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NOTE: A much improved version of this training tutorial is in the process of being written, and a fairly advanced draft is now available from


A guide to the content of the cave surveying course run by the Cave Surveying Group in Yorkshire, November 2011

 This guide is based on survey data recorded during a course run at Bull Pot Farm in Yorkshire using a DistoX and PocketTopo running on a PDA. The survey was done by course members in ‘Bull Pot of the Witches’ and covers the chamber at the bottom of the entrance shaft plus a bit of passage coming up into the chamber from underneath the floor of the main chamber. This provides an example of overlapping passages requiring two scraps in Therion to draw the plan view of the survey. These notes were written by Footleg based on a course structure designed by Andrew Atkinson. This guide goes into more detail than could be covered on the course and extends the course beyond what is usually covered in a weekend. (The eventual plan is to include elevations, but that part is not written at this stage).

The raw data file saved on the PDA is in a file named ‘’ and contains all the data that is required to follow this guide.You can download the data file and examples for each lesson in this course guide here:

Therion Training Data

You will also need a copy of the following applications:

  • PocketTopo (v1.3 or later can be run on a Windows computer or Windows Mobile v6 PDA)
  • Survex (Cave data processing application and viewer)
  • Therion (Cave survey drawing and cave data management application with viewer)

To create the landscape overlay for the survey you will also require:

This course guide was written using the following software versions:

Don’t be put off by the Therion version being listed as the ‘unstable’ version. It just means it contains the latest features which have not had all that much testing yet. But it works fine and contains some features that we want for this course.

Exporting Survey Data from a PDA

The survey data and drawings done underground on the PDA are all saved in a PocketTopo file with the file extension ‘.top’. We need to export this data in a format which captures all the information in plain text files which can be read on any computer. These will be used to import the data into Therion, but also to archive the data in a format that will be readable to people in the future without needing PocketTopo and a computer which can run it.

Open the ‘’ file in PocketTopo (using the File/Open menu). You should see the numbers representing the survey data, and also be able to view the sketches in PocketTopo.

Select File/Export/Text from the menu. On the window that opens, change the location to one of your memory cards (rather than main memory), chose a folder on your memory card and enter ‘bpw’ in the name field. Then click Save. This will save a file named ‘bpw.txt’ to your memory card. This file contains all the survey data (the numbers), including the trip date, declination, and any comments you added.

Select File/Export/Therion from the menu. On the window that opens, change the location to one of your memory cards (rather than main memory), chose a folder on your memory card and enter ‘bpwth’ in the name field. Then click Save. This will save a file named ‘bpwth.txt’ to your memory card. This file contains both the data and drawings in a format which can be imported into the Therion software.

We should now have all our data saved onto the memory card on the PDA. (This is assuming you kept the original .top file on the memory card during the survey trip. It is recommended to do this as it means you can still get your data out if your PDA was damaged while underground. You can still remove the memory card from a dead PDA and read it on another computer. But you will not be able to read from the built in memory of the PDA if the device got broken).

File Organisation

The following procedures are recommendations rather than a required way of working. The important point is to preserve your raw data files because these represent the hard work of the survey done underground. Everything else can be regenerated from these files if you have not lost or edited them. So we are going to put them in their own folder to keep them safe while we work on the drawings. Create a main folder for your project. We called ours ‘Bull Pot’. Inside this create a second folder named ‘PocketTopo’ and copy the files from your PDA memory card into this PocketTopo folder on your computer. You should have the following files in there:

These are the original data files and the intention is that these are never edited directly. We are backing them up to keep them safe. We are now ready to start learning Therion.

Creating a Therion Project

In this lesson we are going to create a new Therion project, import our survey data and create a model of the cave centre-line.

Run the XTherion application. This is the Therion editor. There are three main views in the editor. The text editor, the map editor and the compiler. You can switch between these by selecting the appropriate items from the Window menu, or using the buttons on the toolbar near the top of the application, or by pressing one of the function keys F1, F2 or F3.

Starting with the Text Editor (press F1 to switch to this view), we are going to create the main project file for our survey. Select ‘File/New’ from the file menu to create a new file, then ‘File/Save As’ to save the empty file. Name this file ‘’ and save it in the main folder we created for our project (the folder we named ‘Bull Pot’).

