therion can import cave maps for which you do not have the suvery data [thwiki 6]. To put together scraps drawn on digitized images of maps for which the survay data are not available, you need to have some reference point on each scrap. These can be arbitrary stations, for example, ie, points that you mark on the map (before digitizing it) and treat as a station. To this aim you need:

• to digitalize the cave map with a scanner;
• for each image to define the position of a (virtual) station. This is the station where the map connects with the rest of the cave.
• to define a "survey" with this station, if it does not appear already in another survey of the cave;
• to create a new scrap into which you import the map image as background;
• to define the scale and the rotation of this image in the scrap: if the image has a scale-bar (eg, the 10 m bar), you can use the two little red squares with it, just like for the cross-sections. Place (drag with the mouse) the squares on the ends of the scale-bar and sets the coordinates to (0,0) and (X m,0) where X is the length of the bar, in meters. Remember that the 'x' coordinate points towards east and the 'y' coordinate points towards north.
• draw the scrap, using the scanned image. Add a point of type "station" for the connection station and set it the option "-visibility off", so that it does not appear in the maps;
• use the "join" command to connect the scrap to the others.

This example uses fictitious data Suppose to have a map image file, nodata.gif. Create a new map file, nodata.th2, for the scrap with no survey.

In the data file nodata.th define the "survey" nodata with the single virtual station "1", and "equate" it to a real station of the cave survey. You do not have to define a centerline.

   survey nodata
     input nodata.th2
   endsurvey

   equate 1@nodata 4@s1

   join linea_sx@nodata:0   s1_sxf@s1:end
   join linea_dx@nodata:end s1_dxf@s1:0

In the file nodata.th2 import the image nodata.gif as background. Next create the scrap (id "scrap_nodata"), specify its projection ("plan") in the "Scraps" control, and use the little red squares to define the scale and the orientation of the scrap.

Add a point of type "station" for the virtual station "1". Draw the lines of the scrap and all the other symbols and texts. Finally connect this scrap to the others with the "join" command as shown in the above code.

The figure shows the scrap and the final result.

A final note. If you have the real (geographic) coordinates of two points in the map, you can set the scale (and rotation) of the imported map using the fix command in a survey data file,

   fix point1 X1 Y1 Z1
   fix point2 X2 Y2 Z2

In this way the imported scraps get also vertical information, which makes the map coloring by altitude possible.

The reference points used to join the scraps do not have to be stations. If you have a coordinate grid on the map, you can set the real X,Y coordinates (in some geographic coordinate system, or in an arbitrary map reference system) of two points in the scrap. Use the xtherin map editor, under the Scrap tab. This will calibrate the scrap, ie, defines its orientation and scale.

If you calibrate all the scraps in this way they will correctly placed on the map. If you use a geografic coordinate system for the real coordinates of the reference points, add the -cs option to the scraps (for example -cs UTM32).

For example you digitize the portions of the map. Next you open the xtherion map editor and create new th2 files, one for each image, with its scrap. You calibrate the scrap by dragging the red arrow endpoints on two points of the coordinate grid and by writing the points real coordinates in the Scrap control. You might need to adjust the zoom to see the arrow. Then you draw the scrap with all its features. No need to add station points.

In the source file, or directly in the thconfig file, you write a survey that inputs the map files and with the appropriate join commands. For example the following code is included in the thconfig,

The figure below shows the xtherion map editor opened on a scrap (the top part of the map). The map image has a black grid and the cuts that delimit the end of the portion are marked by the two red dashes, drawn on the map image. The scrap has walls (violet), a pit line (blue) and a water-flow point. The red arrow is the scrap calibration. The two reference points are assigned real coordinates (0,0) and (10, -15), since the X axis points rightward and the Y axis points upward.

The conversion from Scalable Vector Graphics (SVG) to therion format is not straightforward, because the SVG format is a generic format for vector graphics while therion map files are specific for cave information data.

Nevertheless, xtherion can import data from DXF/SVG files. To do this go to the map editor and open a new file. Then select the File->Import menu and choose the svg or dxf file to import.

xtherion splits the centerline into pieces at branch nodes (stations with three or more shots). It does not import the walls (from the DXF file), but it imports the bounding box as a set of four separate lines.

J. Bohnert has written a Python script to convert SVG files to therion th2 format. This script is available on the therion wiki. The organization of cave data in the SVG file depends a lot on personal taste: so is the choices of line widths, and the subdivision of cave data on layers by the type. The external script (or program) has more flexibility, than the simple conversion implemented in therion, and can be easily adapted to different way of organizing the information in the SVG file.

CorelDraw stores line data in relative coordinates. To convert it to absolute coordinates you can load it in Inkscape and then save as plain svg file. In the process you can filter out all the graphic elements you do not need. In plain svg format lines are Bezier cubic curves, with single points (prefixed by "M") or triplets with the point and the control points (prefixed by "C").

Finally you must edit the Python script to adjust the xtherion display zoom (line "##XTHERION## xth_me_area_zoom_to 2", where the value is the zoom percentage), and the scrap scale (ie, the boundaries of the scrap in the -scale option).

The author suggests the following conversion strategy: manually extract all symbols of one kind (e.g. rocks, contour lines) using Inkscape and save them as a separate th2 files with the correct line description. Then combine everything in a single th2 file using a text editor.

Line tracing is an xtherion feature to automatically create therion lines by tracing lines on an map image. You start a new line and select a couple of points on a map line. Next click on the "Line edit" option "Trace line": xtherion start following the map line and adds points to the line. The resulting line usually has very many points; to reduce their number and make it smooth press the "Convert to curve" option.

Line tracing may at times fail as xtherion may get the wrong continuation at a point where another line come close to the one that is being traced. In this case split the line. at the last good point, delete the extra piece, add some points and continue tracing (button "Continue tracing").

The result depends on the quality of the map; it's better if the map lines are neat and with good contrast. If the map line is greyish the tracing easily goes off the line. It works rather well to digitize contours of topographical maps, or to convert old cave maps to therion. It is useless for muddy cave sketches. There are problems with acute angles. The viewing resolution (zoom) also affects the result.

Coverting a line to a curve (button "Convert to curve") works for any line, not just for traced lines. Therefore you can enter a line setting many points and then let xtherion convert it to a smooth curve.

Importing maps Tracing lines