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Elevation-Derived Hydrography Data Acquisition Specifications: Source Raster Data Requirements

By NGP Standards and Specifications

Elevation-Derived Hydrography Data Acquisition Specifications

Source Raster Data Requirements

Return to EDH Data Acquisition Specifications Table of Contents

The DEM used to create the lidar-derived hydrography shall be:

  • Comprised of the best available 3DEP lidar source data or other accepted lidar source data.
  • Seamless within the FDPA boundary.
  • Completely encompassing the FDPA, plus at least 250 meters outside the FDPA, as needed to derive hydrography.

Associated DEM deliverables

  • If 3DEP hydroflattening breaklines are available for the selected elevation datasets, they shall be provided with the elevation-derived hydrography delivery as one polygon dataset in a geopackage.
  • A spatial metadata layer that describes the source lidar data used. See the Elevation Source Spatial Metadata section for more information.
Preparation of Elevation

The DEM used to derive hydrography shall be generated from 3DEP-compliant 1-meter resolution or better Original Product Resolution (OPR) DEMs unless the USGS has approved an alternate DEM source. The DEM shall be in the specified regional CRS (Table 1 and Table 2). It is also acceptable to create the hydro-derivation surface from the 3DEP-compliant lidar point cloud and breaklines.

Source Data
Use of a DEM Source
  1. The 3DEP OPR DEM rasters shall be used as the source DEM if available.
  2. If the 3DEP OPR DEM rasters are not available, the 3DEP 1-meter DEM shall be used pending USGS approval.
  3. If no 3DEP source is available, a USGS approved DEM source may be used.
  4. DEM source requirements.
    1. DEMs shall be mosaicked and projected from the source CRS to the regional CRS (to include Geoid transformation) listed in (Table 1 and Table 2).
    2. A bilinear interpolation method shall be used to spatially transform and resample the DEM rasters to the required CRS and a spatial resolution of 1 meter.
    3. All units (x, y, and z) shall be in meters.
    4. Tiles shall not have NODATA cells between them in the final DEM deliverable.
    5. The final DEM shall include a 250-meter buffer, at minimum, around the entire FDPA. See Appendix 2 of this document for more information.
      1. If sufficient elevation data is not available to complete a 250-meter buffer, complete a DEM to the extent of the available elevation data and note the reason for the variation from the buffer requirement in the collection report.
      2. The DEM bounding extent shall be clipped to the extent of the FDPA.
    6. NODATA areas shall not exist in the final DEM product.
      1. Consult with the USGS if gaps exist or areas are unavailable from 3DEP.
Use of a 3DEP Lidar Point Cloud to Create the Project Area DEM
  1. Requirements outlined in “Deliverables, Bare-Earth Surface (Raster DEM), in the latest version of the USGS 3DEP Lidar Base Specification (LBS) shall be followed when creating a mosaicked DEM from lidar sources.
  2. Lidar point clouds and corresponding hydroflattening breaklines shall be spatially transformed from the source CRS to the regional CRS (to include Geoid transformation) listed in the CRS section (Table 1 and Table 2).
  3. Hydrographic breaklines that correspond to the project area shall be used to hydroflatten the waterbodies.
  4. Hydrographic breaklines from multiple projects shall be merged into one feature class and used to hydroflatten waterbodies in the composite DEM.
  5. Lidar Point Cloud source requirements
    1. The final DEM shall include a 250-meter buffer, at minimum, around the entire FDPA. If sufficient elevation data is not available to complete a 250-meter buffer, complete a buffer with the elevation data available and note the missing buffer in the collection report.
    2. NODATA areas shall not exist in the final DEM product.
      1. Consult with the USGS if gaps exist or areas are unavailable from 3DEP.
Raster Product Characteristics


Raster products shall have the following characteristics:

  1. Number of bands: 1
  2. Cellsize: 1 x 1 meter
  3. Format: GeoTIFF version 1.1 or Cloud Optimized GeoTIFF (COG as GeoTIFF version 1.1).
  4. Pixel type: floating point
  5. Pixel depth: 32 bit
  6. NODATA value: -999999
  7. Lossless Compression: DEFLATE, LERC, LZ77, or LZW
  8. GTRasterTypeGeoKey shall be RasterPixelIsArea
  9. Bounding extents of raster shall be minimized to fit the extent of the 250-meter minimum buffered area.
  10. Pixel alignment shall conform to a grid in the defined CRS based on a model tie point of (0,0) and origin coordinates evenly divisible by the cell size.
  11. Include spatial reference information. See Appendix 5 for additional information.
Overlapping 3DEP Project Data

The choice of which source lidar elevation datasets to mosaic into a project-area elevation surface shall be based on characteristics of the source elevation data. Important criteria to consider when creating mosaicked 1-meter bare-earth DEM for deriving hydrography from two or more overlapping 3DEP projects are listed below.

  1. The elevation source meets 3DEP requirements, in particular:
    1. The lidar point cloud (LPC) is available and coded as “Meets 3DEP” or meets with variance (MwV) in the 3DEP Work Unit Extent Spatial Metadata (WESM),
    2. The Original Product Resolution (OPR) DEM is available and coded as “Meets 3DEP” or “MwV” in the 3DEP WESM.
  2. The data is from the most recent acquisition.
  3. Exceptions may be made for 3DEP lidar projects where individual work units in a project have been published and are “expected to meet” 3DEP requirements but the final vertical accuracy testing cannot be performed until all work units are validated and published.

    Other considerations include:

  • Known variances or limitations (low confidence polygons).
  • Vegetation (leaf-off is preferred) or ground cover (for example snow or ice) at time of collection.
  • Soil saturation.
  • Flooding.
  • Ground detection quality and density.
  • Presence or lack of tidal coordination.
  • Vertical accuracy of the source data.
  • Significant vertical offsets in DEM difference rasters.

Overlap areas shall be resolved based on the characteristics listed in the critical and other considerations above. The USGS will provide further guidance by request if determining the best project area to use within overlapping areas is ambiguous.

If blending between collection areas is necessary, blending shall be restricted to a small overlap area using a distance weighted algorithm. The USGS recommends a 10-meter blend width for lidar-source tiles, and 50-meter blend width if one-third arc-second seamless DEM data must be used to cover areas with restrictions on high-resolution elevation data publication (for example, on military and tribal lands). If significant vertical or horizontal offsets between collection areas are present, request guidance from the USGS for the recommended method to mosaic the datasets.

The seamless DEM for the project area shall be buffered by at least 250 meters outside the FDPA. The buffer area should be extended beyond 250 meters in areas of uncertainty where the watershed boundary is likely to change, such as in flat terrain. Any missing elevation data within the FDPA, including the 250-meter buffer, shall be reported to the USGS.

All methods used to create the mosaicked 1-meter bare-earth DEM shall be documented in the collection report. The collection report should also include a statement indicating whether the mosaicked 1-meter bare-earth DEM, used for the generation of elevation-derived hydrography features, was government furnished or not.

Seamless IfSAR-derived, 5-meter resolution, elevation surfaces are available from the USGS for derivation of hydrography in Alaska. 
 

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