.

You can specify depression points, which is a location where centerlines may terminate (instead of at the edge of the raster). For example, this may be useful if the feature you are extracting is an embankment ridge, so you can specify the end of the embankment as a depression to have the arc ‘flow’ to the end of the embankment ridge.

You can extract all features through a right-click menu on the rasters in the GIS Data folder of the Project Tree or by creating points on a coverage and using their right-click menu. The points that are created will be used as depression points.

Figure 1: All Channel Centerlines Extracted

Figure 2: All Ridge/Embankment Centerlines Extracted

Extracting specific centerlines

The Extract Features tool will extract centerlines from specific starting points. The user creates a coverage with points where he or she would like the centerlines to begin, selects them, and uses the ‘Extract Centerline…’ right-click menu.

When extracting a single centerline, the Extract Features tool finds the closest high accumulation value to the starting point.

Figure 3: Centerline Extracted from Starting Point (in red)

Extracting Channel Bank Features and Cross-Sections

The user needs a Centerline arc and point on the bank feature that he or she wishes to extract. The multi-select tool is needed to select both items and use the ‘Extract Banks…’ right-click menu.

Tip: The user can easily select the arc by holding the ctrl key while dragging across the arc. The Extract Features tool will generate cross-sections that follow the centerline and trace the banks by matching the depth between the selected point and centerline along the cross-sections. See the images below showing how the bank location is defined, how the depth at the bank location is used, and the extracted banks at this depth. If the centerline arc does not have vertices normally distributed close to the raster’s resolution, it will redistribute vertices along the selected arc.

Figure 4: Centerline with point defining bank location

Figure 5: Cross section showing channel depth at user-defined bank location

Figure 6: Bank arcs generated from centerline and user-defined bank location

Extract Features Options

The options vary depending on the selected feature extraction tool.

For extracting all centerlines, the following options are available:

Input raster: The raster used to extract features.

Feature type: Use channel to extract all channel centerlines from the selected raster or Embankment to extract all embankments and ridges from the selected raster.

Threshold area: The minimum flow accumulation value used for extracting a feature.

Pre-processing engine: Use to select TOPAZ or TauDEM for computing flow directions and flow accumulations. Since an inverted raster is used to compute flow directions and flow accumulations when the feature type is set to embankment, the pre-processing engine must be run again when extracting embankment centerlines.

Run pre-processor: Select this button to launch TOPAZ or TauDEM and compute flow directions and flow accumulations. The pre-processor will run in the background and a message will appear when the flow directions and flow accumulations are finished being computed.

Centerline: Select this option to create a new coverage from the extracted features or add the results to an existing coverage. For extracting all centerlines with one or more selected depression points, the following options are available:

Input raster: The raster used to extract features.

Feature type: Use channel to extract all channel centerlines from the selected raster or Embankment to extract all embankments and ridges from the selected raster.

Threshold area: The minimum flow accumulation value used for extracting a feature.

Pre-processing engine: Use to select TOPAZ or TauDEM for computing flow directions and flow accumulations. Since an inverted raster is used to compute flow directions and flow accumulations when the feature type is set to embankment, the pre-processing engine must be run again when extracting embankment centerlines. Note that when you have a depression point (or points) selected, a new set of flow directions and flow accumulations should be generated based on the selected depression point(s).

Run pre-processor: Select this button to launch TOPAZ or TauDEM and compute flow directions and flow accumulations. The pre-processor will run in the background and a message will appear when the flow directions and flow accumulations are finished being computed.

Centerline: Select this option to create a new coverage from the extracted features or add the results to an existing coverage.

For extracting a single centerline, the following options are available:

Input raster: The raster used to extract features.

Feature type: Use “Channel” to extract a single channel centerline from the selected point on the raster or “Embankment” to extract a single embankment centerline or ridge from the selected point on the raster.

X/Y: The selected starting point location (not editable).