We now need to define our cave survey in the .th file. Enter the following text in the editor window:

  survey bpw


What this does is define a cave survey named ‘bpw’ into which we need to put our survey data. We can import this directly from the therion export file that we exported from PocketTopo. Click in between the ‘survey’ and ‘endsurvey’ lines to place the cursor there, and then select ‘File/Import’ from the menu. Select the therion export file we created in the previous lesson (bpwth.txt). When this file is imported, you should see a block of centre-line data has been put between the ‘survey’ and ‘endsurvey’ lines. This data block starts with ‘centreline’ and ends with ‘endcentreline’. Save the file (File/Save).

We now have everything in our file that we need to build our first model of the cave. To do this we need to create a file in the compiler window to describe what sort of output we want to create.

Compiling the Therion Project

Switch to the compiler window (press F3 or use the toolbar button or menus to switch views). We need to create a configuration file to tell Therion what we want it to do with our cave survey data. Create a new file using the ‘File/New’ menu. The default filename will be shown as ‘thconfig’. Change this to ‘thconfig.thc’ and save it in the project folder (which we called ‘Bull Pot’).

You should now be able to click in the top pane on the compiler window and type into the window. Enter the following text:


export model -fmt survex -o bpwth.3d

This pair of instructions are first telling Therion the name of our project file to process (‘’) and then telling it to generate a model of the cave. We are asking for the format of the model to be a Survex 3D file, and giving the filename for the output file as ‘bpwth.3d’. This filename can be anything you like, but here we are using a convention of naming it the same as our project ‘bpw’ plus a ‘th’ to indicate that this file was generated from Therion and not directly from Survex.

We have now provided all the information that Therion needs to generate some output. Click the compile button (looks like a black cog wheel in the toolbar) or press F9. If everything is in good order then you should see a green OK bar in the pane on the right, and a load of output log text in the lower pane of the compiler window. You should also find a new file named ‘bpwth.3d’ has appeared in your project directory. Double click to open this file in Aven (the Survex cave viewer) and you can rotate and zoom your model to see your cave. You can also launch the Therion viewer ‘Loch’ from the Therion group on your start menu and open the .3d file in that.

Adding additional information to the survey data

The essential numerical data for the cave centre-line was automatically imported from PocketTopo, but it is generally good practice to include some additional data to record who the surveyors were. It is also helpful to label cave entrances as these can then be highlighted on the models. If you switch back to the text editor view (F1) and look at the centre-line data block you will see that the date of the survey has already been imported. Below the date is a data line detailing the order that the numerical data is provided in. We are going to insert some more information between these two lines. So click to place the cursor at the end of the date line and press the ‘Enter’ key a couple of times to create a couple of blank lines after the date. The enter the following lines:

  team "Joe Bloggs" instruments
team "Jane Bloggs" notes
team "Tim Bloggs" dog

These lines provide the information on the surveyors. The person reading the instruments, the person recording/drawing the survey on the PDA (notes) and the person who did all the running around setting stations etc. (the dog). For traditional tape measure surveys you can also specify a team member followed by the word ‘tape’. There are other roles and also many other pieces of information you can provide (what types of instruments were used, author, copyright information, etc.). Take a look in the Therion Book for details of all the supported roles and other information that can be added in a centreline data block (look for ‘centreline’ in the table of contents). Note that the team members names are only allowed to contain one space character, so use the forward slash '/' character for additional spaces or your project will generate an error when you try to compile it. (e.g. "Joe Horatio/Bloggs" will appear on the final survey as "Joe Horatio Bloggs".)

We also want to indicate which survey stations are at entrances to the cave. In our case none of our stations were at a real entrance, but for demonstration purposes we will pretend that station 1.8 was at the cave entrance. We can indicate this with the following line:

  station 1.8 "main ent." entrance

Note that we have to provide a comment or this line does not work. We have put the comment ‘main ent.’ in quotes. If you do not want a comment then you still need the double quotes, but can enter them without any text between them.

Now recompile the project (F9 or click the black cog wheel toolbar button). Open the model in the viewer again and now you can turn on the option to view entrances and will see the entrance highlighted in the viewer.

