Wednesday 19 February 2014

Archaeological drawing symbols in QGIS

QGIS style tools for vector layers allow us to build custom symbols for archaeological drawings (plans and sections). In this post I will present some preliminary examples of customizable styles for graphically representing hachures and soil texture (clay, silt, sand and so on). For each, the .qml style files are downloadable from (section programming): you can put them in your QGIS project and/or add to QGIS style manager for testing and modifying.
Using vector styles makes the drawing more automatic and fast, because is no more necessary to draw manually each individual symbol. Furthermore, vector styles enable to change quickly the size of symbols when the scale changes: in such way there is no need to scale any single object.
The bulding of a new style is managed by QGIS style window: with button “Add symbol layer” is possible to make a new symbol joining 2 or more individual symbols. All symbols settings (dimension, angle, offset, etc.) can be changed in the same window.

In plans, the edges of holes and ditches and the differences in level are represented with hachures. They are composed by an isosceles triangle and a line: the triangle marks the highest part of the feature, where the slope begins; the line represents the length and direction of the slope, with the end of the line showing the bottom of the slope. I created 3 different styles of hachure for point layers (hachure_pn), linear layers (hachure_ln) and polygon layers (hachure_pl).
For representing the differences in level, for example at the top of wall, I create a shapefile of points connected to an attribute table with two numeric columns named “angle” and “length”: the first column records rotation angle of hachures, the latter records the length of hachure's lines (where lines are not required, the column value will be always “0”). Then I draw points in the place where there are differences in altitude, I load hachure_pn style, I rotate each points with “Rotate Point Symbols tool” (in advanced digitizing toolbar) and I write the line length I desire in attribute table.
All settings are customizable: from “Marker – Advanced – Rotation field” you can set attribute columns for angle; from “Vector Field Marker” you can set attribute columns for angle and length; from “Simple marker” you can set triangle dimensions and offset: usually I give an offset that is half of triangle dimension for fitting the point to the line.
Hachure_pn style can be used also for linear or polygon features.

If I have to draw a scarp, I create a linear shapefile and I load hachure_ln style. It's possible to modify dimension of triangles, space between symbols, width of line, etc. changing parameters in QGIS style window.

For representing a ditch or a hole, I create 2 polygon shapefiles, one for top perimeter of the feature, one for the bottom limits. The top polygon is drawn by hachure_pl style and the bottom polygon is filled by white color: in the layer tree the bottom polygon must stay over the top polygon for covering the hachure's lines that exceed the slope. Like above, dimensions, space between symbols, offset and so on are editable in QGIS style window.

Soil texture
I post some examples of archaeological layers filled with symbols representing soil components: clay, silt, sand, silt-sand.

You have to pay attention to the settings of the these styles. Many symbols are set in map units and not in absolute measure (i.e. millimeters) to better fit the variation of scale during the display. For printing, all style settings must be changed according to the plot scale; for this purpose printing tests are recommended. Finally, I suggest to export in .svg format and modify the layout in Inkscape for adjusting the last details.
In future I hope to post other examples of archaeological styles. But if you have other ideas or you want to contribute with your personal QGIS archaeological styles, we could create a shared library of archaeological symbols for QGIS.

Denis Francisci

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