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Adding X-Sections

In this lesson we will learn about a new type of scrap, used for adding cross sections to our survey. Each section is drawn in a separate scrap, and these are then incorporated into the survey using a special point type.

First we need to create a new scrap for drawing one of the cross sections. We are going to draw the section at survey station 1.7, so we will identify the scrap using the name of our survey, followed by ‘SX’ for scrap+x-section. As we will have a x-section scrap for every section we draw, we also need to add the station name to make the id for each scrap different to all the others. So set the id field for the new scrap to ‘bpwSX1.7’. This scrap is neither a plan or elevation drawing, so we need to set the projection field to ‘none’. Here is the section we are going to draw, and the scrap information panel.

Note that a x-section scrap only contains a single survey station, so Therion cannot work out the scale from the distance between stations. This is where the scale information in the scrap information panel becomes crucial. Here the values have already been entered correctly because we set the scale when we created the first scrap from the PocketTopo drawing back in lesson 3. All new scraps will be created with the same scale as the last scrap in the same .th2 file, which is why we set it up right at the start. Now we can draw our section. Use wall lines for the outline, and rock-border lines for the boulder. It is not required to add the survey station to the scrap, but doing so enables the position of the station to be shown on the section in the finished survey drawing. It also clearly associated the section with the station in the data.

Now we have drawn the section we need to include it on the survey. We do this by indicating the plane of the section on one of our plan scraps. So select the scrap containing the plan drawing of the part of the cave where our section is located. Now draw a straight line through the cave passage to indicate the plane of the section. Do this by clicking without dragging so that we get a straight line. Change the line type to ‘section’. Make sure the yellow tick on the line points in the direction which our section was drawn facing. In our case this is down towards the bottom of our sketch. We have arrows at either end of the section line on the final drawing, or at both ends. We will choose both here, so we need to add the text ‘-direction both’ to the options field for the line. If we only wanted an arrow at one end we would use ‘-direction begin’ or ‘-direction end’ to say which end of the line the arrow should be drawn.

Now we need to add a point to show where we want the actual x-section to appear on the plan. Add a point near to the end of the section line, and set the point type to ‘section’. We need to specify which section scrap we want to be drawn at this position, so set the options field for the section point to ‘-scrap bpwSX1.7’. This will draw our section on the plan at the position we placed the point. Here is how the plan PDF looks now.

Normally we would draw a section facing in the direction looking up the page, so that the section as drawn on the page has the same left-right orientation as the passage on the plan. Ours is the reverse way round here because in the cave we drew these sections while surveying South (i.e. facing down the page). We can easily fix this in Therion. We just add the option ‘-flip horizontal’ to the section scrap. So select the scrap where the section was actually drawn (bpwSX1.7) and add this option to the options field in the scrap information panel. We also need to reverse the direction of the tick on our section line so that the arrows on the ends now point up the page.

There is one more neat trick we can do with the section line. If we add curve control points to the points at each end of the line and position these just outside the passage walls then the section line will only be drawn up to the positions of these nodes. This is done by ticking the check boxes either side of the word ‘smooth’ for each line point. We only need to tick the box that shows the control handle over the line (rather than the one beyond the end of the line). Then move the control node so that it is close to but just outside the wall as shown here.

The section line and reversed section should now look like this.

We can add more sections to the survey by creating a new scrap for each one and the drawing a section line and section point on the plan scrap where we want them to appear. Try adding a few more to practice what we have covered in this lesson.