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Inkscape extension to export a survex (.svx) file
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README.md
farcountry_ulsaj89.png
farcountry_ulsaj89.svg
loneranger_cpcj6-2.png
loneranger_cpcj6-2.svg
mossdale_ulsaj89.png
mossdale_ulsaj89.png.points
mossdale_ulsaj89.pos
mossdale_ulsaj89.svg
mossdale_ulsaj89.svx
mossdale_ulsaj89.tiff
svx_output.inx
svx_output.py

README.md

Survey data reconstruction

The tool here adds an Inkscape option to save a drawing to a Survex (.svx) file. A typical workflow starting from a scanned drawn-up survey image is described below. Use-cases might include:

  • producing a plausible length estimate for a drawn-up survey;
  • georeferencing a drawn-up survey (see below);
  • generating a skeleton from an existing drawn-up survey, to hang data off in a resurvey project;
  • giving 'armchair cavers' something useful to do.

A couple of example surveys (Inkscape traced drawings) are also included.

Installation

Copy the files svx_export.py and svx_export.inx into your local Inkscape extension folder (eg $HOME/.config/inkscape/extensions/ on unix, or %APPDATA%\inkscape\extensions\ on Windows).

That's it – there are no additional dependencies and this should work on most platforms. In particular it has been tested to work with Inkscape 0.91 (r13725), on both Linux and Windows.

Usage

Usually the drawing will be made by tracing over a scannned image of a drawn-up survey. The following conventions are observed:

  • by default, all red (poly)lines are converted to traverses in the survex file;
  • by default, a single green line line determines the orientation (default S to N);
  • by default, a single blue line line determines the scale (eg from the scale bar);

Lines of any other color are ignored, as are other drawing objects. The default colors can be changed (see below), for example to red-blue-black if red-green-blue gives problems.

In Inkscape, the option to save to a Survex (*.svx) file should appear under 'File → Save As…' or 'File → Save a Copy…'. This brings up a dialogue box, with:

  • in the 'Parameters' tab:
    • length of scale line (in m);
    • bearing of orientation line (in degrees);
    • tolerance to equate stations (in m);
    • an option to restrict conversion to a named layer;
  • in the 'Colors' tab, options to change the default line colors;
  • in the 'Help' tab, a reminder of the expected drawing conventions.

If an Inkscape layer is specified by name in the 'Parameters' tab, then only those (red) (poly)lines belonging to that layer are exported. The orientation and scale lines are picked up irrespective of the layer.

Traverses generated from (poly)lines are captured in separate *begin and *end blocks in the survex file. The Inkscape path id is used for the block name, so traverses can be matched up to paths in Inkscape. The survey as a whole is wrapped in a top level *begin and *end block with a name derived from the Inkscape docname (or, if export is restricted to a given layer, the name of that layer).

Within each traverse, survey legs are generated in the format

*data normal from to tape compass clino

In this format:

  • The from and to stations are generated automatically.

  • The tape length is generated using the scale line as reference, which will usually be traced over a scale bar in the survey. The true length that the scale line represents is specified in the 'Parameters' tab in the export dialogue box.

  • The compass bearing is generated using the orientation line as reference, which for example will be traced over a North arrow in the survey (in the direction S to N). If a North arrow is not used the true bearing represented by the orientation line can be specified in the 'Parameters' tab in the export dialogue box.

  • The clino reading is set to zero.

Survey stations that are within a certain distance of each other are given as an *equate list at the start. The distance tolerance to select these is usually small (eg 0.2m) and can be adjusted in the 'Parameters' tab in the export dialogue box. Stations which feature in the *equate list are automatically exported out of the underlying *begin and *end block by the appropriate *export commands.

The python script svx_output.py can also be used standalone at the command line. The required modules in the Inkscape global extensions direcory should be made discoverable though: these are inkex.py, simplepath.py, and simplestyle.py. The simplest way is to copy these .py files to the same directory that contains svx_output.py. Command line options can be found by doing ./svx_output.py --help.

Workflow

A typical workflow using Inkscape might be as follows:

  • start a new Inkscape document;
  • import (link or embed) a scanned survey as an image;
  • optionally, lock the layer containing the image and create a new layer to work in;
  • then do the following:
    • trace the scale bar in blue, making a note of the distance this represents,
    • trace an orientation feature in green (eg a North arrow, from S to N),
    • trace the desired survey traverse lines in red,
    • adjust the positions of nodes which are supposed to represent the same survey station until they are coincident (within the tolerance);
  • save the Inkscape file to preserve metadata;
  • do File → Save a Copy… and select 'Survex file (*.svx)';
  • choose a file name and click on Save;
  • in the dialogue box that appears, in the 'Parameters' tab, fill in at least the length of the scale line to correspond to the length of the traced scale bar;
  • click on OK to generate the .svx file;

Notes:

  1. 'File → Save a Copy…' is preferred to 'File → Save As…', since it prevents Inkscape from thinking that the actual Inkscape drawing is of file type .svx.