It is also very useful to give the location of the cave, because it can then be positioned with respect to other locations in the world. This is required if you want to position the cave on Google Earth, or under a surface landscape overlay, or in the same model as other nearby caves. We can specify the position of a station using the fix command. We will deal with coordinate systems later when we build a KML model to view in Google Earth. But for now we will just give the coordinates for the position of one of our stations. The entrance station makes most sense because we can position it using a GPS. Until we specify otherwise, the units of any coordinates we use are assumed to be metres. So we will enter the position using the OS Grid reference within the OS grid 10km square where our cave is located, based on the GPS position and altitude of the entrance station. The order has to be Easting, Northing, Elevation. Our cave entrance is at approximately the OS grid location SD 662 812 and at around 290m above sea level. So to specify this grid location in metres we enter the following line:

  fix 1.8 66200 81200 290

Now if we recompile our project, our Survex model will give us the correct grid coordinates and elevation for any station when we view it in the Aven viewer. More importantly if we were to include a second cave in the same model then both caves would be positioned in the correct location relative to one another so we could see how far apart the nearest parts of each cave are to one another.

Therion can output a variety of different models. We are going to generate one more to complete this lesson. The default model for Therion is the lox model. This is the model which the Loch viewer is mainly designed for. So we need to switch to the compiler window again and enter on the following line to instruct the compiler to build a lox model:

  export model -o bpw.lox

Now if we compile the project again we should find a new output file ‘bpw.lox’. Double click on this file in Windows Explorer to open it in the Loch viewer. We can view a solid 3D model now by turning on some of the options in the Loch viewer. Select ‘Tools/Options’ from the menus in the Loch viewer and select the option to ‘Extrapolate only files without walls information’. Click OK and then reload the lox file in the viewer (just click the toolbar button near the left end of the toolbar showing twin green arrows). Now we should see solid tubes representing our cave passages. These are derived from the survey leg lengths at this stage. But later as we develop our drawing of the cave the model will improve as we provide more information.

The final project files for this lesson have been copied into the Lesson 2 folder of the supplied project data zip file. So if you are unsure about anything or find your project is not working then use these files as a reference to how the complete project should look at this stage. In the next lesson we will start drawing the plan sketch for our cave survey.

Drawing the plan outline for the Chamber

In this lesson we are going to create a new sketch file in our Therion project by importing our pocket topo plan drawing into the Therion editor, and linking it to our cave centre-line.

With the project files created in the previous lesson open in the XTherion application, we are now going to import the plan sketch from the files we exported from PocketTopo. Open the Map Editor (press F2 to switch to this view), and create a new file using the ‘File/New’ menu. Save this file in the project folder (which we called ‘Bull Pot’). The file type for sketches is .th2, so we will name this file ‘bpw.th2’.

Now from the ‘Edit’ menu, select ‘Insert Image’. In the dialog window which appears, change the ‘file types to show’ from ‘Pictures’ to ‘PocketTopo therion export’ in the list selector near the bottom right of the dialog box. Now navigate to the folder where we put our files exported from PocketTopo, select the ‘bpwth.txt’ file and click ‘Open’. You will be prompted to confirm some ‘XVI Properties’. Make sure that ‘Plan’ is selected because we want to import the plan sketch. Leave the scale (1:200), resolution (200dpi) and grid spacing (1.0m) settings at the default values as given here in brackets and click ‘OK’. You should now see the plan sketch drawing you did in PocketTopo displayed in the map editor.

Drawings in Therion are made up of parts called scraps. For very simple caves it may be possible to draw the entire plan survey in a single scrap, but in most cases more than one scrap will be needed. There are some basic rules to follow in deciding how to break a survey up into scraps.

  1. No scrap can overlap itself. So if your cave contains any passage which lies over or under another passage then you need to draw each of the overlapping passages in their own scrap.
  2. Scraps cannot be too big. How big is too big? Well you find out if you try to draw too much cave on one scrap because Therion will generate an error when you try to generate the survey. If this happens then you can always split the drawing up into more than one scrap at that point. So there is no need to worry about this problem now. Our survey for this tutorial will not get that big.
  3. For features such as colour by altitude, the entire scrap will be shaded the same colour, so it is a good idea to start a new scrap if there is a significant change in height in the part of the cave you are drawing. A good rule is to start a new scrap if the height has changed by over 5 metres from one part of your scrap to another. But use your judgement for what works best for your particular cave. Try to join scraps at places where there is no passage detail other than walls because Therion will do the best job then of creating a seamless join between them.

We start by creating a new scrap to draw the cave walls for the main chamber in our sketch. Down the right hand side of the map editor window are a number of panels with blue bars separating them. Scroll to the top of the screen and click on the ‘Objects’ panel heading to open that panel. At this stage if should just contain a single line of text saying ‘end of file’. This panel shows all the objects in our drawing, including scraps. So our first job is to create a new scrap. You can do this by clicking on the ‘Insert new scrap’ icon in the toolbar above this panel, or using the shortcut key combination ‘CTRL + r’.

