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.
Mass points are point height measurements; they become nodes in the TIN network. Mass points are the primary input into a TIN and determine the overall shape of the surface.
TINs allow you to model heterogeneous surfaces efficiently by including more mass points in areas where the surface is highly variable and fewer in places where the surface is less variable. The example below shows mass points categorized by the height attribute.
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)
Breaklines, shown below, are lines with or without height measurements. They become sequences of one or more triangle edges. Breaklines typically represent either natural features, such as ridgelines or streams, or built features, such as roadways. There are two kinds of breaklines: hard and soft.
HARDLINES: Hard breaklines represent a discontinuity in the slope of the surface. Streams and road cuts could be included in a TIN as hard breaklines. Hard breaklines capture abrupt changes in a surface and improve the display and analysis of TINs.
SOFTLINES: Soft breaklines allow you to add edges to a TIN to capture linear features that do not alter the local slope of a surface. Study area boundaries could be included in a TIN as soft breaklines to capture their position without affecting the shape of the surface.
Hulls are polygons represent surface features with area—such as lakes—or boundaries (hulls) of separately interpolated areas. Hulls could define the shores of individual islands in an archipelago, or the boundary of a study area.
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.
Polygon surface feature types