Cross Section at Endpoints

Generate a cross section defined by two user-specified endpoints using NLDI pygeoapi processes.

Overview

The NLDI pygeoapi service provides a cross section at endpoints algorithm,
which constructs a cross-sectional transect between two user-provided points.
This tool is part of the Python-based geoprocessing suite (nldi-flowtools,
nldi-xstool) surfaced through the OGC API – Processes interface.

This process complements the single-point cross section tool by allowing
explicit specification of both ends of the transect.

How the Algorithm Works

To construct a cross section between endpoints:

  1. The user supplies two coordinates, representing the endpoints of
    the desired transect.
  2. If necessary, internal hydrolocation steps determine their positions
    relative to the NHDPlus flowlines.
  3. A transect line is generated between the two points.
  4. Elevation or hydraulic geometry is sampled along that transect to
    produce the cross section output.

Because the algorithm operates on user-specified endpoints, it can represent
features such as:

  • Wide channel sections
  • Floodplain cross sections
  • Custom transects for modeling or design

Input and Output Characteristics

The source blog post does not include detailed schemas, but it supports the
following general description:

Inputs may include:

  • Two point coordinates
  • Projection or snapping parameters

Outputs include:

  • A line geometry representing the transect
  • Associated elevation or profile attributes

These outputs are suitable for hydraulic or geomorphic analysis workflows.

Notes & TODOs

  • TODO: Clarify whether endpoints must be snapped to flowlines before processing.
  • TODO: Add geometry details (two-point straight line, multi-vertex line generation rules).
  • TODO: Document error handling for invalid or hydrologically inconsistent endpoint pairs.