The focus in the panels on the right should jump to the ‘Scraps’ panel when you create a scrap. Here we can give it an identifying name in the ‘id’ box. By default it will be named something like ‘scrap1’. It will really help later to use a system for naming scraps. We are drawing a plan in this scrap, so start the name with the name of the cave (or part of the cave for larger surveys). End the name with ‘S’ (for scrap) followed by ‘P’ (for plan), finishing with a number for this scrap. So we will name our first plan scrap for Bull Pot of the Witches ‘bpwSP1’. Finally we need to tell Therion what type of scrap this is. So select ‘plan’ in the ‘projection’ box below the ‘id’ field.

Before we start drawing, it will help to tell Therion what the scale of our sketch is. Therion can work this out for scraps which contain more than one survey station (because Therion knows how far apart the survey stations are). But when we come to draw cross-sections at stations there will only be one station in each scrap. So we will set the scale now. You will see the scale fields in the scrap information panel where we set the scrap id and type. The scale fields require a pair of points to be identified on the drawing, and the real world scale for these points to be entered below. You will see there is a button with the text ‘Scale’ on it on the scraps information panel. If you click on it then a red bar appears at the bottom of the window indicating we are in ‘scale scrap’ mode. We can now click in two places on the drawing to mark the start and end of the scaling arrow. Using the grid shown in blue on the screen, click on the bottom left corner of the grid, and then again on the point that is 10 square across and 10 squares up from the bottom left corner. You should now see a red arrow between these two points.

The picture scale points fields should now be showing numbers approximately from 0.0,0.0 to 395,395. Due to the fact we imported our drawing from PocketTopo we can actually work out the exact values that these fields should be set to. So set the values to 0,0 and 393.7,393.7 and click the ‘Update scrap’ button. The red arrow should now be perfectly aligned with the blue grid origin and the point 10m across and 10m up. So we also need to set the real scale points values to indicate the coordinates of these points in the real world. i.e. 0,0 10,10 with units set to ‘m’. Our completed scrap information box should look like the one shown above.

Next we need to mark the survey stations on our drawing. The Therion map editor gives us a helping hand here because our drawing was imported from PocketTopo. Switch to ‘insert point’ mode by clicking on the blue dot icon in the toolbar.

The mode status bar at the bottom of the screen should turn red and display ‘insert point’. Our survey stations are already indicated on the drawing as black dots. Click on one of these. It should turn blue and in the points info box on the right you should see a survey station point has been created automatically and given the station name from our PocketTopo survey. We are still in insert point mode, so click on the remaining survey stations along the main survey legs along the chamber. Do not include the stations on cross sections or on the side passage coming up from below. We will be drawing these in later scraps. The stations we want to include in this first scrap are 1.2, 1.3, 1.4, 1.6, 1.7 and 1.8 only. Remember if you do anything you did not want to then you can step back through your actions by selecting ‘Edit/Undo’ from the menu, or press Ctrl+z to undo. Once we have finished inserting points, exit the insert mode by pressing the ‘Esc’ key. The mode status bar should turn green again.

Now we can start to draw the walls. We do this using lines. In Therion a wall line has an inside and an outside, indicated with a small yellow tick mark on one end of the line. The ticks should all point into the ‘open air’ inside the cave passage, not into the solid rock. If a line is drawn the wrong way around then we can reverse it to put the tick on the correct side. But we can avoid having to do this for all our lines by drawing around the chamber in an anti-clockwise direction. Switch to ‘insert line’ mode by clicking on the ‘Insert new line’ button on the toolbar.

The mode status bar should turn red and display the message ‘insert line point’.

Drawing lines in the Therion map editor takes a bit of practise. Each line consists of a series of points, and each point has a pair of control points attached to it to control the curve of the line between the points. Click and hold down the mouse button at the point where you want to start drawing a new line, then while holding the mouse button down, drag the mouse in the direction you want to the line to head. This will drag out the control points from the point itself. Release the mouse button and click where you want to place the next point defining the line. Again drag the mouse while holding the button down. You should start to see how the position of the control points determines how the line curves between the points. Repeat this process until the entire line is defined. Then press the ‘Esc’ key to exit ‘insert line point’ mode. You can now select your line with the mouse and edit it. Click on any point and drag to reposition that point. The control points will appear for any selected point, and you can then click on these to move them. This method of drawing smooth curved lines enables fine control of the line appearance. Practise drawing some lines and editing them. Remember you can undo any step by pressing ‘Ctrl+z’. Or you can select an entire line and delete it by pressing ‘Ctrl+d’.

