National Hydrography Dataset (NHD) Data Dictionary Reachcode Tables

By NGP Standards and Specifications

The data dictionary for reachcode tables in the National Hydrography Dataset.

NHDReachCodeMaintenance : Table used to maintain links for backward compatibility with FOD-based systems.

Name	Definition	Type	Allow Nulls	Length	Domain	Default Value	Comments
GNIS_ID	A permanent, unique number assigned by the Geographic Names Information System (GNIS) to a geographic feature name for the sole purpose of uniquely identifying that name application as a record in any information system database, dataset, file, or document	Text	True	10			GNIS_ID = "null" if no name is associated with the feature. Removed from WBD as of version 2.3 schema.
GNIS_Name	The Geographic Names Information System (GNIS) assigned proper name, specific term, or expression by which a particular geographic entity is known.	Text	True	100
OBJECTID	Internal feature or event number.	OBJECTID	False
Permanent_Identifier	A 40-character globally unique ID (GUID) value that uniquely identifies the occurrence of each feature or event in The National Map.	Text	False	40
ReachCode	Unique identifier. The first eight digits are the WBD_HUC8. The next six digits are randomly assigned, sequential numbers that are unique within a HUC8.	Text	True	14			For event feature classes, this value is the ReachCode on which the event is located.
ReachSMDate	Reach spatial modification date.	Date	True	8			The ReachCodeMaintenance table is updated by the NHD Update Tool. When a geometric edit is performed on a feature that contains a ReachCode (NHDFlowline or NHDWaterbody), the NHDReachCodeMaintenance table is updated to the current date to reflect the last spatial modification (SM) date of the associated ReachCode. Any previous records for that Resolution (Medium or High, depending on the target feature class resolution) are overwritten. The Permanent_Identifier field is unique to this table and is not related to NHDFlowline or Waterbody features. The GNIS_ID and GNIS_Name are updated, as these are unique per ReachCode. MValue updates are not considered spatial modifications and thus do not trigger ReachSMDate updates. For events (NHDPointEvent, NHDLineEvent, NHDAreaEvent), when an event is created by the Hydrography Event Management (HEM) Tool, the tool pulls the current ReachSMDate from the ReachCodeMaintenance table for the ReachCode the event is associated with. So, if underlying hydrography features are updated, and events are not, the ReachSMDate in the event table will be out of date in relation to the current hydrography. Users can either synchronize their events using the HEM Tool or they may not, depending on their needs. For example, a user may not want to synchronize their events to the current hydrography if a study was performed on a certain version of the NHD.
Resolution	Source resolution.	Long Integer	False		Resolution		Currently NHD is available as separate resolutions.Domain values:Local > 1:12,000High 1:24,000/12,000Medium 1:100,000

NHDReachCrossReference : Information that tracks changes, over time, to reach codes.

Name	Definition	Type	Allow Nulls	Length	Domain	Default Value	Comments
ChangeCode	Type of change to the reached feature.	Text	True	4
NewHUCode	Hydrologic Unit of feature after migration.	Text	True	8			Used if the flowline was originally assigned a reach code based on an incorrect subbasin assignment.
NewReachCode	Reach code after the change.	Text	True	17
NewReachDate	Date new reach code was assigned (yyyymmdd).	Date	True
NewUPMI	Upstream Marker Index of OldReach (RF-3-Alpha Only).	Text	True	5
OBJECTID	Internal feature or event number.	OBJECTID	False
OldHUCode	Hydrologic Unit of feature prior to migration.	Text	True	8			Used if the flowline was originally assigned a reach code based on an incorrect subbasin assignment.
OldReachCode	Reach code prior to change.	Text	True	17
OldReachDate	Date old reach code was assigned.	Date	True	8
OldUPMI	Upstream Marker Index of OldReach (RF-3-Alpha Only).	Text	True	5
Process	Name of the process that created the reach code change.	Text	True	6	Process Domain		Many processes to create the NHD were done on the National Database and these have standard codes.
ReachFileVersion	Reach File Version in which the change became effective.	Text	True	10	ReachFileVersion Domain