The QGIS Line Direction Histogram Plugin

rose

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
    rose rose90_1
  • All the direction bin sectors will have the same size (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 positions of the direction bins are shown graphically.

    Direction bins illustration and options
    specbins
  • The user can choose if the histograms shall be “orientation” neutral (0-180 degrees instead of 0-360 degrees).

    Orientation neutral Not orientation neutral (with twice the number of bins)
    rose rose36_5_360
  • 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 by 200). – added in version 1.3
  • The histogram can be copied to the clipboard. – added in version 1.4
  • If the plugin window is resized, the direction histogram is also resized.

Tiling

Added in version 2.0.

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.

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

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

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

Added in version 2.4.

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

Direction mean
Not or. neutral Orientation neutral
  medium low high
dirmeannon dirmean dirmeanlow dirmeanhigh
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 sectors of the bin that corresponds to the mean direction with colour according to the strength of the trend (white for direction neutral, full colour for maximum strength).

Saving the tile mean directions to CSV

Added in version 2.6.

When saving the tile mean directions to CSV, the result is a CSV file with three columns:

  • The first column (“ID”) is an ID that identifies the tile. This ID is stable for a tile polygon layer, so mean directions for different datasets can be compared if the same tiling layer is used.
  • The second column (“Direction”) is the direction of the mean direction sector (for the direction neutral case) or the mean direction vector (for the non-direction neutral case). The unit is degrees, and the range is 0 to 180 for the direction neutral case and 0 to 360 for the non-direction neutral case. 0 is grid north, 90 is grid east, 180 is grid south.
  • The third column (“Strength”) is the strength of the trend. The value range is from 0 to 1. 1 corresponds to the case when all line segments have this direction (non-direction neutral case), or all line directions are in the interval defined by the current sector (direction neutral case).

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 four columns:

  • The first column (“StartAngle”) contains the start angle of the direction bin.
  • The second column (“EndAngle”) contains the end angle of the direction bin.
  • The third column (“Length”) contains the accumulated lengths of the line segments that fall within the bin.
  • The fourth column (“Number”) contains the number of line segments that fall within the bin.

”.” 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 following options are available:

  • The user can specify if only selected features are to be used. This is the default if the layer has selected features.
  • The user can specify the number of direction bins (the default is 8).
  • The user can specify an angle offset (clockwise from north) for the direction bins (the default i 0).
  • The user can choose to ignore the “orientation” of the lines. In that case, two lines with opposite directions will end up in the same direction bin (this is the default).
  • The user can specify an output CSV file for either the (over all) histogram, or the tile mean directions (in case a tiling layer is used)
  • The user can specify if line segment length shall be used for weighting the bins (this is the default).
  • The user can choose to use the logarithm to define the radius of the sectors. The default is not to use the logarithm.
  • The user can choose 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).
  • The user can choose to produce a point layer (“RoseDiagrams”) styled with rose diagrams according to a tiling by a selected polygon layer. For this option, it is also possible to specify the location for storing the generated SVGs (that are used for styling the rose diagram layer.
  • The user can specify what to include in the output graphics: The (background) rings - the rings are useful for comparing the lengths of the sectors; The direction histogram - the distribution of directions over a number of direction bins; The direction mean.

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 falls into, and the length is added to the accumulated length for the bin.

Polygons are split into its rings, and the line geometry of each ring is 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). 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.

distancemean

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.

normalised

Versions

The current version is 2.6

  • 2.6
    • Added graphical output flexibility (#23)
    • Added CSV output for the tile mean directions (#24)
  • 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.