Pre-processing engine: Use to select TOPAZ or TauDEM for computing flow directions and flow accumulations. Since an inverted raster is used to compute flow directions and flow accumulations when the feature type is set to embankment, the pre-processing engine must be run again when extracting embankment centerlines.

Run pre-processor: Select this button to launch TOPAZ or TauDEM and compute flow directions and flow accumulations. The pre-processor will run in the background and a message will appear when the flow directions and flow accumulations are finished being computed.

Maximum feature length: The maximum length for the centerline feature. If the edge of the raster is reached before the feature length is reached, the feature stops at the edge of the raster.

Centerline: Select this option to create a new coverage from the extracted feature or add the results to an existing coverage.

For extracting banks, the following options are available:

Input raster: The raster used to extract features.

Use the depth that is: This option is used when the matching depth is found at multiple points along one side of the cross-section.

1. “Closest to centerline” finds the matching depth on each generated cross section that is closest to the centerline.
2. “Closest to previous” finds the matching depth on each generated cross section that is closest to the previous distance from the centerline, starting at the user-specified bank point location.
3. “Furthest from centerline” finds the matching depth on each generated cross section that is furthest from the centerline.

Pre-processing engine: Use to select TOPAZ or TauDEM for computing flow directions and flow accumulations.

Run pre-processor: Select this button to launch TOPAZ or TauDEM and compute flow directions and flow accumulations. The pre-processor will run in the background and a message will appear when the flow directions and flow accumulations are finished being computed.

Maximum distance from centerline: The generated cross section lines that are used for extracting the banks are equal to twice this distance. If the edge of a cross section is encountered before the matching depth is found on a cross section, a bank line is drawn at this distance from the centerline.

Banks: Select this option to create a new coverage from the extracted bank or add the results to an existing coverage.

Smooth bank lines: Select this option to smooth the bank lines after they are generated.

Cross-sections: Select this option to create a new coverage from the generated cross sections or add the results to an existing coverage. NOTE: If the name of the cross-section coverage matches the name of the bank coverage, the cross-sections and banks will be placed in the same coverage.

Best Practices

Digital dams: Digital dams, usually artifacts from stamping channels or from unprocessed LIDAR data, cause problems determining flow directions on the raster. Note in the image below how the deep channel comes to an end before reaching the edge of the raster.

Figure 7: Digital dams

The pre-processing engines (TOPAZ and TauDEM) fill these digital dams prior to computing flow directions. This can be a problem because the centerline may stray from the thalweg and the computed matching depth will be based on the filled raster instead of the original raster. This results higher elevations along the channel above the digital dam.

Fix any digital dams by trimming the raster above the digital dam (there’s a right-click command on rasters to trim them using a polygon in the map module) or modifying the elevations downstream of the digital dam to match the correct stream thalweg elevations and remove the digital dam (there’s a right-click command on a raster to edit a raster’s elevations using the elevations on a selected arc).

Placing points for extracting bank features: Place the point for extracting a bank feature closest to the area that is most important. For example, if a section is being used for bridge analysis, place the bank point close to the bridge. The bank lines stay closest to original feature at the original point location.

If the extract bank tool misses a section of your bank, try moving the starting point to the missed section of the bank and run the tool again.

Extracting centerlines for braided streams: Sometimes when extracting centerlines for braided streams, the extracted centerline may not follow the actual stream location because of digital elevation model inaccuracies. See the image below where two points were used to extract stream centerlines:

Figure 8: Extract Features following separate channels

In this case, you would extract both centerlines and manually merge the arcs into a single arc after both centerlines have been extracted.

‘Use the depth that is’: If you have a single well defined channel with a strong bank feature, use the ‘closest to centerline’ option. If there are multiple floodplain bank features, Extract Features tool may catch a different floodplain feature and using the ‘closest to previous’ option will provide the best result. If there are multiple channels or a braided stream and the user wishes to catch the widest extents of the channel, use the ‘furthest from centerline’ option.