Once you have got a feel for drawing lines, we can draw our first wall. Going anti-clockwise around the outside of our drawing, we are going to draw the walls using smooth curved lines. Starting at the bottom right and going up the right side of our PocketTopo sketch, place line points to draw a smooth curved line matching the line of the wall. When you get to the top of the sketch where there is an open on-going passage we need to start a new line. So end the line by pressing the ‘Esc’ key. Your drawing should look something like this.

The Therion map editor draws all line types with the same appearance, so we need to check that the line type we have drawn is a wall line. Make sure your line is selected, then check the ‘Lines’ information panel on the right. If the type is not already set to wall then change it here now. All new lines drawn will be of the type you last selected here. Continue to draw the walls. You can start each new line by pressing ‘Ctrl+l’. This puts the map editor into ‘insert line’ mode, or ends a line if already in ‘insert line’ mode and keeps in that mode ready to draw another. When you get back to the start, exit ‘insert line’ mode by pressing the ‘Esc’ key to end the last line. Don’t forget to click ‘Save’ when you are finished drawing.

That completes drawing the walls for this lesson. If you are unsure how the sketch should look at this stage then find the bpw.th2 file in the Lesson 3 folder and open that in the Therion map editor to see what our drawing should look like. The final part of this lesson is about generating a survey from our drawing.

Back in the text editor (press F1) we need to include our drawing in our project. We do this by adding a line just below the ‘survey ...’ line at the top of the .th text file to include our .th2 file. The start of our project file should look like this:

  survey bpw

input bpw.th2

Now that we have included our drawing file, we can define a map containing the scraps in this drawing. We only have one scrap at the moment, so our map will just contain the one scrap. We need to give the map a name. As this is a plan map we are drawing, the convention we recommend is to name it after the cave, followed by ‘M’ for map, and ‘P’ for plan. So we are naming our map ‘bpwMP’ for this example. Inside the map definition we put the name of our scrap. So directly below the ‘input...’ line we add the following:

  map bpwMP

Finally we need to edit the configuration file to tell it to create some output for this map. Switch to the compile window (F3), and add the following line to the configuration file:

  export map -projection plan -o bpw.pdf

Here we are giving the instruction to create PDF document for our map. As we only have one map in the project we do not need to specify the name. We just indicate that it is a plan, and the filename we want to give the output file. Compile the project (press F9) and if everything has been done correctly then you should see the green ‘OK’ bar and find a file ‘bpw.pdf’ has been generated. Open this to view your Therion survey!

There is one more thing that has changed in our output now. The 3D model of the cave will now use the passage walls we have drawn to generate a better model. Open the bpw.lox file and take a look. Now instead of displaying thin cylinders along the survey legs, you should see a much better 3D representation of the cave passage we have drawn.

Drawing symbols and other line types

In the last lesson we learned how to draw lines, and to check the line type. Now that our survey has the main walls drawn, we need to add some feature details. The Southerly part of our cave passage is a wide bedding plane at floor level, with a walking size section down the middle where there is a phreatic roof development. See the cross sections on the PocketTopo sketch. We can draw this step up in ceiling height using the ‘ceiling-step’ line type. Conveniently when we drew our walls we placed a point on the wall line exactly where this ceiling step feature joins the wall, so we can draw our line from this node. Start a new line, and click on the wall node where we want to start it. Draw the new line along the maroon line on the PocketTopo sketch as shown below. When the line is finished, set the type to ‘ceiling-step’ in the lines information panel.

Note the direction of the yellow tick at the start of the line. This indicates which side the marks along the line will be drawn when we render it. The UIS symbol for ceiling steps is a line with ticks pointing to the side of the line where the ceiling is lower, so our tick is pointing the right way. If it was not then we could reverse the direction of the line by ticking the ‘reverse’ checkbox in the lines information panel. Compile the project again and view the PDF. (Note that you have to close the PDF before you can recompile the project each time or you will get an error because the PDF file is locked by the PDF viewer). You should see the ceiling step line looking like this.

