Currently, a self-contained, executable Windows installer is provided on this webpage. The installer can be downloaded from here. It provides both Inkscape and the extensions themselves. Previous versions of Inkscape therefore have to be removed from your system first.
Alternatively, it is possible to use the standard Inkscape distribution from http://www.inkscape.org and install the extensions manually. This is the only feasible option in case of Linux or MacOS
Use Object / Symbols to open the symbol library dialog. Select the "Cave plan" symbol set. To place a symbol on your drawing, simply drag it from the library dialog onto canvas. If you don't see any Symbols item in the main menu, you probably have an older inkscape version (earlier than 0.49). If you can open the symbol library, but you don't actually see the Cave plan symbol set, you probably don't have my extensions installed. In both cases I suggest uninstalling whatever inkscape version you have and re-installing it using my installer.A few remarks concerning particular symbols follow.
The Alt+1/2/3 keyboard shortcuts simply call relevant options from the Effects / Speleo menu. You can call these options "manually", by choosing an appropriate menu item. This has an advantage in that a configuration dialog opens. You can tune a few parameteres before the effect is applied. The configured parameters will be also used next time you press the keyboard shortcut.
Note: step lines/driplines are a bit cheated. After using Alt+1/Alt+2 you actually end up with two objects on the drawing: one is a line defining the symbol shape, and the other is a series of characters from a special font wrapped around that line (read more on that font). This can lead to misunderstandings. For example, if you need to tune the shape of your step, you have to select the line for editing and not the text object. Sometimes you have to use Shift+D before switching to node edit tool in order to find the path defining how the text is bend. Note that after you find the right path, the symbol direction can also easily be reversed.
To import Survex files into Inkscape, simply use File / Open or File / Import and select a .3d file.
The importing process keeps the original hierarchy of prefixes contained in the file. Every prefix will have its own group that will contain its sub-prefixes as sub-groups. Using standard Inkscape group selection features, it is possible to move around this hierarchy and, for example, change style of particular prefixes.
Data from PocketTopo has to be saved in the Therion format (in Pocket Topo: Export / Therion...). The file name needs to end with .the (!)
The import filter works similarly to the 3d import. Centerline shots and splay shots are put in separate groups. Strokes of each color are also groupped together. By selecting the imported drawing and clicking Extensions / Speleo / Group to Layer(s)... the group structure can easily be turned to a structure of Inkscape layers. Conversely, Extensions / Speleo / Sublayers to group can revert this operation.
The input filter is able to generate a projected elevation from extended elevation stored in the PocketTopo file. In general, because of fundamental reasons, such a conversion is impossible for all cases. Therefore the resulting image only makes sense along shots orientated close to parallel to the projection surface. If the extended elevation has side passages, it is often impossible to automatically determine which drawing stroke belongs to which branch. In such a case, the stroke is duplicated and transformed multiple times, for every branch. This produces drawing artifacts that need to be sorted out by the surveyor. Besides, the module is buggy and distorts splay shots far off the extended elevation plane. Therefore, please use these converted sketches only as a drawing aid and refer to your measurements - ie. import a 3D file with your splay measurements as well and not just the sketch!
Use Extensions / Speleo / Render grid. By default, the origin is located at top left corner of the page. To have it placed in another position, create a temporary rectangle and align its top-left edge to the point where the origin is desired. Select the rectangle and then run the grid extension.
Use Extensions / Speleo / Scalebar... to generate a scale bar.
First of all, because it is free software. Everyone on an expedition who brought their laptop can legally install it and start drawing clean what they had surveyed. It runs on all common platforms - Windows, Linux as well as MacOS X
Second thing is it is open software, which means its source code is available for improving to any hobbyst or professional programmer with some caving background. It is relatively easy to create custom extensions, such as those advertised above on this page. These extensions make it easy to:
This is harder to answer. Generally I agree that Therion is better suited to produce cave maps than probably any generic vector drawing software, such as Inkscape. The only problem with Therion, actually a very serious one, is it is somewhat hard to use and even more difficult to teach.
Recommended Inkscape preferences (File / Inkscape preferences... or Ctrl+Shift+P)
Transforms - Definitely unset Scale stroke width. Scrolling - consider Left mouse button pans when Space is pressed, Mouse wheel zooms by default. Tools - Pen - consider Create new objects with: Last used style
Inkscape has numerous hotkey bindings. It is advisable to read the list of available keyboard commands (google: inkscape keyboard shortcuts), especially since some of the program features are available only as keyboard commands.
While obviously to publish drawings you should use the PDF export, it is most advisable to store your editable copies as Compressed Inkscape SVG. There is no quality loss compared to Inkscape SVG. By storing your drawings in another format, you will almost certainly lose your layer structure.
You should definitely become familiar with Inkscape's object groupping features. One most important thing is that in order to edit something within a group you do not have to un-group the group. You can enter the group (and then any internal groups it has etc.) by double-clicking or right clicking on the group and selecting "Enter group ...".
Program features worth investigating: Edit / Paste in place (extremely useful for moving objects mistakenly positioned in wrong layers); Edit / Paste style; Making copies by hitting space while dragging objects; Path / Difference (useful for making islands in lake shapes); Object / Align and distribute....
In case of conventional symbols, it is wise to use Edit / Clone / Create clone instead of standard Copy & Paste. Clones are identical copies of objects that can have their own position, scale and orientation. The advantage over plain copies is that changing the shape of the original immediately alters all the clones. This makes perfect sense if you, for example, decide at some point that you would like to make all clay (~) symbols smaller. The good thing is that copying a clone with Copy & Paste also yields a clone, which means you do not need constantly remember to use Create clone instead of Ctrl+C/Ctrl+V.
If you installed the jext extensions, check out the File / New / cave and File / New / cave_symbols options. They create, respectively, a document with a predefined layer set and a document with all the UIS symbols generated from the SpeleoUIS font. The latter is handy for copy-pasting symbols into your cave map. Both documents can be modified by overwriting cave.svg and cave_symbols.svg in the share/templates subdirectory of your Inkscape installation.