  2. If Survex complains with an error 'Survey not all connected to fixed stations', then it is most likely a pair of supposedly coincident survey stations are not close enough together (they may have been overlooked). To diagnose this:

    • increase the tolerance until Survex no longer complains and the .svx file processes properly;
    • open the .svx file in a text editor and examine the list of *equate commands to see which stations were too far apart;
    • return to Inkscape and fix the problem.
  3. The automatically generated *equate list may contain superfluous entries if more than two path nodes are at the same position. The fastidious can correct this by hand afterwards but it doesn't affect the processing of the survex file.

  4. It is suggested that magnetic N not be corrected for using the 'bearing' setting in the 'Parameters' tab. Instead add a *calibrate declination <angle> line by hand to the top of the survex file, where the angle is positive for declination W. The NOAA website can be used to obtain declination data for a given location and year. The design choice to export legs as tape and compass readings, rather than as cartesian changes in easting and northing, was made precisely to accommodate this.

  5. Depth information can be added by hand by editing the .svx file. A convenient way to do this is to change the survey data type to *data cylpolar from to tape compass depthchange. The final column (which originally contained zero clino entries) can be edited to reflect the depth change between survey stations, whilst preserving the tape and compass entries which are presumably correct if taken from a drawn-up survey in plan view.

  6. A large survey can be dealt with by partitioning the (red) (poly)lines into different, named layers, keeping the same scale bar and orientation line, but generating a different survex files for each named layer. These survex files can be stitched together using a master file as illustrated below. To facilitate this, the top level *begin and *end block is given the name of the layer rather than a name derived from the docname (and by convention this should also be the name of the .svx file). Some hand editing has to be done to match up the corresponding stations in the different survey data files: in particular it is necessary to export these stations through the *begin and *end blocks in each file.

Examples

The file loneranger_cpcj6-2.svg is a tracing of a survey of the Lone Ranger series in Link Pot (Easegill) where the PNG image (loneranger_cpcj6-2.png) – originally published in CPC Journal 6(2) – is taken from CaveMaps. The scale line end-end distance is 30 m, so 'Length of scale line (in m)' in the 'Parameters' tab would be set to 30.0.

Similarly, farcountry_ulsaj89.svg is a tracing of a survey of Far Country in Gaping Gill where the PNG image (farcountry_ulsaj89.png) – originally published in the ULSA '89 Journal – is likewise taken from CaveMaps. In this case the scale line end-end distance is 200 ft, so the 'Length of scale line (in m)' in the 'Parameters' tab would be set to 60.96. To get an idea of the work involved here, the generated Survex file contains 176 survey stations, joined by 180 legs, with a total length of around 1.5km of passage: it took less than 30 mins of drawing in Inkscape to do.

Multiple layers

The loneranger.svg tracing can also be used to illustrate the use of named Inkscape layers to break a survey into smaller pieces. The section from Matchbox aven to Tonto aven has been placed in a layer named matchbox, and the remaining passage beyond Tonto aven in a layer named silverstream. Thus the survex files matchbox.svx and silverstream.svx can be generated by exporting twice, using the option in the 'Parameters' tab in the export dialogue to restrict conversion to each of these named layers in turn. (By convention, the survex file name should be chosen to match the name of the top level *begin and *end block, here set by the layer name.) These survex files can each be processed to a .3d file individually, but to stitch them together we note that Tonto aven is station 8 in path4288 in matchbox.svx, and is also station 0 in path4290 in silverstream.svx. Hence the following short survex file (here called combined.svx) will do the job:

; combined.svx 
*include matchbox
*include silverstream
*equate matchbox.path4288.8 silverstream.path4290.0

As it stands this doesn't quite work because we have not yet exported these stations from the underlying survex *begin and *end blocks. This has to be done by hand: in matchbox.svx, add station 8 to the list of stations exported from the path4288 block, and add a line *export path4288.8 just below the top level *begin matchbox statement (both are needed, as stations must be exported from each layer in turn). A similar pair of edits is required in silverstream.svx. Then one should be able to process combined.svx to a .3d file.

