Lidar Base Specification Appendix 2: Hydro-flattening Reference

Appendix 2. Hydro-Flattening Reference

The subject of variations of light detection and ranging (lidar)-based digital elevation models (DEMs) is somewhat new and substantial diversity exists in the understanding of the topic across the industry. The material in this appendix was developed to provide a definitive reference on the subject only as it relates to the creation of DEMs intended to be integrated into the standard national DEM available through the U.S. Geological Survey’s (USGS’s) The National Map. The information presented in this appendix is not meant to supplant other reference materials and is not to be considered authoritative beyond its intended scope.

As used in this specification, “hydro-flattened” describes the specific type of DEM required by the USGS National Geospatial Program (NGP) for integration into the standard national DEM available through The National Map. Hydro-flattening is the process of creating a lidar-derived DEM in which water surfaces appear and behave as they would in traditional topographic DEMs created from photogrammetric digital terrain models (DTMs). A hydro-flattened DEM is a topographic DEM and is not to be confused with hydro-enforced or hydro-conditioned DEMs, which are hydrologic surfaces.

Traditionally, topography was depicted using contours on printed maps and, although modern computer technology provides superior alternatives, the contour map remains a popular and widely used product. The standard national DEM available through The National Map was initially developed as a topographic DEM from USGS contour maps and it remains the underlying source data for newly generated contours. To ensure that USGS contours continue to present the same type of information as they are updated, DEMs used to update the standard national DEM available through The National Map must also possess the same basic character as the existing standard national DEM available through The National Map.

A traditional topographic DEM such as the standard national DEM available through The National Map or the earlier National Elevation Dataset (NED) represents the actual ground surface, and hydrologic features are handled in established ways. Roadways crossing drainages passing through culverts remain in the surface model because they are part of the landscape (the culvert beneath the road is the man-made feature). Bridges, man-made structures above the landscape, are removed.

For many years, the source data for topographic raster DEMs were mass points and breaklines (collectively referred to as a DTM) compiled through photogrammetric compilation from stereographic aerial imagery. The DTM is converted into a triangulated irregular network (TIN) surface from which a raster DEM could be generated. Photogrammetric DTMs inherently contain breaklines that clearly define the edges of waterbodies, coastlines, and single- and double-line streams and rivers. These breaklines force the derived DEM to appear, and contours to behave, in specific ways: water surfaces appear flat, roadways are continuous when on the ground, and rivers are continuous under bridge locations; contours follow waterbody banks and cross streams are perpendicular to the centerline.

[Note: DEMs developed solely for orthophoto production may include bridges, because their presence prevents distortion in the image and reduces the amount of postprocessing for corrections of the final orthophotos. These are special-use DEMs and are not relevant to this specification.]

Computer technology allows hydraulic and hydrologic (H&H) modeling to be performed using digital DEM surfaces directly. For these applications, traditional topographic DEMs present a variety of problems that are solved through modification of the DEM surface. The “DEM Users’ Manual” (Maune, 2007) provides the following definitions related to the adjustment of DEM surfaces for hydrologic analyses:

Hydrologically Conditioned (Hydro-Conditioned).—Processing of a DEM or TIN so that the flow of water is continuous across the entire terrain surface, including the removal of all spurious sinks or pits. Whereas “hydrologically-enforced” is relevant to drainage features that are generally mapped, “hydrologically-conditioned” is relevant to the entire land surface and is done so that water flow is continuous across the surface, whether that flow is in a stream channel or not. The purpose for continuous flow is so that relations/links among basins/catchments can be known for large areas. This term is specifically used when describing Elevation Derivatives for National Applications (EDNA), the dataset of NED derivatives made specifically for hydrologic modeling purposes.

