Polygons from Arcs Tool

Polygons from Arcs

The Polygons from Arcs tool converts all arcs in a coverage to polygons based on specified parameters.

This tool is designed to simplify the creation of linear polygons to represent features such as channels or embankments. The feature extraction operations create stream or ridge networks. These features can be used to position cells/elements in a mesh/UGrid to honor these features. However, it is often useful to represent such linear features as Patch polygons to allow or anisotropic cells—elongated in the direction of the feature.

Each arc in the input polygon will be converted to a polygon. The following applies during the creating process:

• The polygon for isolated lines/arcs will consist of offset lines in both directions from the arc. The ends of the polygon will be perpendicular to the end segment of the arc.
• If two arcs are connected (end to end), the orientation of the two polygons will be averaged so that the polygons share and "end".
• If three arcs/lines join at a node, the two that have the most similar direction will be maintained as a continuous feature. The third will be trimmed back to not encroach on the polygons of the other two. If more than three arcs/lines join at a single location, the two arcs with the most similar direction will define the preserved direction. All other arcs will be trimmed back to not encroach.
• If a single arc closes in a loop, the resulting polygon closes on itself. This polygon will not function as a patch in SMS. This workflow is not recommended.
• The tool performs a check to determine if the polygons from two unconnected overlap/intersect each other. This is reported in the progress dialog.

Input Parameters

• Input coverage – This can be any map coverage in the project. The arcs in the coverage will be used to guide polygon creation.
• Average element/cell width – This defines the average width of the segments projecting perpendicular from the arc. The units (foot/meter) correspond to the display projection of SMS.
• Number of elements/cells (must be even) – This defines the number of segments projecting in each direction from the centerline. Because it is projected in both directions, it must be even.
• Bias (0.01-100.0) – This provides control of the relative length of the segments across the feature. In this case, it is actually a double bias because both sides of the feature are biased from the outer edges to the center. Therefore, a bias less than 1.0 results in segments that are shorter at the center. A bias greater than 1.0 results in segments that are longer at the center. Specify a small bias to improve representation of the channel bottom or embankment crest and a larger bias to increase resolution (representation) or the outer edge (shoulder or toes) of the feature. See images below for examples.

Output Parameters

• Output coverage – Specify the name of the coverage to be created that will contain the generated polygons. The intent of this coverage is to be incorporate into a mesh generation coverage for the domain. The resulting coverage will have the Area Property coverage and will need to be changed to the desired coverage type.

Current Location in Toolbox

Coverages/Polygons from Arcs

Examples

Example 1 – 2 segment wide channel

Example 1 Single arc converted to polygon with 2 elements wide

In this case the average element/cell width was set to "w". Since each half has one cell, both segments are "w". The total width is therefore "2.0 w". Since there are only 2 segments, the bias value has no impact in this case.

Example 2 – 4 segment wide channel with bias greater than 1.0

Example 2 Single arc converted to polygon with 4 elements wide with bias of 2.0

In this case with an average segment length (element/cell width) of "w" the total width is "4.0 w". Since the bias is 2.0, the center segments are twice as long as the outer segments.

Example 3 – 4 segment wide channel with bias less than 1.0

Example 3 Single arc converted to polygon with 4 elements wide with bias of 0.5

In this case with an average segment length (element/cell width) of "w" the total width is "4.0 w". Since the bias is 0.5, the center segments are half as long as the outer segments.

Example 4 – 6 segment wide channel with bias greater than 1.0

Example 4 Single arc converted to polygon with 6 elements wide with bias of 2.0

In this case with an average segment length (element/cell width) of "w" the total width is "6.0 w". Since the bias is 2.0, the center segments are twice as long as the outer segments.

Example 5 – End to End Arcs to Polygons

Example 5 Two arcs connected end to end converted to polygons

The offset at the junction of the two arcs is adjusted to be perpendicular to the average of the two arcs. The two polygons would create a continuous channel.

Example 6 – Tributary Arcs to Polygons

Example 6 Two centerlines merging into a single centerline converted to polygons

At the junction of more than two arcs, the two that are closest to linear are assumed to be the main channel and the polygons for those two are treated just like example 5. The other arcs connected to this junction are treated as a tributary. The arc is still converted to a polygon, but any vertices on the arc within two widths of the junction are ignored to allow room for a transition between the channels to occur. (Note: it is anticipated that in the future this will be modified to allow for T or Y type merging of the polygons for junctions of three arcs.)

Example 7 – Parallel Arcs that Result in Overlapping Channel Polygons

Example 7 Two centerlines resulting in overlapping polygons

If two arcs being converted to polygons result in overlapping polygons, the tool reports this issue using one of the overlapping arc indices. It is the responsibility of the modeler to adjust the arcs and convert to non-overlapping polygons, or clean up the overlapping polygons manually.

Error message from Example 7

Related Tools

• Trim Coverage
• Bridge Footprint