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introduction_definitions_and_data_structure_dem
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Table of Contents
Types of objects
- Continuous fields
- elevation, rainfall
- represented by raster data model
- Discrete objects / features
- lines, points, areas with attributes
- represented by vector data model as geometry with attribute table or object based
- can be also represented by raster data model (streams, roads, landuse??? - we have vector, but whatever)
DEM
- is a digital model or 3D representation of a terrain's surface created from terrain elevation data
- DEM can be represented as a raster (a grid of squares, also known as a heightmap when representing elevation) or as a vector-based triangular irregular network (TIN). The TIN DEM dataset is also referred to as a primary (measured) DEM, whereas the Raster DEM is referred to as a secondary (computed) DEM. The DEM could be acquired through techniques such as photogrammetry, lidar, IfSAR, land surveying, etc.
- The values X, Y are referred to a coordinate system, usually a cartographic projection system
the most important elements
- contour lines
- VIPs (very important points)or singular points
- top of peaks
- bottoms or shinkholes
- structural lines
- define linear elements with altitude values associated to each vertex
- network of ridges, streams
- breaklines
- edges of cliffs
- areas of constant altitude
- lake
- clipping zones
- area what defines the outer limits of DEM
- empty areas, without heigh information
- data error, snow
Mass Points (spot points) and Breaklines
- Mass points are irregularly spaced points, each with an x/y location and a z‐value, typically (but not always) used to form a TIN or Terrain. They are normally generated by automated methods, e.g., by LiDAR or IFSAR scanners or photogrammetric auto‐correlation techniques. When generated by automated methods, mass point spacing and pattern depend on characteristics of the technologies used to acquire the data.
- Breaklines are linear features that may be used to maintain the smoothness or continuity of a surface (soft breaklines) or to describe a change in the smoothness or continuity of a surface, especially at the intersection between two surfaces with distinctly different slopes (hard breaklines)
- Hydro breaklines are commonly used to define the land‐water interface. This is especially relevant with LiDAR because LiDAR elevations on water are unreliable, i.e., sometimes the LiDAR measures the top of the water, sometimes it penetrates somewhat below the water surface, sometimes there is spectral reflectance, and often water absorbs the LiDAR pulse and there is no return. For lakes and double‐line streams, it is common to use delineate hydro breaklines so that all LiDAR points within hydro features are classified to a special hydro class for water.
DEM Mapping of Hydrographic Features
- hydro-enforcement
- Hydro‐enforcement is a modification of the traditional topographic DEM (Hydro‐flattened DEM) that produces hydrologic surfaces that are fundamentally different at a functional level. Hydrologic surfaces are identical to topographic surfaces in many respects, but differ significantly from a topographic DEM (where roadways over culverts are included in the surface as part of the landscape). From the hydrologic perspective, these roadways create artificial impediments (digital “dams”) to the drainages and introduce sinks (un‐drained areas) into the landscape.
- hydro-flattening
- Hydro‐flattening is the process of creating a LiDAR‐derived DEM in which water surfaces appear / behave as they would in traditional topographic DEMs created from photogrammetric digital terrain models (DTMs). A hydro‐flattened DEM is a topographic DEM and should not be confused with hydro‐enforced or hydro conditioned DEMs, which represent hydrologic surfaces.
- hydro-conditioning
surfaces
topographic
- Stereo DTM
- built from masspoints and breaklines
- much coaser resolution than lidar
- most notably, the flat water surfaces
- pure LIDAR
- DEM created only with lidar points
- surface contains extensive triangulation artifacts (“tinning”)
- cause by the absence of lidar returns of the water, breakline contrains that would define buildings, water, etc
- estecically and cartographicaly unacceptable by the most users
- hydro flattened
- Intent is to support the development of a consistent, acceptable character within the NED, suitable for contouring
- Removes the most offensive pure lidar artifacts: those in the water
- constant elevation for waterbodies
- wide streams and rivers are flattened bank-to-bank and forced to flow downhill
- The accuracy is zero
- building voids are too costly to correct
- full breaklines
- removed artifacts from building voids
- refines the delineation of roads, single-line drainages, ridges, bridge crossing, etc…
- requires the development of large number of additional detailed braklines
- a higher quality topographic surface, but significantly more expensive
- not cost effective for NED
hydrologic surface
- hydro enforced
- Surface used by engineers in Hydraulic and Hydrologic (H&H) modeling
- not for traditional mapping (contours, etc…)
- hydro conditioned
- similar to hydro enforced, but with filled sinks
- flow is continuous across the entire surface
data structure
- TIN
- Quadtree
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