Elevation-Derived Hydrography Data Acquisition Specifications: Attribute Table Structure Requirements

The attribute table structure is described in Table 4 and Table 5. An important component of the elevation-derived hydrographic features is that they have three-dimensional (3D) geometry (Table 3). Each feature type must be z-enabled, with z-values assigned to each point, vertex, and node. The feature classes use 3D geometry, pointZ, polylineZ, and polygonZ.

• Feature class (FClass)—A one-digit short integer code describing the hydrographic use of a feature (Table 9).
  • A “1” is used when a feature is collected to meet the elevation-derived hydrography acquisition specifications. An example of a feature with an FClass = 1 is a point feature defined as a sink. An example of a line feature with an FClass = 1 is a stream that is too narrow to be represented as a polygon. An example of a polygon feature with an FClass = 1 is a wide river, or a lake/pond.
  • A “2” is used when a feature is outside of the collection criteria. An example is when the features add too much complexity and will not be used for network connectivity. A line representing a drainage ditch in a neighborhood may be collected, but not used for network connectivity. 
  • A “9” is used when a feature is not a hydrographic feature and is outside the 3DHP collection requirements. A line representing a levee is an example of a feature that may be useful for elevation processing but is not part of the hydrography network. 
• Feature class for elevation (EClass)—A short integer code describing how the features are used in elevation surface treatments (Table 10).
• Feature code (FCode)—A long integer field containing a coded value for the type of hydrographic feature represented (Table 8). For instance, a Lake/pond has an FCode of 39000, and a Stream/river has an FCode of 46000.
• Description (Desc)—A 250-character free-text field with a text description of FCode, or it can be used for user-defined features not included in the domain list. Not required.
• Elevation source data (Source)—A 128-character free-text field with a text description of the source data used for deriving hydrography. For most features ‘Lidar’ or ‘IfSAR’ can be specified.  If a source other than elevation is used, it should be noted. This may be required in areas of DEM limitations (see Digital Elevation Model (DEM) Limitation section for definition), or where ancillary data is used (Table 14). 
• Hydrography delineation method (Method)—A 250-character free-text field with a text description of the method used for deriving hydrography. Examples are software, models, or digitizing techniques. Ancillary datasets can also be noted here.
• User-defined code (UserCode)—A 25-character free-text field with a code designated by the acquisition entity to identify features collected outside the scope of features described in this specification. It is intended to be used as a key to join tables with attributes outside of this specification. Not required.
• Free-text space used for comments (Comments)—A 250-character free-text field for user comments. Not required.
• User-defined code (UniqueID)—A 50-character free-text field that stores a unique identifier for each feature in the dataset. The UniqueID is intended to be stable throughout multiple deliveries of a dataset within a project area. The UniqueID field shall be populated by the data producer prior to delivery of data to the USGS.  UniqueIDs allow communication with data producers by providing a tracking system for individual features.
• DEM Limitation (Limitation)— A short integer one-digit code that indicates when hydrographic features do not meet vertical or horizontal requirements because of elevation source data limitations (Table 7). 
• Flow Direction Determination (FlowClass)— A short integer one-digit code used in the line feature class to describe whether a feature's flow direction can be determined (Table 6). In most cases, features shall be digitized in the direction of flow and shall flow downslope (FlowClass = 1). In very flat, wetland, or tidal areas, it is often impossible to determine the direction of flow (FlowClass = 0).  Water may occasionally flow uphill, such as when a pump is used or in rare natural situations. A segment’s z-values may flow in an upslope direction in cases where water enters a depression on the landscape and must rise to exit the depression. When flow is known to initiate from a lower elevation and discharge to a higher elevation, FlowClass = 2.