Internet Archaeol 1. Gillings & Goodrick. 2.4.3

2.4.3 Co-ordinate Systems

One very important issue in using VRML to implement archaeological models concerns the nature of the co-ordinate system utilised by the format. VRML uses a right-handed, three dimensional Cartesian co-ordinate system. The positive X-axis is to the right, the positive Y-axis up, and the positive Z-axis out towards the viewer.

Figure 7. VRML co-ordinate system This system can cause problems when translating from the standard Easting, Northing, Elevation systems used in archaeological recording, resulting in models which literally stand on their side. This conceptual difference in the structuring of the co-ordinate systems can, however, easily be corrected by rotating the object using a rotated viewing angle or by using a transform to rotate the whole scene.

Figure 8. Rotating 90 degrees around the x-axis to correct for co-ordinate differences.

Figure 9.Camera position and orientation to correct for co-ordinate differences.

What must be realised, however, is the fact that whilst VRML files do exist within a single conceptual 'world' co-ordinate system where the scene is assembled, any transformation undertaken (e.g. movement, rotation or scaling) is referenced not to a single co-ordinate system origin, but to the last referenced co-ordinate system, creating in effect a new local system. Fuller details of this process can be found in the full VRML 1.0 specifications.

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