A THREE - DIMENSIONAL VISUALISATION OF ARTHUR'S SEAT

2.0 Introduction

Arthur's Seat is an ancient volcano situated about one mile from the centre of the city of Edinburgh towards South - East. Its top lies 823 feet above sea level (251 m). It was created in the Carboniferous Period. It consists of five vents, three portions of the cone and many dikes and sills. The most impressive sill is called The Salisbury Crags Sill which spreads along the west edge of the hill. The volcano is built mostly of the lava rocks - basalt and mugearite - but also cementstone, red sandstone, tuff - some with limestone, agglomerate and beds of ash containing bombs and blocks of basalt. During the Pleistocene, Arthur's Seat was covered by an ice-sheet which created the present topography - westward cliffs and easterly slopes [4] - a "crag and tail" feature.

Certain plants found a suitable habitat on the basaltic rocks and so many small animals and birds. They often are a subject of a thorough study of many local scientists [3].

Arthur's Seat is also one of the main recreational areas in Edinburgh with its spectacular view over the whole city, and the Firth of Forth towards the North Sea.

A: Plotting

X, y and z co-ordinates are obtained from an aerial photograph of Arthur's Seat in scale of approximately 1:4000 . A pair of photographs of this region is plotted on an analytical stereoplotter. In addition, several ground control points are collected in a regular manner. GCPs - ground control points are early identifiable feature with a known location which is used to give a geographic reference to a point within a map or an remotely sensed image. They are used in the geometric correction of spatial data sets [1].

B: Scanning

A photograph and a historical map of the area of interest are scanned on the EPSON scanner using a Photoshop LE program. In order to minimize the size of the files not losing any important features, a different resolution is used 300 d.p.i for a photograph. and 200 d.p.i. for a map. Resolution is a value which can be determined by a user to change the quality of an output. It specifies the distance between sample points used to generate the view. It is important to realize that as the surface resolution increases the memory requirements and time of processing increase as well.

The first stage of merging the images involves altering the brightness and contrast due to achieve the best results in a small resolution. Because of the size of the map, it is scanned in six pieces which at the last stage are put together using the canvas size tool. With the image magnified (ZOOM) it is possible to align the pieces of the image accurately.

Both pictures are saved in a TIFF format in two files, p_as and as_1891 .

C: Digitising

Some features from the historical map must be digitized to build the breaklines on the model. The Summagraphics MICROGRID 2 digitizer is used. Arc and label point coverages are produced with ARCEDIT as two separate files; as_polygon and as_1891dig. The as_polygon file consists only of one polygon which covers the whole area of the model and is later used for CLIP function within the "CREATE TIN" process. As_1891dig file includes a road which goes around the hill, the edges of The Salisbury Crag and higher points of the hill. After digitizing, the topology is built in ARCEDIT, a tool supported by ARC/INFO. ARCEDIT can be used to:

• create new coverages by digitising

• correct errors in scanned or digitized maps

• add and modify attribute data

• create and edit INFO files

• annotate features

• rubber sheet coverages for edge matching and improved coordinate accuracy

• snap features and transfer attributes to features in other coverages

4.1.2 Data processing

Processing the data takes place on the UNIX system. The major reason is the availability of ARC/INFO Version 7.0.3 on UNIX as opposed to ARC/INFO Version 6.x on VMS Workstation which is more restricted in terms of the possibilities.

Because all the files created by processes of plotting, scanning and digitizing are on VMS, they all must be exported into the UNIX platform in the first instance.

All files must also use a real co-ordinate system, so before any process is done, the co-ordinate system must be changed from the digitizer units to the appropriate coordinate system - Greatbritain_Grid where units are meters.

ARC/INFO provides two tools REGISTER and RECTIFY to complete this task.

A: Registration

This process "initiates an interactive program that allows the user to georeference an image"[7]. This means that from both the image p_as and a coverage data as_1891dig a series of links is added that joins the image locations to their corresponding map coordinates. The links are the Ground Control Points (see 4.1.1 A). Once an acceptable registration is established, the transformation parameters can be written to an ASCII file.

B: Rectification

The two images - a historical map and a photograph before they are clipped together - must be subject to a process of rectification. The process of rectification is important as it removes the distortions in the imagery caused by the motion of the Earth, curvature of the Earth, motion of the plane, instrument errors and panoramic distortions. The photograph is rectified in ARC/INFO using p_as file and as_1891dig file as a reference files.

4.2 GIS Environment

4.2.1 ARCTIN

A Triangulated Irregular Network (TIN) is a set of adjacent non overlapping triangles computed from irregularly spaced points with x, y coordinates and z values. It is the surface modeling package used to create, store, analyze and display surface information. The tin models stores the topological relationship between triangles and their adjacent neighbours. The TIN software package consists of specialized ARC and ARCPLOT commands and AMLs which are integrated with the entire ARC/INFO [6].

ARC/INFO is based on a command line interface. The commands can not be saved so it is necessary to edit and build up an AML procedure (see Appendix E)..

A: Building a TIN - using CREATE TIN

TIN surface models can be created from a wide variety of data sources, e.g. contour maps existing tins, lattices. For the purpose of the project, data are collected from ARC/INFO point, line, and polygon coverages and from photogrammetry stereoplotter data. Data are checked on the accuracy and completeness in case there are some missing or wrong. If this is the case, data sets must be completed and corrected. To do so it is necessary to collect more data for some of the areas of the model. For example, point data or arcs with z-values are collected on imposed places. When the arc coverage is used, the process of interpolation is stopped along the linear features like edges, ridges or borders of the lakes, to enhance the appearance of the surface in the desired location. This type of the line is called a hard breakline [6].

Before producing a view it is necessary to specify the viewing environment. This step is divided into three parts: establishing the surface, orientating the view and specifying where and how the surface view will be drawn. Many of the commands have default values but for the project's purpose it is better to establish them yourself.

The next step in building tin is a surface draping. When a surface view is already generated, various ARC/INFO geographic data models can be draped. These include, e.g.

• meshes of lines at specified intervals
• point, line, or polygon coverages
• images
• contour lines
• views shaded by elevation change and sun illumination angles
• graphic files
• GRID

B: Exporting TIN into Vista Pro

To present the model with some additional landscape features, TIN must be exported into Vista Pro package. This process caused several problems so a special conversion program had to be written.

Firstly a lattice is created then it is converted into a DEM USGS format to be readable by Vista Pro. The commands are following: TINLATTICE - converts TIN into a LATTICE and DEMUSGS - to convert a LATTICE into a DEM with USGS format.

4.2.2 Vista Pro

Vista Pro is a three-dimensional landscape simulation program used by artists and designers as well as by teachers and those interested in the study of topography [5].

It simulates real and imagery landscapes but also allows extraction of some data from the DEM files. Creating the simulation of the terrain gives the opportunity to add some weather effects like clouds - with the percentage of the cloud cover or cloud color and haze being specifying. The color control palette can also be used to change the color of the sky, cliffs, bare earth, vegetation, water and snow. Trees or buildings can be drawn with marking the tree level.

The use of this package can be almost completely automated through the Vista Pro's scripting language. The most common use of scripts is the animation. It can be created with the MakePath Flight Director module. MakePath creates smooth motion paths based on the control points that are placed on the map. The user has an option to choose a vehicle e.g. jet to fly-through the image, dune buggy to run over the landscape.

4.3 Implementation of the model on the WWW