Registered Feature Sources

Overview of NLDI feature sources, discovery methods, and indexing concepts.

Introduction

The NLDI organizes indexed information into feature sources, each representing
a dataset that has been linked to the NHDPlusV2 hydrologic network.
Feature sources can be discovered programmatically and explored through the
NLDI API.

Feature sources may represent various hydrologic or environmental datasets, such as:

Streamgages
Water quality sampling locations
Watershed outlets

Each feature within a source is accessed using a unique {featureID}.

Discovering Feature Sources

Feature sources can be listed using the getDataSources operation in the
NLDI’s Swagger interface, which returns all available {featureSource} collections
that the NLDI knows about.

Using a feature source, all items within that source can be retrieved via the
getFeatures operation.
In the updated API, requesting a feature source with no filters returns all features
in that collection—a significant change that enables client-side feature inspection
and map-based selection UIs.

Example pattern:

/linked-data/{featureSource}

Feature IDs

Every record in a feature source collection is assigned a {featureID}.
These IDs identify real-world hydrologic features such as monitoring stations,
observation wells, or catchment outlets.
They can be inspected directly via the getRegisteredFeature endpoint.

Special Feature Source: `comid`

The comid feature source corresponds directly to NHDPlusV2 catchment polygons
and their associated flowlines.
It exposes two useful capabilities:

Direct access to catchment and flowline features
A position endpoint that returns the catchment containing a given lon/lat
using a point‑in‑polygon lookup

The comid source forms the backbone of NLDI navigation and indexing.

Catchment Indexing and Hydrographic Addressing

A key concept in understanding feature sources is the difference between:

Catchment indexing — assigning a feature to the NHDPlus catchment
polygon that physically contains it.
This is used broadly by the NLDI and is straightforward to compute.
Hydrographic addressing — assigning precise hydrologic network
locations using reachcode + measure pairs (“street name + house number”).
These identifiers enable more accurate routing along hydrographic networks.

Hydrographic addresses may be available for some feature sources, depending on
the dataset and indexing method used upstream of the NLDI.

Querying Feature Sources by Point

The updated API adds the ability to query a feature source by providing a
longitude/latitude point encoded in WKT format.