Hydrologically Enforced (Hydro-Enforced).—Processing of mapped waterbodies so that lakes and reservoirs are level and so that streams flow downhill. For example, a DEM, TIN or topographic contour dataset with elevations removed from the tops of selected drainage structures (bridges and culverts) so as to depict the terrain under those structures. Hydro-enforcement enables hydrologic and hydraulic (H&H) models to depict water flowing under these structures, rather than appearing in the computer model to be dammed by them because of road deck elevations higher than the water levels. Hydro-enforced TINs also use breaklines along shorelines and stream centerlines, for example, where these breaklines form the edges of TIN triangles along the alignment of drainage features. Shore breaklines for streams would be 3-D breaklines with elevations that decrease as the stream flows downstream; however, shore breaklines for lakes or reservoirs would have the same elevation for the entire shoreline if the water surface is known or assumed to be level throughout. See also the definition for “hydrologically-conditioned” that has a slightly different meaning.

Hydro-enforcement and hydro-conditioning are important and useful modifications of the traditional topographic DEM, but they produce hydrologic surfaces that are fundamentally different at a functional level. Hydrologic surfaces are identical to topographic surfaces in many respects but they differ significantly in specific ways. In a topographic DEM, roadways over culverts are included in the surface as part of the landscape. From a hydrologic perspective, however, these roadways create artificial impediments (“digital dams”) to the drainages and introduce sinks (undrained areas) into the landscape. Similarly, topographic DEMs obviously cannot reflect the drainage routes provided by underground stormwater systems, hence topographic DEM surfaces will invariably include other sinks. For topographic mapping, sinks are of no consequence—it is actually desirable to know their locations—but they can introduce errors into hydrologic modeling results.

As lidar has largely replaced photogrammetric DTMs as the primary source of data for DEMs, a new complication has been introduced. Unlike the DTM, lidar data consist solely of mass points; breaklines are not automatically created during lidar data collection. In spite of the substantially higher density of mass points collected, lidar points alone are limited in their ability to precisely define the boundaries or locations of distinct linear features such as waterbodies, streams, and rivers. The lack of breaklines in the intermediate TIN data structure causes triangulation across waterbodies, producing a water surface filled with irregular, unnatural, and visually unappealing triangulation artifacts. These artifacts are then carried into the derived DEM and ultimately into contours developed from the National Elevation Dataset (NED). The representation of random irregular water surfaces in the NED is wholly unacceptable to the USGS–NGP and to users of the NED and its derivatives.

To achieve the same character and appearance of a traditional topographic DEM (or to develop a hydrologically enforced DEM) from lidar source data, breaklines must be developed separately using other techniques. These breaklines are then integrated with lidar points as a complete DTM or used to modify a DEM previously generated without breaklines.

Hydrologic DEMs usually require flattened water surfaces as well, hence the breaklines required for hydro-flattening the topographic DEM can be equally useful for all DEM types as well (see Note below). Additional breaklines (and lidar point classifications) are needed to efficiently generate hydro-enforced DEMs. If properly attributed, breaklines for all DEM treatments can be stored in a single set of feature classes.

The use of breaklines is the predominant method used for hydro-flattening, though other techniques may exist. The USGS–NGP does not require that breaklines be used for flattening but does require the delivery of breaklines for all flattened waterbodies and any other breaklines developed for each project (see the “Hydro-Flattening” section in the main report and this appendix for additional information).

[Note: Civil engineers and hydrologists may have requirements for the accuracy of water surface elevations. With respect to elevation data, the USGS–NGP’s interest is in accurate and complete representation of land topography, not water surface elevations. Topographic lidar is known to be inconsistent and unreliable in water surface measurements, and water surface elevations fluctuate with tides, rainfall, and changes to man-made controls. It is therefore impractical to assert any accuracy for the water surface elevations in the NED, and the USGS–NGP imposes no requirement for absolute accuracy of water surface elevations in lidar and DEM deliveries.]

References Cited

Maune, D.F., 2007, Definitions, in Digital elevation model technologies and applications—The DEM users’ manual, 2nd edition: Bethesda, Md., American Society for Photogrammetry and Remote Sensing, p. 535–564.