Georeferencing

Georeferencing refers to assigning a co-ordinate system to a map, in this case for example to a scanned hard copy of a survey. The actual steps require identifying so-called Ground Control Points (GCPs), which are identifiable features on the map for which actual co-ordinates are known. If the survey has a grid, or multiple entrances, these can be used. Frequently though the survey may only have one entrance, and just a scale bar and North arrow. In this case one can use the survex export plugin to generate co-ordinates for additional GCPs in the cave. To do this one can either trace over the passages and export the survex file, or more simply trace a fake centerline which simply connects the extra GCPs to the entrance. Check the generated .svx file can be processed, and then add a *fix command to fix the entrance GCP to the known co-ordinates. Processing this file allows you to extract the co-ordinates of the GCPs.

For example suppose one wanted to georeference the ULSA 1989 Mossdale survey, which is included here as mossdale_ulsaj89.png. The original of this was downloaded from CaveMaps.
This version has some material cropped out and has been reduced to a binary (2-colour) image. The image is imported into inkscape and the scale bar (500ft = 152.4m) and North arrow traced (conveniently, the survey uses true North, so magnetic declination need not be corrected). We then add a single line connecting the entrance to a distant identifiable feature in the cave, for example the small chamber shown at the end of the Stream End Cave passage. This line will be exported as the survex centerline. The resulting inkscape drawing is included here as mossdale_ulsaj89.svg. The exported survex file, modified as described below, is mossdale_ulsaj89.svx.

We now need to establish the co-ordinates of the entrance, as the first GCP. To do this one can of course make a site visit with a GPS, or perhaps more conveniently make a virtual site visit using MagicMap website and use the 'Where am I?' option to find the entrance is at (E, N) = (401667, 469779). This is the full 12-figure grid reference in the OSGB36 co-ordinate reference system. The 6-figure NGR SE 016697 given on the survey is correct of course, but as it locates the entrance only to within a 100m square it's not really accurate enough. An alternative to using Magic Map is to use Google Earth or Google Maps with satellite imagery to find the WGS84 location of the entrance is (N, W) = (54°7'26", 1°58'34"). These can then be converted to OSGB36 co-ordinates.

We add these entrance co-ordinates to the survex file which now contains

*begin mossdale_ulsaj89
*fix entrance 401667 469779 425
*entrance entrance
*equate entrance path4342.0

*data normal from to tape compass clino

*begin path4342
*export 0
  0   1 1383.46  113  0
*end path4342

*end mossdale_ulsaj89 

I have adopted the convention of exporting the entrance (station 0 in path4342) out to the top level, and equating it to a new station named entrance. Also the altitude (1400ft = 425m) has been added, although irrelevant for present purposes. On processing by cavern and 3dtopos the result is

( Easting, Northing, Altitude )
(401667.00, 469779.00,   425.00 ) mossdale_ulsaj89.entrance
(401667.00, 469779.00,   425.00 ) mossdale_ulsaj89.path4342.0
(402940.48, 469238.44,   425.00 ) mossdale_ulsaj89.path4342.1

(saved as mossdale_ulsaj89.pos). The last line gives the co-ordinates of the GCP at the end of Stream End Cave. We can now proceed to georeference the survey, for example using QGIS. One point to note is that the relevant co-ordinate system is

OSGB 1936 / British National Grid    EPSG:27700

It is the European Petroleum Survey Group (EPSG) number that really identifies this. The final georeferenced survey, here provided as mossdale_ulsaj89.tiff in GeoTIFF format, can be directly imported into a GIS platform such as QGIS, and superimposed on Google satellite imagery, or the Environment Agency LIDAR data, for example.

The survex centreline data can also be imported into a GIS platform by using the cad3d survex tool to export the .3d file to .dxf. The centreline in this .dxf file can be extracted for example by running the following GDAL command, which generates a GeoJSON file:

ogr2ogr -f "GeoJSON" outfile.json infile.dxf -where "Layer='CentreLine'"

As an alternative to the full 12-figure OS grid reference, one can use the more conventional NGR 10-figure grid reference provided the co-ordinate systems are specified in the .svx file. For example, from our work above the Mossdale entrance is at NGR SE 01667 69779, and the following can be used

*cs OSGB:SE
*cs out EPSG:27700

*begin mossdale_ulsaj89
*fix entrance 01667 69779 425
*entrance entrance
*equate entrance path4342.0

*data normal from to tape compass clino

*begin path4342
*export 0
  0   1 1383.46  113  0
*end path4342

*end mossdale_ulsaj89 

Processing this file (here provided as mossdale_ulsaj89.svx) by cavern and 3dtopos gives the same result as above, but note this may require a recent version of survex to handle the *cs commands. The survex documentation is a bit vague on how to use the *cs command so this is probably not the only way to do this.

Copying

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.

Copyright

This program is copyright © 2015 Patrick B Warren.

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