digitalterrainmodelling: 1 digital terrain model also known as –digital ground model (dgm) &...

DigitalTerrainModelling: 1

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



• 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.



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


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)


Regular Grid Method

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


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


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)


Regular Grid Method

• ambiguities may occur in the linear interpolation process

• software solves the problem quite arbitrarily and human intervention may sometimes be required


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



• 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



• contour positions are determined automatically using linear interpolation techniques

• the program first calculates the entry points of the contour lines (along the boundaries)



• 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


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:


(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.


(1) Formation of boundaries and breaklines

(con’t)

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


(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


(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


(3) Define Surface Plotting Parameters

(con’t)

• 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


(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 user’s requirements


(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


3D Digital Map


Digital Map


Alignments & Profiles

digitalterrainmodelling: 1 digital terrain model also known as –digital ground model (dgm) &...

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