Elevations

From Fs_wiki

A Theory for elevations

This feature is not yet part of Tunnel, and is at purely a design phase.

The main idea is that to have an intermediate structure that is not the same as the flattened drawing of the elevation (extended or projected), but which can be considered a flattened drawing in pieces. Cross sections as well as the elevation survey both belong as parts of the same structure.

Imagine a the plan of a simple cave with a few interconnective passages. Draw a network of paths down the middle of the passages which join at the junctions. These are like the linear paths you walk or crawl along when you explore the cave. They are not the same as the network of the centreline, which tends to go in artificially straight lines and or zig-zags to convenient survey stations.

Imagine you plant a vertical pole at each junction and hang an imaginary paper curtain along each of the paths, gluing the ends to the poles.

When you flatten the curtain against the page, its horizontal length is the horizontal length of the path. The vertical heights of the floor and ceiling are unambiguous; they are the height of the floor and ceiling at each point on the path. It will be possible to edit this elevation drawing on the screen while at the same time being able to see the plan, so that as your cursor moves right and left across the sketch, it can be shown at the relevant position on the path. This means you will be able to make your pitch drop-offs line up correctly with pitch boundaries on the plan.

Passage cross-sections are simply built upon the straight line path connecting one wall to another. Where they intersect the main elevation structure, the floor and ceiling should coincide.

Pitch cross-sections don't fit into this structure, although they could be formalized as a series of pitch holes in the plan.

The resulting structure is something that could be printed out in pieces and assembled using scissors and tape. Flattening this into a single large image can be done in many ways and depends on the style, the orientation of the cave, and what is chosen to be left out. There could be more than one flattened presentation of the same elevation structure.

The simplest special case is as follows.

Imagine a long stream passage where there is a short oxbow connecting across a meander. The length of the oxbow is XX% of the length of the stream it bypasses.

If XX is above 90%, we could certainly stretch the oxbow or shorten the stream passage to make the two super-imposed passage drawings connect. It would depend whether the main route followed the stream, or cut through the oxbow, which way round gave a more representative picture.

If XX is below 50%, we could cut the oxbow in the middle and leave a gap that said that the two ends are supposed to join. Alternatively, if the oxbow route was the main route, we could introduce a double fold into the stream passage to use up the slack. Double-folds are not always representative since a meander normally looks like a single fold. A third alternative is to include a fold in both the stream passage and the oxbow, but at different distances from the join, so that the entire underlying direction of the elevation changes from going left-to-right to right-to-left.

Extended elevations are subject to many artistic choices. With a projected elevation, the distortion of each portion of the elevation structure is defined exactly according to the meandering of the path. There is then a considerable amount of clearing up to do when the path is going near parallel to the direction of projection, and passage cross sections may need to be substituted in these cases.

Please post any further questions or points for clarification here.