3D Terrain Modelling

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G. Argyris, I. Kapageridis and A. Triantafyllou

Technological Educational Institute of Western Macedonia,

Department of Geotechnology and Environmental

Engineering, Greece


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Introduction

A number of important studies, ongoingenvironment monitoring efforts and mining androad construction projects in the lignite basedelectrical power production area of Amyntaio

Ptolemais in North West Greece require anaccurate and up to date model of the topography.

In recognition of the importance of such a model,a project was initiated by the Laboratory ofMining Information Technology and GIS

Applicationsin the Department ofGeotechnology and Environmental Engineeringof the Technological Educational Institute ofWestern Macedonia.


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Project Aims

The project aims at the generation of a

large terrain model of the wider area of

North-West Greece in the EGSA87

coordinate system that can be used by avariety of computer software (CAD, GIS,

Modelling) and hardware (PC, PNA, etc).

This model will be used as the basis for a

number of studies including air pollutionmodeling and resource/reserves

calculations for lignite deposits in the area.


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Map Scanning

The data collection process involvedscanning and digitising existing maps,produced by the Hellenic MilitaryGeographical Service, using specialised

hardware and software. A professional drum scanner, the Kip

Starprint 6000, was used for mapscanning.

The scan resolution was 400 DPI resultingin each map occupying 200 to 250 Mb ofdisk space. The scanned maps werestored in Tagged Image File Format (TIFF).


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Scanned Raster Images

Original Bitonal


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Images Before Insertion

Bitonal Inverted


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Image Insertion to Real World

Coordinates

The raster maps were inserted into anAutoCAD / Raster Design drawing.

A Match operation was performed, which

applies linear correlation, adjustinginsertion point, scale and rotation asnecessary, to align the raster maps withknown points in the vector drawing.

Two source points were selected on theraster map and two correspondingdestination real world coordinate points inthe drawing.


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Georeferencing with Rubbersheeting

The point matching procedure is notsufficient to bring all points on the map totheir true real world coordinates.

This was particularly important in this

project as the source maps were in theHATT projection system which is notorthogonal and their coordinate grid linesare not parallel to each other.

In order to bring the entire raster map tothe correct coordinates, a very usefulRaster Design tool was used calledRubbersheeting.


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Rubbersheeting in Detail

Rubbersheeting uses a set of matched controlpoints, consisting of source points in the rastermap and destination points in the drawing.

These points can be specified by establishing

a grid of destination points, to which thesource points are matched.

The grid of destination points represented thepoints of intersection between actualcoordinate grid lines shown in each map.

A total of 588 grid points per map werematched to control points (28 horizontal x 21vertical coordinate grid lines).


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Rubbersheet control points


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3D Contour Tracing

Major and minor contours from the raster maps weretraced using a special contour follower tool in RasterDesign.

This tool provides a semi-automated procedure forcontour tracing in three dimensions giving more controlto the user over other fully automatic raster to vector

conversion packages. The user adjusts parameters such as major and minor

contour interval, layer allocation, and digitized contourline properties.

Contours are traced one by one and once a contour is

completely traced the user verifies or modifies thecalculated contour elevation.

The process is fairly time consuming but the end result isa set of very accurate, high resolution 3D contour lines.


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Contour Parameters & Tracing


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Hatt to EGSA87 Transformation

The contours from Raster Designwere exported to a DXF file and thenimported into Vulcan 3D software for

further processing and modeling. 3D transformation matrices were

produced for each map using eightpoints, four original co-ordinate triples

and four transformed co-ordinate triples. The transformed contour layers were

saved into a vector database in Vulcan.


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Traced 3D Contours


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Optimised Triangulation Modelling

The EGSA87 contour lines from allmaps were used to generate anoptimized Delaunay triangulation model.

Special filtering of long-edged andsmall-angle triangles was performedalong the outer boundaries of the modelto remove triangles linking very distant

points. The algorithm was optimized by the

calculation of spur s t r ings.


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Spur Strings for Flat Spot Elimination

Spur strings represent thenatural breaklines thatform at the top of the hillsand the bottom of valleys.

Spur strings are

incorporated into thetriangulation model tokeep these areas frombeing flat.

This forces the

triangulation to triangulatebetween contour lines inplaces where it maytriangulate betweenpoints on the samecontour line

Without spur strings

With spur strings


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3D View of Optimized

Triangulation Model

The complete triangulation model of the

area currently covered included 338,458

nodes and 533,105 triangle faces. The final model of the entire North West

Greece is estimated that, once

completed, will consist of over 2 million

triangle faces.


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Incorporation of Surface Workings

Data from several vector maps collected byconventional surveying and digitalphotogrammetry of existing lignite mine pitswere incorporated into the digitized data togenerate a more realistic and updated viewof the area.

All vector maps were converted to EGSA87before their insertion to the naturaltopography contours.

Each excavation was modeled using thestandard Delaunay triangulation algorithm.


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Aerial and Satellite Imagery

Registration Several images of the lignite production area

of Amyntaio and Ptolemais were collectedfrom Google Earth, georeferenced inEGSA87 coordinates and draped on thetriangulation model using Vulcan.

These images are low resolution and notalways up to date and were used only to showthe potential of image draping.

Georeferencing these images included

nominating control points that could beapproximately located on both the images andthe underlying triangulation model.


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Triangulation Model


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Model with Raster Image in Plan View


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Contours in 3D


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Conclusions

The lignite production area of Amyntaio -Ptolemais is constantly monitored for its airpollution levels and explored for new ligniteproduction areas.

The availability of a complete terrain model

that can be used by these and otherapplications is crucial. The project described inthis paper was realized to cover this need.

The procedure described involves different

software packages that work well together toproduce an accurate, easy to update modelthat can be imported and used by other CAD,GIS and modeling applications.


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Thank You for Your Attention

The data and model can be downloaded from:

http://airlab.teikoz.gr/geope/labs/mineitlab.htm

3D Terrain Modelling

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