# The QGIS Line Direction Histogram Plugin¶

## Functionality¶

The QGIS Line Direction Histogram Plugin creates a rose diagram that can be used to investigate the distribution of the directions of line segments of a line or polygon vector dataset.

The accumulated lengths of the line segments for each direction bin determines the shape of the histogram. Alternatively, the number of segments can be used (no weighting on line segment length).

Line and Polygon vector layers are supported, including multigeometries.

Feature selections are supported.

The number of direction bins for the histogram can be specified.

Fewer bins More bins

All the direction bin sectors will have the same width (same number of degrees covered). This type of angle histogram is also called a “rose diagram” or “rose plot”.

An angle offset (positive or negative - clockwise or counter clockwise) for the direction bins can be specified.

The effect of changing the number of bins and the angle offset are shown graphically.

Direction bins illustration and options

The user can choose if the histograms shall be “orientation” neutral (0-180 degrees instead of 0-360 degrees).

Orientation neutral Not orientation orientation neutral (twice the number of bins)

- A direction histogram (or rose diagram) is displayed, showing the distribution of the directions according to the chosen bins.
- The histogram can be saved to a CSV file.
- The histogram can be saved as PDF (100 mm by 100 mm) and SVG (200 pixels by 200 pixels).
- The histogram can be copied to the clipboard.
- If the plugin window is resized, the direction histogram is also resized.

### Tiling¶

There is an option available for producing at point layer styled using rose diagrams (using SVG files) according to a tiling specified using a polygon layer. The illustration below shows the result of using a regular grid polygon layer for tiling.

- The polygon layer can be chosen.
- The location for storing the SVG files can be specified.
- The SVG files are not deleted - the default location is the system temporary file directory / folder.

Warning

CRS transformations are not performed, so the tiling layer should have the same CRS as the input layer. A warning is given if the CRSs are different.

All the other options are respected when generating the rose diagrams for the tiles.

Note

Due to a QGIS issue with SVG file caching (#13565: “modifying a svg already cached doesn’t invalidate the cache, renders as version initially loaded during a session”), the SVG files have to be stored using unique file names. This produces a lot of SVG files that are not deleted by the plugin.

### Direction mean¶

An indication of the direction mean (direction and strength) can be added to the rose diagrams.

The user can choose the base colour for the direction mean indication (added in version 3.1.0).

Direction mean | |||
---|---|---|---|

Not dir. neutral | Orientation neutral | ||

medium | low | high | |

- Non-orientation neutral
- The direction mean is visualised by a “vector” in the rose diagram, with length corresponding to the strength of the trend.
- Direction neutral
- The direction mean is visualised by filling the bin with colour according to the strength of the trend (white for direction neutral, full colour for maximum strength).

## The displayed histogram¶

The displayed histogram is normalised, so that the maximum value of the direction bins will result in a sector with a maximum length, and the lengths of the sectors of the rest of the bins are scaled proportionally.

## The saved histogram (CSV)¶

The saved histogram is a CSV file with three or six columns.

Three columns for exporting only directional trends
(**Mean direction (for tiling)**):

- “Id” is the system generated ID that identifies the tile
- “Meandir” contains mean direction in degrees.
- “Strength” contains strength of the mean direction. From 0.0, for even distribution, to 1.0, for maximum skew).

Six columns for exporting everything (**Histogram**):

- “StartAngle” contains the start angle of the direction bin.
- “EndAngle” contains the end angle of the direction bin.
- “Length” contains the accumulated lengths of the line segments that fall within the bin.
- “Number” contains the number of line segments that fall within the bin.
- “Meandir” contains mean direction in degrees.
- “Strength” contains strength of the mean direction. From 0.0, for even distribution, to 1.0, for maximum skew).

“.” is used as the decimal separator in the CSV file.

The CSV file is accompanied by a CSVT file that describes the data types of the CSV file columns.

## Options¶

- The user can specify if only selected features are to be used (if is no selection, all features will be used). If the input layer has selected features, using only the selected features is the default.

### Bins¶

**0-180 (“orientation” neutral)**defines if the “orientation” of the line segments should be ignored. Ignoring means that two lines with opposite directions will end up in the same direction bin (this is the default).**Number of bins**specifies the number of direction bins to use (the default is 8).**Angle offset**specifies an offset (clockwise from north) for the direction bins (the default i 0).

### Tiling¶

- With
**Use tiling**turned on, the user can choose a**Polygon layer for tiling**the input layer. With tiling in place, a point layer is produced (where each point is the centroid of a tile). A rose diagram is produced for each tile, and the point layer is styled with these rose diagrams. For this option, it is also possible to specify a location for storing the generated SVGs. This makes it possible to save the rose diagram point layer and use it in other projects

### CSV Export¶

- The user can specify an output CSV file for the (over all) histogram.
- In the case of tiling, there is a choice between
**Histogram**(for exporting the histogram) and**Mean direction (for tiling)**(for exporting the directional trends only). Without tiling, the histogram is exported.

