Sometimes it is nice to see a cross section through several passages at once, to see how they relate to each other. Therion draws cross sections one at a time, and does not appear to have a dedicated way to draw multiple cross sections at once, positioned in the correct place relative to each other.

To see how to draw a single cross section, see the Therion tutorial chapter on Cross sections, and Footleg's tutorial “Lesson x: Adding X­Sections”.

The simplest method is to draw these cross sections normally, and move them into positions relative to each other, and draw a border around them to show that they are grouped together. However, this can be difficult to achieve, as the positions relative to each other need to be manually calculated or approximated.

For a more complete and accurate approach, read on.

Decide where you want to draw your cross section. Select a station on each passage that the cross section will intersect. It is quite possible that these will be in different survey data files. Not to worry, just select good representative stations. These do not have to be perfectly below each other, and they do not have to perfectly intersect the cross section line. Just select a station where you like the cross section, somewhere near where you want your cross section line to be drawn.

Work out what bearing your cross section will be facing. For example, a cross section through passages all running approximately north-to-south on a bearing of 355 degrees, would probably have its cross section facing 355 degrees, while its cross section line would be drawn east-to-west on a bearing of 85 degrees. 355 is the important number here.

This is the laborious bit.

If your cave is extremely simple (then why are you doing this sort of cross section?!) you might just be able to render the entire survey on a projection, and be able to visually identify the stations and splays you are interested in. However, it is quite likely that there will be too many stations and splays to identify the right one, so this section assumes that you will need to render just the relevant stations and splays.

Most often, by the time you want to draw multiple cross sections at once, you are dealing with data in multiple separate survey files. That is OK, and it doesn't really change the approach.

Create a new .th file which will contain the data representing only those stations. Give it a “survey somename” that you will use to reference it, which must not clash with the existing surveys. Copy the survey station's splay legs from their original survey data .th file, into this new file. Yes, it means some data is duplicated, but they are normally only splay legs, so they do not affect the cave's measured length. If you also need to copy survey legs between stations (you don't normally need to), use “flags duplicate”-“flags not duplicate” around them to make sure they do not add to the overall cave length.

You will also need to copy any configurations that influence the data. This will include any instrument calibrations, and any dates which affect declination. Since you will probably be copying stations from multiple surveys, you might need to create multiple “centreline” sections in the survey, one for each station, containing the appropriate calibration and splay data. If all the station names are different (lucky you!), you do not need to rename them. If two of them are the same by chance, you can rename one of them, eg. 7a. Alternatively, you can put a “survey passage2” around each of them, with an appropriate subsurvey name.

Open the master survey file that contains all of the passages that you will be working with. In a simple cave with just one data file (unlikely!), this will be the main data file. Tell it to “input somename.th” - your new data file. Then use “equate 7@somename 7@realsurvey” to equate the points in your new cross section survey, with their real points in the main data. Now all of the splays in your new data file should perfectly match the existing ones, and be indistinguishable from them in the rendered survey.

Create a new “thconfig” file, which will project the new survey data on the correct bearing - 355 for the demonstration.

source "master.th"
select mysurvey@master
export map -projection [elevation 355] -fmt xvi -output "section2.xvi"

Run it to export the XVI file.

The easiest way to use this file, is to create a new .th2 file for each set of cross sections that you are creating. In XTherion, open the map editor (F2). Create a new file and save it somewhere appropriate. Edit menu - Insert image, and select the XVI file that you exported. Draw your cross sections the same way as you normally would draw a single cross section. This does mean that you will be creating new cross sections, not using an existing sketch or cross section as a drawing aid. More details about that later. Tip; you can even set the scale of your cross sections to something different than the main survey, just by setting the scrap's scale to the wrong values - eg. a 10 metre x 10 metre scale bar could have its “real scale points” set to 20×20 instead.

Note that Therion may become confused if cross sections do not have walls on all sides of them (eg. if some are open to the surface). If this is the case, create a wall line to close the wall loop, and set it to “-visibility off”.

You may want to draw a border around the entire set of cross sections, to show that they are related spatially to each other. A regular “border” line with “-clip off” may be used for this purpose.

You do not need to mark the stations in the cross sections. If you decide to do so anyway, you can use “-name” to reference the real stations, but note that you will need to reference them by their complete name, taking into account whichever survey or subsurvey they are defined in.

A significant failing of this approach is that it cannot colour the passages correctly when using coloured scraps (eg. coloured by altitude). For this reason, you will probably want to render the cross section scraps without any colours at all.

If you do not plan on marking the “station” points in the cross section, it does not matter which at which level you “input” the .th2 file that you create the cross sections in. This is the easiest way, so it can just be “input” in any of the existing .th files which are being used to draw the survey. In your cross section's .th2 file, create a new scrap in the file, with its scale set properly. Give it a single “station” point linked using “-name” to a real point in the survey which it is going to be “input” into. Give it a “section” point for it to draw the section relative to, and set the “-scrap” option to point to the cross section scrap. Use “input” to add it to the relevant survey, and add the new scrap (not the cross section scrap) into your “map” for that survey.

It should now be rendered into the final survey, positioned relative to the station you selected.

If you are marking multiple stations in the cross sections, you need to input the .th2 file into the master survey file instead, since that is the level where the stations can all be referenced.

This is not so easy.

Existing cross section drawings are in their own scraps, and these cannot be simply positioned relative to each other (but if there were a way, it could massively simplify this whole process!). Normally, any existing cross section will have been drawn in its existing .th2 file.

There are two approaches. One is to leave them in their own files, and the other is to copy them into a combined file.

In the .th2 file that uses the projected XVI image, create a new scrap in the same projection as the main survey (usually plan). Include this scrap in your map. Give it a single “station” point to show what the cross sections should be positioned relative to - this will be somewhere outside the plotted points and splays. Draw any borders around the station markers in the XVI file, which you want to surround the group of cross sections. Now add “section” points on or near the station markers of the XVI file, telling it to use the right “-scrap” for each station.

Note though that Therion positions cross section scraps relative to the points and linepoints within that scrap, *not* relative to the station within it. Normally, stations are not in the exact centre - horizontally and vertically - of their passage. Therefore you will normally need to adjust the position of your “section” points so that they are slightly offset from the station, causing the passages to be positioned more correctly compared with their station. This is a manual process, and you may need to temporarily add a visible point type on the station until you have finished adjusting the position.

Use a text editor to copy the lines and points from the existing cross section's scrap, into the newly created multiple-cross-section scrap (which will normally be in different .th2 files). Move the lines and points into the correct positions in XTherion's map editor, using the existing section's “station” and the new “station” position as references for the move-from and move-to.

Note that this relies on all the existing cross sections to have been drawn facing the same direction. If not, they will need to be manually redrawn facing the correct direction.