NGP Standards and Specifications

Lidar Base Specification: Digital Elevation Model Surface Treatments

Lidar Base Specification 2021 rev. A

DEM Surface Treatments
The character of lidar-based DEMs differs from traditional DEMs derived through stereo photogrammetry, necessitating additional treatment of the lidar-based topographic surface. Flattening water surfaces is the primary target of current treatments.

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Bridges

Hydro-flattening

 

Bridges

  • All instructions and requirements regarding the use of breaklines also applies to non-hydrographic terrain generation below bridges.
  • Any breaklines used to enforce a logical terrain surface below a bridge shall be considered a required deliverable.
  • The bare-earth surface below the bridge shall be a continuous, logical interpolation of the apparent terrain lateral to the bridge deck.
  • Where abutments are clearly visible, the bare-earth interpolation shall begin at the junction of the bridge deck and approach structure. Where this junction is not clear, the contractor shall use their best judgement to delineate the separation of below-bridge terrain from elevated bridge surface.
  • Streams, rivers, and water bodies meeting the criteria for hydro-flattening shall be monotonically continuous where bridge decks have been removed.
  • Bridges, as defined in the glossary, shall be removed from the bare-earth surface.

 

Hydro-Flattening

Hydro-flattening pertains only to the creation of derived DEMs from lidar points and breaklines. Hydro-flattening makes no changes to the geometry of the originally computed lidar points. Breaklines developed for use in hydro-flattening may also be used to support classification of the point data. 

  • Bare-earth lidar points (serving as mass points) that are in close proximity to any breakline shall be classified as Ignored Ground (class 20) and shall be excluded from the DEM generation process when the breaklines are included. This process prevents unnatural surface artifacts from being created between lidar points and breakline vertices. The proximity threshold for reclassification as Ignored Ground is at the discretion of the data producer, but in general should not exceed twice the ANPS.
  • The requirements for hydro-flattening are listed below. These requirements also define the minimum features for which breaklines shall be collected and delivered.
    1. Inland Ponds and Lakes
      • Waterbodies with a surface area of 0.8 hectare (ha; 2 acres) or greater (approximately equal to a round pond 100 m in diameter) at the time of collection shall be flattened.
      • Flattened waterbodies shall present a flat and level water surface (a single elevation for every bank vertex defining the waterbody's perimeter).
      • The entire water surface edge shall be at or below the immediately surrounding terrain (the presence of floating waterbodies will be cause for rejection of the deliverable).
      • Long impoundments such as reservoirs, inlets, and fjords, whose water surface elevations decrease with downstream travel, shall be treated as streams or rivers.
    2. Inland Streams and Rivers
      • Streams and rivers of a 30-m or greater nominal width shall be flattened.
      • Streams or rivers whose width varies above and below 30 m will not be broken into multiple segments; data producers will use their best professional cartographic judgment in determining when a stream or river has attained a nominal 30-m width.
      • Flattened streams and rivers shall present a flat and level water surface bank-to-bank (perpendicular to the apparent flow centerline).
      • Flattened streams and rivers shall present a gradient downhill water surface, following the immediately surrounding terrain.
      • In cases of sharp turns of rapidly moving water, where the natural water surface is notably not level bank-to-bank, the water surface will be represented as it exists while maintaining an aesthetic cartographic appearance.
      • The entire water surface edge shall be at or below the immediately surrounding terrain.
      • Stream channels shall break at culvert locations leaving the roadway over the culvert intact.
      • Streams shall be continuous at bridge locations.
      • Bridges in all their forms shall be removed from the DEM.
      • When the identification of a structure as a bridge or culvert cannot be made definitively, the feature shall be regarded as a culvert.
    3. Non-Tidal Boundary Waterbodies
      • Boundary waterbodies are waterbodies that contain some or all of the DPA.
      • Boundary waterbodies may be any type of waterbody but are virtually always large in area or width.
      • A boundary waterbody shall be represented as a polygon that follows the shore throughout the project and is then closed using arbitrary line segments as needed across the waterbody. Boundary waterbodies do not include the natural far shoreline.
      • The water surface shall be flat and level, as appropriate for the type of waterbody (level for lakes, gradient for rivers, and so forth). It is not expected that ponds <0.8 ha (2 acres) or streams <30 m in width would be used as boundary waterbodies, thus it is expected that all boundary waterbodies will be hydro-flattened.
      • All landward water surface edges shall be at or below the immediately surrounding terrain.
      • Unusual changes in the water surface elevation that may take place over the course of the collection (for example, different river stages due to increased or decreased discharge from an upstream dam) shall be documented in the project metadata.
      • Unusual changes in water surface elevation shall be handled as described in Tidal Waterbodies.
    4. Tidal Waterbodies
      • Tidal waterbodies are defined as any waterbody that is affected by tidal variations, including oceans, seas, gulfs, bays, inlets, salt marshes, and large lakes.
      • Tidal variations during data collection or between different data collections will result in lateral and vertical discontinuities along shorelines. Because it is the USGS–NGP’s intent for the DEM to represent as much ground as the collected data permit, lidar ground points are not to be removed for the sake of adjusting a shoreline inland to match another shoreline.
      • Likewise, adjusting a shoreline outland will create an equally unacceptable area of unmeasured land in the DEM. It is recommended that, to the highest degree practical, collections are planned to minimize tidal differences at the land-water interface.
      • In addition to meeting the requirements for inland waterbodies listed in “1. Inland Ponds and Lakes” and “2. Inland Streams and Rivers” (above) as appropriate, the treatment of tidal waterbodies shall also meet the following requirements:
      • Within each tidal waterbody, the water surface shall be flat and level for each different water surface elevation.
      • Vertical discontinuities within a tidal waterbody resulting from tidal variations during the collection are considered normal and shall be retained in the final DEM.
      • Horizontal discontinuities along the shoreline of a tidal waterbody resulting from tidal variations during the collection are considered normal and shall be retained in the final DEM.
      • For projects located in coastal areas, cooperating partners may impose additional requirements for tidal coordination.
    5. Islands
      • Permanent islands 0.4 ha (1 acre) (approximately equal to a round island 72 m in diameter) or larger shall be delineated within all waterbodies.