Hydrologic Event Management
Add Intro
Overview
HEM Point of Contact: Michael Tinker, mdtinker@usgs.gov. For general information, contact us at hem@usgs.gov.
The Hydrography Event Management (HEM) Tools are a set of shared components to allow for creation, management, and refresh of event data that is referenced to hydrography data in the NHDinGeo format.
Why are the HEM Tools important to the NHD?
The National Hydrography Dataset serves as a framework for geospatial data representing the surface water of the United States. The features that make up the NHD have a basic set of attributes that describe these features. These features may have additional information linked to them describing other characteristics relating to hydrology, water quality, biology, regulations, and a host of other data. This linked information greatly contributes to the power of the NHD to serve not just as spatial data, but also as an information system, and further as a base for analysis and knowledge. The information may come in many forms and from many sources. Standardizing all of this information to conform to the NHD model is impractical. The information must be seen as a module that can plug into the NHD. The best way to do this is to reference the information to the geospatial data using a link. In the NHD the basis for this link is the ReachCode, a permanent identifier for all features to which information can be linked. Since the NHD is not just geospatial data, but also a linear flow network, it makes sense to link much of this information to that linear system. It is possible to create addresses on that linear system to describe where that information is located. These are known as linear addresses and also as linear referencing. The linked information is known as an “event”.
The events can be a point on the linear system, a length on the linear system known as a linear event, or whole or partial polygons linked to the NHD polygons. Besides being a highly modular, and therefore flexible, approach it is also very effective and efficient for data management because it does not duplicate geometry.
Events can be any kind hydrologic observation---a stream gage, dam, withdrawal point to irrigation, a fish ladder, biologic sample locations, or point sources of pollution can all be represented by point events on the NHD network.
Generic tools are available for linear referencing, but do not fully exploit the potential of the NHD model. To do this, the Hydrography Event Management (HEM) tool has been developed to give users a full suite of capability in creating and managing events in the NHD. This ability to manipulate and manage events ensures that events can be utilized to the fullest extent possible and therefore maximize the effectiveness of the NHD concept.
Web Services
HEM Web
HEM Web (Hydro Event Management Web service) is a set of National Map geospatial services that allow any organization to create, edit, and display geospatial markers, called Events, that are referenced, or indexed, to the National Hydrography Dataset (NHD). Events can represent nearly any water-related feature, such as sampling locations, streamgages, scenic or impaired stretches of river, fish passage barriers, toxic spills, or put-ins/take-outs for boats. Events are linked to the NHD by way of linear referencing, which is a network addressing system. The HEM Web services allow a user to click a point or line on an NHD flowline, or draw a polygon on an NHD waterbody, and the services return the address, or network location, on the NHD. With HEM Web services, organizations can build their own event indexing application.
To learn how your organization can use these services, contact us at hem@usgs.gov.
To see a sample of the services see:http://edits.nationalmap.gov/hem-soe-docs/
HydroLink Tool
The web-based HydroLink Tool, which utilizes HEM Web services, is now live for use by anyone with an ArcGIS Online organizational account (including partners outside of the Department of Interior). The HydroLink Tool is a linear referencing tool for both the NHD High Resolution (NHD HR) and also the medium resolution NHDPlus V2. The tool generates and records linear reference locations (ReachCode and Measure) for point data, linking information simultaneously to both hydrography networks. The tool works with any kind of hydro-referenced data, such as fish sampling locations, streamgages, water rights, boating access, etc.
The HydroLink Tool allows a user to upload a point shapefile, utilizing ArcGIS Online for data management and tool functionality. The tool consumes the uploaded data as a service from ArcGIS Online. The point data is displayed alongside both the NHD HR and the medium resolution NHDPlus V2. The tool enables the user to work through a queue to link points to a flowline on the NHD HR, NHDPlus V2, or both. After linking is completed, the resulting data can be downloaded--with added attributes of ReachCode and Measure for both hydrography networks, the COMID for NHDPlus V2, and verified point locations (i.e. latitude and longitude).
The HydroLink Tool requires an organizational account with ArcGIS Online, such as a USGS, U.S. EPA, or U.S. Forest Service account. All ArcGIS licenses include access to an ArcGIS Online organizational account, so anyone with an ArcGIS license should have access. The tool’s homepage provides information on how to create a USGS account (for USGS users only.)
The HydroLink Tool was built by the USGS and utilizes the USGS HEM (Hydro Event Management) Web services. The HEM Web services are a set of National Map geospatial services that allow any organization to create, edit, and display geospatial markers, often called “events”, that are linked to the NHD.
The USGS HydroLink Tool is available at: https://maps.usgs.gov/hydrolink/
Feedback for the Hydrolink Tool is encouraged and can be sent to Andrea Ostroff at aostroff@usgs.gov.
USGS National Map services, including the HEM Web services, are available at: https://edits.nationalmap.gov/arcgis/rest/services
If you would like to include HEM Web services into your own application,a HEM Web developer's guide and a sample application using the HEM Web services are available at:
https://edits.nationalmap.gov/hem-soe-docs/
Hydrolinked Data in ScienceBase
ScienceBase is a platform for data sharing, cataloging, and discovery. One of the main features of ScienceBase is that it automatically generates free Web Mapping and Web Feature services.
ScienceBase is a creation of the USGS Core Science Analytics Synthesis and Libraries group. It was designed as a platform for scientists to share data. ScienceBase serves as a catalog for data and metadata, and automatically generates web services for files that users upload, such as Web Mapping Services, or Web Feature Services. These services can be used in a web map, such as ArcGIS Online, or may be used by any number of web applications. ScienceBase also serves as a search and discovery application for scientific data. Items in ScienceBase can be discovered by searching within ScienceBase, or by searching from an internet search engine.
Items in ScienceBase are organized in communities. A community is a designated project space in ScienceBase. It appears in ScienceBase as a folder, and can contain any number of child items: individual records, folders, and links to ScienceBase items outside the community. A community can be private or visible to the public. Editing permissions are limited to users associated with the community.
Hydro linked data (aka events) can be nearly any kind of data you can link to the NHD such as flow-volume, velocity, temperature, water chemistry, pollution control classifications, aquatic species habitat, recreation designations, or water rights. If your organization has hydro linked data referenced to the NHD, ScienceBase allows you to share it with the world.
USGS encourages organizations that link their data to the NHD to share their data. To that end, we now encourage organizations to post their hydro linked data to our community, the National Hydrography Dataset Linked Data Registry. Another option is that your organization can create your own community in ScienceBase, which would allow your to curate your own data and community. We designed our community with examples of FGDC-compliant metadata for hydro linked data, recommended keywords and tags to make your hydro linked data discoverable, links to other communities related to the NHD, as well as links to tools for creating FDGC-compliant XML metadata documents.
If you would like to learn more about sharing your hydrolinked data in ScienceBase, please contact Mike Tinker (mdtinker@usgs.gov).
Visit the top-level search page in ScienceBase at https://www.sciencebase.gov/catalog/
Visit the National Hydrography Dataset Linked Data Registry at https://www.sciencebase.gov/catalog/item/530d0115e4b08f991722dce3
HEM Help and Documentation
Importing and Synchronizing Area Events
- Synchronizing with HEM tool
- Batch Synchronizing Point Events
- HEM Area Import and QC
- Training data: NHD131687.7z
For help, please contact hem@usgs.gov