Once we have set a line type, any new lines we draw will be of the same type by default. So it makes sense to draw all the ceiling step lines together, before moving on to other line types. The line for the other side of our ceiling feature does not join the wall at a line point. There are two ways we can handle this. We could just draw our ceiling step line across the wall at the point we want it to join. Therion will not render the ceiling step line on the survey outside the boundaries of the wall by default. So the ceiling line will just stop at the wall even though we drew it crossing over beyond the wall. The alternative approach is to insert a new point into the wall line at the position where we want to end our ceiling step line. To insert a point, select the wall line, and then look in the lines information panel on the right. At the bottom there is an ‘edit line’ item. Click on this and a menu drops down. Select the ‘Insert point’ menu item and then click and drag at the position on the wall line where you want to insert the point. It is important to drag to pull out the curve control points. Otherwise the point will cause a sharp corner in the line. You can adjust the curve around the new point to make the wall line look right. Remember you can edit any point in a line by clicking on it to select it, and then drag it to a new position or adjust the control points to control the curve. If you mess it up, remember you can undo any number of steps by pressing Ctrl+z. Now we can draw the second ceiling step line.

Next we will look at the chimney dropping into our passage on the left hand wall. This poses some interesting problems which will teach us a number of useful things. Firstly, there is a wall line drawn through the middle of the feature. So we will need to break the line.

This is done by selecting the point on the line where we want to break it into two lines. Then from the ‘Edit line’ drop down menu in the line information panel, select ‘split line’. Now there are two lines meeting at the point in the middle of our feature. So select the point at the end of each of them in turn and then click and drag it to the edge of the feature. Adjust the control points on the end points and the next point along each line to get the curves aligned with the wall lines on the sketch. The tidied up wall lines should look like the image on the right (above).

Now we can draw the feature. Start a new line (Ctrl+l to put the editor into new line mode), then click and drag on one of the points at the end of the wall lines to start the new line from one of these wall end points. Draw around the chimney feature, making sure you also put a point on top of the other wall end line and continue to close the loop back on the starting point. The editor will automatically exit line drawing mode when you close the line into a loop. Set the line type to ‘floor step’ and compile the survey.

The images above show how our closed line looks, and how it was rendered in the PDF. There are two problems. The first is that the tick was pointing outside the loop in our case above. This means the floor step tick marks in the PDF appear outside the loop and not inside like we wanted. To fix this we simply need to reverse the line by ticking the ‘reverse’ check box in the line information panel. The other problem is that only half our floor step line was rendered. This is because the survey drawing is clipped by the outside of the wall lines. So only symbols and detail lines which are inside the boundary of our cave are rendered. Half our chimney is outside the boundary because Therion joins across gaps in the boundary wall lines with a straight line between the wall ends. This is straight through the middle of our chimney feature. We can turn off this clipping for our floor step line by adding the option ‘-clip off’ in the options field in the line information panel. Here is how it should all look when we have fixed these problems.

This has illustrated some useful lessons in line editing and clipping, but technically we have just drawn a pit in the floor and not a chimney going up. We could change the line type to ceiling step if this was an unexplored aven. But in this case we entered the cave down this chimney and there is a small passage coming in at the top of it. So we can actually redraw it properly to illustrate this. See if you can use what you have learned to make these edits yourself. Split the line forming the loop so we can set the line types separately for different parts and draw the side passage coming in to a floor step (a climb down) at the top of the chimney. Here is how it might look.

Some symbols are represented by points rather than lines. In the main part of our chamber we have some slope arrows and water flow symbols to draw. Switch to ‘insert point’ mode (Ctrl+p) and click on the top end of one of the slope arrows on the PocketTopo sketch. A new point should be created there. As we were last creating survey points the point type will probably be a new survey point. We can change the type in the Points information panel. The point type we need for a slope arrow is ‘gradient’. We also need to give our arrow a direction (they should point down the slope). This is done by ticking the ‘orientation’ checkbox in the information panel. You should see an arrow has been drawn on your new point. You can now click and drag on this arrow to rotate it around the point to orient it correctly to show the direction of the slope. The orientation arrow is only shown in the map editor while the point is selected. So as we add more slope arrows we should set the orientation of each one as we go or it is difficult to check they were all drawn correctly. As we are still in ‘insert point’ mode we can click on the other positions where we want slope arrows, adjusting the orientation of each one as we add them.