### Output¶

**Include direction trend**will include the calculated direction mean in the rose diagram(s).If

**0-180 (“orientation” neutral)**is not checked, a line that shows the average direction vector is added. If not, the sector that has the highest mean direction value is given a background colour for its full length. The amount of colour indicates the strength of the direction trend (from white, for an even distribution, to 100% colour if all the line segments have a direction that belongs to this sector). For the “orientation” neutral option, the sector bins are transparent.**Color**chooses the base colour for the direction trend.**Include direction histograms (rose diagrams)**includes the rose diagrams (disabled if**Include direction trend**is not checked).**Include percentage circles**turns on percentage circles in the rose diagrams.

### Rose diagram tuning¶

Under the rose diagram in the dialog, there are some options that can be used to tune the rose diagrams:

**No weighting on length**determines if line segment length shall be used for weighting the bins (this is the default).**Logarithmic**allows the logarithm of the accumulated lengths to define the radius of the sectors. The default is not to use the logarithm.**Area proportional**can be checked to have the*area*of a sector of the histogram be proportional to the accumulated amount for that sector. The default is that the*length*/*radius*of a sector is proportional to the accumulated amount (histogram like behaviour).

## Implementation¶

The calculations of the histogram is performed in a separate thread. Each line geometry is traversed from start to end. For each segment of the line, the angle and length are calculated. The angle is used to determine which bin the segment belongs to, and the length is added to the accumulated length for the bin.

Polygons are split into its rings (the outer ring and zero or more inner rings), and the line geometries of the rings are used for the calculations.

### Mean direction¶

- Non-orientation neutral
The normalised mean direction vector (

**dist_mean**) is calculated using vector summation from the bins (sectors), not from the original lines. Each sector is represented by a vector (**sector**) with length equal to the total length of the line segments in the sector. The middle of the bin’s sector is used as the angle for the sector vector. The result vector is normalised by dividing by the sum of the bin / sector line lengths.This means that the mean direction is very sensitive to the number of bins - the more bins, the more precise will the mean direction be.

- Orientation neutral
The mean direction is found by calculating the magnitude of the “direction trend” for each of the bins.

The “direction trend” for a bin, B, is calculated using the values for all the bins within 90 degrees offset from the angle of B. For each bin (including B), the angle between that bin and B is calculated (using the angles of the middle of the bin sectors), and the cosine of that angle is multiplied by the bin value.

The “direction trend” of B is the sum of these values. The bin with the largest value is taken to represent the direction mean.

The value is then normalised to a [0..1] scale using the the sum of the bin values (

*total_sum*) as the maximum value, and the value that had been obtained (*even_dist_value*) if the distribution of line segment lengths among the bins had been even, as the minimum value.

## Versions¶

The current version is 3.1.1.

- 3.1.1
- Eliminated the superfluous “Line Direction Histogram” submenu under the Vector menu
- Changed “Tiling Layer” to “Tiling” in the GUI
- Documentation update

- 3.1.0
- Fixed issue with geometry conversion for tiling (#27)
- Added possibility to specify the colour for the direction trend (#28)

- 3.0.1
- Fixed issue with plugin icon not showing in the QGIS user interface (#25)
- Fixed issue with help not showing (#26)

- 3.0.0
- Support for QGIS 3

- 2.5.1
- SVG export fixed (#21)

- 2.5
- User interface modified to allow use on smaller screens (#20)

- 2.4
- Added directional mean indicators to the rose diagrams (#14)
- User interface change from toolbox to tab for options

- 2.3
- Added the logarithm option (#17)
- Fixed CSV output (#16)

- 2.2
- Fixed angle offset issue (#15) and added some circular statistics output (#14)

- 2.1
- Fixed issue #13 (problems with multipart geometries)

- 2.0
- Added option to generate a point layer with rose diagrams based on tiles provided through a polygon layer (#10, #11)
- Set the default state for “selected features only” based on the presence of a selection (#12)

- 1.6
- Fixed progress bar issue (#9)

- 1.5
- Added help button

- 1.4
- Copy to clipboard button added (#8)

- 1.3
- PDF and SVG export added
- Area-proportional sectors option introduced

- 1.2
- Fixed issue #1 (on update of min/max for angle offset)
- Fixed issue #2 (divide by zero when no features in layer)
- Fixed issue #3 (effect of “no weighting” checkbox)

- 1.1:
- Selected features option introduced
- Unweighted option introduced
- Multigeometry support
- CSV file header row added
- CSV angle offset bug fixed
- User interface fixes and updates

- 1.0: First official version.

## Citation¶

Would you like to cite / reference this plugin?

Tveite, H. (2015). The QGIS Line Direction Histogram Plugin. http://plugins.qgis.org/plugins/LineDirectionHistogram/.

Bibtex:

```
@misc{tveitesde,
author = {Håvard Tveite},
title = {The {QGIS} Line Direction Histogram Plugin},
howpublished = {\url{http://plugins.qgis.org/plugins/LineDirectionHistogram/}},
year = {2015--2020}
}
```