National Hydrography Dataset (NHD) Data Dictionary Reachcode Tables
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 |