Water flow arrows work exactly the same way as slope arrows. Click on one of the water flow arrows on the PocketTopo sketch and change the point type to ‘water-flow’. Remember to tick the orientation checkbox again and set the direction of the arrow. Add a few water flow arrows to indicate where the stream flows down the chamber. Compile the project again and look at the PDF. Now it is beginning to look like a proper cave survey. There are plenty of point types and line types to explore in Therion. We can add some more to our survey later. But sometimes it is not practical to draw in every single rock or grain of sand. In the next lesson we will learn how to create areas and use symbol fills.

Adding fill areas

Some survey symbols fill an area rather than appear at a single point. In this lesson we will create some areas with symbol fills. Typical area symbols show detail of what is on the floor. e.g. Water, mud, sand, debris and boulders. Our main chamber area has boulders all over the floor, so rather than draw them all we will define the area we want them to cover and then assign a symbol to fill that area. An area consists of a series of lines defining the boundary of an enclosed area. These can be lines of the various different types which appear on the survey, or can be invisible lines defining the perimeter of the area without actually appearing on the finished drawing. Like other symbols, an area fill will be clipped by the cave walls in the scrap. So you can just draw an area over the top of the passage, passing outside the walls. When the drawing is rendered only the parts of the area inside the walls will be filled with the area fill symbol.

We could join the lines defining the border of the area onto points on the wall lines, making the wall lines part of the area boundary. But this requires us to split the wall lines at the points where we want to join on the lines defining the other parts of the area boundary. It also makes it harder to spot that there is an area fill defined because the area boundary is less obvious in the scrap when viewed in the map editor. Here is an example showing three ways we could define an area. On the left we have split the wall lines at points and then started and ended two new border lines at these points on the walls. In the middle we have drawn two lines crossing over the walls to define part of the passage as an area. Note that the wall lines are included in the set of lines defining the area. On the right we have drawn just a single line in a loop across the passage to define the section of passage inside the loop as an area. All the lines defining the area are coloured red here in each case.

All these cases would render the same because Therion will only draw symbols inside the passage walls. But it is strongly recommended to use the third of the three methods shown. This is because lines defining an area all get assigned unique id codes, and if you later split any of the lines the area is very likely to get broken as the id codes do not get copied onto both segments of the line. So to avoid risk of messing up the areas later we should as a rule avoid using lines which are serving another purpose in the sketch (like the walls) in our area definition. It is safer to draw the area with a single line, so this is what we will do here.

We will draw the line more carefully than in the rough illustrations above, so that it properly marks out the parts of our passage where the floor is covered with boulders. Once the line has been drawn, set the line type to ‘border’ to indicate that the line is just a border between different areas of the cave. Next click the ‘new area’ toolbar button (or press Ctrl+a).

The map editor should now be in ‘insert area border’ mode as shown by the text in the red status bar. We define the area by clicking on each line which surrounds the area in turn, working around the area either clockwise or anti-clockwise. In our simple case here we only have one line to click on. You should see the line id being added to the ‘Areas’ information panel (identified by long id codes in a text box, with ‘end of area’ at the end of the list).When the line has been added to the area, press the ‘Esc’ key to exit ‘insert’ mode. You can change the area fill type in the ‘Areas’ information panel. Set it to ‘blocks’. Compile the project and check the rendered drawing. The area should be filled with blocks, but the border line is also drawn as a solid thin black line where it crosses the passage. We want to hide our border line, so select it and enter ‘-visibility off’ into the ‘options’ field in the ‘Lines’ information panel. Now when you render the drawing the border lines should not appear.

Sometimes it is not possible to draw an area border with a single line. When two areas sit side by side they need a border line in common. This is because it is only passage walls which clip a symbol fill. So if we drew two overlapping circles then each would display their symbol fill in the part of the areas which overlap, as shown here.

Instead we need to draw an area joined onto the line of another. In our example we will draw a pool in the passage at the bottom right of our sketch, and a mud/clay floor in the main passage at this end of the cave. When we add the areas in this case we need to add all the lines which make up the area. Be careful to only click on each line once so that the id is only added once in the ‘Areas’ information panel. If you added one twice by mistake you can delete it by clicking on the id, and then using the ‘Delete’ button in the information panel. If there are more than three lines making up the boundary of an area then you must add them in order going around the boundary.

Here is how our new areas look in the map editor and the rendered drawing. Note that we have made the line bordering the pool visible, but hidden the line bordering only the mud area.

