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 Terrain Modeling

     Terrain Modeling

     Elevation Data

A terrain surface can be characterized in many ways depending on the interest and perspective of the final user.

     Digital Elevation Model (DEM)

     A digital elevation model (DEM) contains elevations at points arranged in a raster data structure, a regularly spaced x, y grid, where the intervals of Δx and Δy are normally in linear units or geographic units. The z-values in a DEM represent the height of the terrain, relative to a specific vertical datum and void of vegetation or manmade structures such as buildings, bridges, walls etc.The DEM, because it is a raster data structure similar to a digital image, is an efficient format for storage, analysis, rendering, and visualization.
Automatic generated DEM

     Digital Terrain Model (DTM)

     A digital terrain model (DTM) data structure is also made up of x,y points with z-values representing elevations, but unlike the DEM, these may be irregularly or randomly spaced mass points. Direct observations of elevation at a particular location can be incorporated without interpolation, and the density of points can be adjusted so as best to characterize the actual terrain. Fewer points can describe very flat or evenly sloping ground; more points can be captured to describe very complicated terrain. In addition to mass points, the DTM data structure often incorporates breaklines (further defined below) to retain abrupt linear features in the model. A DTM is often more expensive and time consuming to collect than a DEM, but is considered technically superior for most engineering analyses because it retains natural features of the terrain.


     Triangulated Irregular Network (TIN)

     A triangulated irregular network (TIN) represents terrain with adjacent, non-overlapping triangular surfaces. A TIN is a vector data structure generated from the mass points and breaklines in a DTM. TINs also preserve abrupt linear features and are excellent for calculations of slope, aspect, and surface area and for automated generation of topographic contours, which are all important functions to the flood study engineering. Storage formats for TINs are more complex than either DEMs or DTMs, because the relationship of elevation points and triangular surfaces must be preserved within the data structure.
   Tin from automatic generated DEM

     Contour Lines

     Contours are isolated lines that connect points of equal elevation based on a vertical datum; they are the traditional method for representing a three-dimensional surface on a two-dimensional map. Contours lines are also excellent for map interpretation, but inferior to DEMs, DTMs, and TINs for computer display and analysis.
      1 m interval contours lines

     Elevation points

     An elevation point is the vertical distance of a point or object above or below a reference surface or datum (generally mean sea level); elevation refers to the vertical height of land.

DEM/DTM inputs automatically generated will not produce a proper representation of the terrain. So they need to be edited by modifying, adding or deleting points and by capturing breaklines wherever slope changes are visible.

Gisbox conforms to the proper usage of hard and soft breaklines in all its DTM/DEM editing assignments. This produces a very good terrain representation resulting in superior quality products, for topographical mapping as well as for orthophoto generation.