Digital Terrain Model

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Digital Terrain Model

• also known as
• digital ground model (DGM)
• digital height model (DHM)
• a method of representing the surface of the earth
  numerically, using three-dimensional coordinates
  of a series of closely spaced points

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Digital Terrain Model

• these coordinates are stored in the database of a
  mapping system and are manipulated by the system
  to produce a contoured plan of the ground
• such a model is extremely useful in the design
  and construction process, as it permits quick and
  accurate determination of the coordinates and
  elevation of any point.

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Digital Terrain Model

• data for the model are acquired by a variety
  methods, e.g.
• radial positioning method
• aerial photogrammetry method or
• satellite positioning method

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Modelling Techniques

• most mapping systems are capable of producing
  plan view of the ground and of contouring the
  plans
• each package although has its own methodology but
  the characteristics of each are fundamentally
  very similar
• 2 methods are commonly used
• regular grid method
• triangular irregular network (TIN)

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Regular Grid Method

• on a point location survey, the points are
  usually surveyed
• along a pre-set grid or in a random fashion

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Regular Grid Method

• for the latter case, the software will
• select a regular grid of squares or rectangles to
  fit over the randomly coordinated points and
• via interpolation, calculates the height of the
  ground at each intersection
• these grid intersections are called nodes

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Regular Grid Method

• Interpolation process
• searches for the nearest 5 or 6 points to each
  node and use their heights to compute a height
  for the node
• when the height of each node has been determined,
  a linear interpolation is carried out along all 4
  sides of the square and contour positions are
  computed (indirect contouring)

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Regular Grid Method

• ambiguities may occur in the linear interpolation
  process
• software solves the problem quite arbitrarily and
  human intervention may sometimes be required

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Triangulated Irregular Network

• a method adopted almost universally in DTM
  systems
• the coordinated points are used as the basis of a
  triangular grid to form the surface of the ground
  model

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Triangulated Irregular Network

• the program uses the coordinated points to create
  a set of triangles which are as near to
  equilateral as possible in the given
  circumstances
• Delaunay Triangulation Method

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Triangulated Irregular Network

• contour positions are determined automatically
  using linear interpolation techniques
• the program first calculates the entry points of
  the contour lines (along the boundaries)

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Triangulated Irregular Network

• the exit points are then calculated along the
  internal sides of the triangles (entry points for
  subsequent triangles)
• finally the interpolated points are joined by
  smooth curves to form the final contour positions

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Triangulated Irregular Network
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Plotting the Plan

• upon completion of either the regular grid method
  or TIN, the plan is displayed on the monitor
  (VDU) and the package awaits further instructions
  from the user
• if contour lines are not required, user can
  simply quits the program OR ELSE the following
  steps are followed

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(1) Formation of boundaries and breaklines

• User must define boundaries and breaklines
• Boundary -
• a set of points forming the perimeter of the
  database area
• user is required to define these points or else
  the program may create contour outside the
  confines of the surveyed area.

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(1) Formation of boundaries and breaklines (cont)

• Breakline -
• a line having a constant grade, e.g. surface of a
  paved road or the top or bottom of a bank
• they have to be defined to avoid making incorrect
  interpolations across them, otherwise the contour
  plan will not reflect these abrupt changes in
  ground surface

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(2) DTM Generation

• form the DTM
• software creates Delaunay triangulation and
  interpolates the positions of the contour lines
  along the triangle sides
• user has no control over this operation

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(3) Define Surface Plotting Parameters

• choose surface plotting parameters from a menu
  which includes
• range of heights in the database
• lowest and highest possible values of contour
  lines using any selected contour interval

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(3) Define Surface Plotting Parameters (cont)

• user may choose to plot any or all of the
  following features
• contours, triangles, boundaries or breaklines
• major and minor contour intervals
• straight or curved contour lines on the plans

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(4) Plot Editor Option

• set the plot parameters
• scale of the plan
• origin of grid
• grid interval
• size of paper and layout drawing
• to the users requirements

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(5) Plotting

• when satisfied that all the plotting parameters
  have been correctly chosen, the user generates
  the plot and the packages causes the plan to be
  drawn ti these specifications either on the
  screen or plotter

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3D Digital Map
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Digital Map
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Alignments Profiles