Elevation-Derived Hydrography READ Rules: Canal/Ditch

By NGP Standards and Specifications

An artificial open waterway constructed to transport water, irrigate or drain land, connect two or more bodies of water, or serve as a waterway for watercraft (Figure 38, Figure 39, and Figure 40).

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Image: Shadroe Canal Weir — **Figure 38.** Shadroe Canal Weir, Cape Coral, Florida, as an example of a canal. Photograph by Shane Prorok, U.S. Geological Survey.

A graphic with labels and lines pointing to the shortest and longest axes of a polygon representing a canal ditch. — **Figure 39.** Diagram showing shortest and longest axes of a Canal/ditch feature.

Two images depicting canal/ditch features. — **Figure 40.** Courtland and Superior Canals near the Nebraska-Kansas state line, shown as examples of canal/ditch features. Source data from the NHD (U.S. Geological Survey, 2020), provide examples of hydrographic feature types but may not have the same density and other characteristics of elevation-derived hydrography.

Delineation

The limit of Canal/ditch is the top of the banks of the artificial waterway.

Representation Rules

Delineate features as points, lines, or polygons based on their area or length along different axes (Table 1).

**Table 1.** Canal/ditch Representation Rules.
Kind of feature object	Area	Shortest Axis	Longest Axis
0-dimensional (point)	--	--	--
1-dimensional (line)	--	less than 50 ft (15 m) in lidar; less than 98 ft (30 m) in IfSAR	--
2-dimensional (polygon)	--	greater than 50 ft (15 m) in lidar; greater than 98 ft (30 m) in IfSAR	--

Special Conditions

To accommodate variations in the shortest axis of Canal/ditch:

For lidar-based hydrography
- The 3DEP breaklines used for hydroflattening canal and ditch locations on the elevation surface may be used as Canal/ditch polygon features in elevation-derived hydrography.
- Canal/ditch features greater than 15 meters across should be delineated as 2D (polygon) features, while those less than 15 meters across should be delineated as line (1D) features. Occasionally, the Canal/ditch may narrow or expand beyond these thresholds and adjustments shall be made based on the following guidelines:
  - If a narrowed section in a Canal/ditch polygon is less than 15 meters wide for less than 1000 meters, keep it as a polygon.
  - If a narrowed section in a Canal/ditch polygon is less than 15 meters wide for more than 1000 meters, convert it to a line feature.
  - If a widened section of a Canal/ditch line is greater than or equal to 15 meters for less than 1000 meters, keep as a line feature.
  - If a widened section in a Canal/ditch line is greater than or equal to 15 meters for more than 1000 meters, convert it to a polygon feature.
For IfSAR based hydrography
- Smoothed 3DEP breaklines used for hydroflattening canal and ditch features on the elevation surface in IfSAR may be used as Canal/ditch polygon features in elevation-derived hydrography.
  - If a narrowed section in a Canal/ditch polygon is less than 30 meters wide for less than 2000 meters, keep it as a polygon feature.
  - If a narrowed section in a Canal/ditch polygon is less than 30 meters wide for more than 2000 meters, convert it to a line feature.
  - If a widened section in a Canal/ditch line is greater than or equal to 30 meters for less than 2000 meters, keep as a line feature.
  - If a widened section in a Canal/ditch line is greater than or equal to 30 meters for more than 2000 meters, convert it to a polygon feature.

Data Extraction

Canals and ditches can be an important aspect of a hydrologic network, particularly in low relief areas. The problem with many interconnected canals as a part of a hydrologic network is that it becomes difficult to navigate the stream network. The direction of flow in canals and ditches is not always apparent and may vary based on weather or season. Creating a network that flows in a downstream direction, with z-values descending from upstream to downstream, is challenging in areas when many low-slope segments. Additionally, culverts can be difficult to identify in smaller canal systems, leading to isolated networks. An area in the Coastal Plain of North Carolina with many visible canals in orthoimagery and in the lidar surface is shown in Figure 41.

A split overhead image with othoimagery and lidar of an area with an abundance of canal/ditch features. — **Figure 41.** An area with an abundance of Canal/ditch features. A, orthoimagery; B, lidar.

Figure 42 shows an example of features present in the NHD (delineated in black) and additional Canal/ditch features added to an elevation-derived stream network (represented in purple). Features tagged as “A” are canals that extend existing canal networks, help guide drainage patterns around a built environment, and are therefore substantial additions to the network. The features tagged as “B” are less than 300 meters long, are within a field that is agricultural, and do not add a substantial amount of information about drainage patterns because the Canals/ditch features are flowing into larger Canals/ditch features that are part of the hydrologic network. These two canals/ditches (B) do not meet the current criteria for capture as an elevation-derived hydrography feature and will not be added to the NHD. However, if local/project needs require, these features may still be collected and identified using the UserCode attribute as described previously.

Elevation-Derived Hydrography READ Rules: Canal/ditch Features Part of the Hydrographic Network — **Figure 42.** Canal/ditch features as part of a hydrographic network. “A” denotes features in purple that should be included in the network. “B” denotes features in purple that should be removed from the network.

Capture Conditions

If Canal/ditch is named, or

if Canal/ditch is greater than or equal to 984 feet (300 meters) along the longest axis,

then capture.

If Canal/ditch is needed to provide network connectivity,

then capture.

If Canal/ditch is within agricultural fields and drains to another Canal/ditch or other hydrographic features,

then do not capture (see following exception).

The exception is if a project has a special need for canals and ditch features that are within agricultural fields and drain to another Canal/ditch or other hydrographic feature, then these features shall be coded with an FClass=2 to indicate that the hydrographic feature is captured outside the collection criteria of the elevation-derived hydrography specifications.

Note that a hydrographic network shall not be broken if features are excluded.

Attribute Information

FClass 1— Hydrography feature defined within the collection criteria of the elevation-derived hydrography specifications.
FCode 33600—Canal/ditch (an artificial open waterway constructed to transport water, irrigate or drain land, connect two or more bodies of water, or serve as a waterway for watercraft).
EClass 2— Linear feature that follows elevation surface.

FClass 2— Hydrography feature captured outside the collection criteria of the elevation-derived hydrography specifications.
FCode 33600—Canal/ditch (an artificial open waterway constructed to transport water, irrigate or drain land, connect two or more bodies of water, or serve as a waterway for watercraft).
EClass 0—Not used to create elevation derivatives.

FClass 1— Hydrography feature defined within the collection criteria of the elevation-derived hydrography specifications.
FCode 33600—Canal/ditch (an artificial open waterway constructed to transport water, irrigate or drain land, connect two or more bodies of water, or serve as a waterway for watercraft).
EClass 11—Polygon created using a hydroflattening breakline.

FClass 1— Hydrography feature defined within the collection criteria of the elevation-derived hydrography specifications.
FCode 33600—Canal/ditch (an artificial open waterway constructed to transport water, irrigate or drain land, connect two or more bodies of water, or serve as a waterway for watercraft).
EClass 12—Polygon created from edited hydroflattening breakline on flattened elevation surface.

FClass 1— Hydrography feature defined within the collection criteria of the elevation-derived hydrography specifications.
FCode 33600—Canal/ditch (an artificial open waterway constructed to transport water, irrigate or drain land, connect two or more bodies of water, or serve as a waterway for watercraft).
EClass 13—Polygon created for non-flattened waterbody features in the DEM.

Source Interpretation Guidelines

Do not capture rivers that have been channelized to control flooding or erosion, or to maintain flow for navigation as Canal/ditch. See “Stream/river” section (for example, Los Angeles River is a large, channelized river, and coded as Stream/river).