Adding additional scraps

Our cave has a passage coming from under part of the chamber and up into the chamber through a hole in the floor. We cannot include both this passage and the chamber in one scrap because a single scrap is not allowed to include overlapping passages. So in this lesson we will learn how to add additional scraps into our drawing. Once you can do this you will be able to draw up surveys for even the largest cave systems.

There is no reason not to draw multiple scraps in the same drawing file, unless it is becoming difficult to see what is happening in the editor. Therion will render the final survey the same whether the scraps are in one or several .th2 files. For our cave we will draw the new scrap in the file we are already using for the chamber. Start by inserting a new scrap by clicking the toolbar button (or press Ctrl+r). Name the scrap ‘bpwSP2’ and set the projection to ‘plan’ following the same steps as we covered in lesson 3.

If you look at the ‘Objects’ information panel at the top of the right hand pane, you should see there are now two scraps defined. Each starts with a line ‘scrap - <name>’ where <name> is the name of the scrap, and ends with the line ‘end scrap’. All the objects in each scrap appear between these lines. So our new scrap contains nothing at the moment. If you select one of the start lines for a scrap then everything in that scrap is coloured blue in the map editor. Select the ‘scrap - bpwSP2’ line and everything should turn yellow in the drawing because nothing is in this scrap yet. Now the new scrap is selected in the objects window, any new lines or points we draw will be put into the new scrap. Start by adding a couple of survey station points so that Therion will know where to place the scrap on the survey. The procedure is exactly the same as we covered in lesson 3. Next draw the wall lines and the floor step. Remember to check that the ticks on the wall lines point into the passage, and that the tick on the floor step points down the drop. This should be enough for now to add the new scrap into the survey. Here is how our scrap looks in the map editor.

Switch to the text editor and add the new scrap into the map:

  map bpwMP

Now compile the project again to see how it looks. You should see that the floor step line has been clipped because it curves outside the area bounded by the two wall lines. To fix this step the option ‘-clip off’ for the floor step line. The plan survey drawing should look better now, but the passage passing underneath the floor of the chamber is not shown underneath. We need to break the map into two parts so that Therion renders one scrap over the top of the other. This is done by adding the word ‘break’ in between the two scraps in the map.

  map bpwMP

Here is now this should have rendered.

Now the floor step appears greyed out when the pit where the passage goes off is actually in the chamber floor. So we will move the floor step line into the chamber scrap. Select the floor step line and then look at the ‘Objects’ information panel. We want to move it from the position in the second scrap to inside the first scrap. Find the line number of the ‘end scrap’ line for the first scrap, enter that number into the box next to the ‘Move to’ button and click the button. The line should have been moved up to above the ‘end scrap’ line for the first scrap. You can select the scraps in turn by clicking on the ‘Select next scrap’ button in the toolbar above. Now the floor step line should be rendering in the finished drawing as solid black. Ideally we should also draw the opposite side of the pit to clearly indicate that this side passage enters up from a hole in the floor.

Finally in this lesson a quick note on pillars or loops in passages. We will extend the passage in our new scrap to include a loop to illustrate this. Here is what we will draw.

Note the rock pillar, or loop in the cave passage highlighted in red. When we compile this we do not get a green OK bar on the compiler page. Instead the bar stays Orange and displays ‘Warning’. This is telling us that Therion issued a warning. We can find it in the log file produced with the other output files. The file is called ‘therion.log’. Open this file in a text editor and search for the word ‘warning’. You should find the following:

  “warning -- bpwSP2@bpw -- multiple scrap outer outlines not supported yet”

The warning tells us which object caused the warning. This is where our naming convention for objects really helps us, because we can tell it is in a plan scrap (SP) in the part of the cave named bpw. For really big caves with many scraps this will help identify where the problem is. In this case it is our new scrap, and the warning is because our scrap has two outlines. One is the rock outside the passage, and the other is the pillar of rock inside the loop in the passage. If we look at the 3D model (.lox file) we see that this part of the model does not look right. The walls in this scrap are not showing up in the model as expected. The way to fix this is to indicate that the lines making up the inside of the loop are surrounding a section of rock inside the outer boundary of the cave walls in this scrap. We do this by adding the text ‘-outline in’ to the options field for the wall line inside the loop. Compile the project again and this time the warning is not generated and the 3D model shows the walls correctly.

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.