Line styles in Earthlight are defined by a plethora of parameters. On the screenshots below you can see most of them.

Colour & Width

The first tab allows you to define colour and transparency of the line, its width, the units used to calculate the width, and pattern.

Components – in this window you can see all the components of the line style. Components can be moved up and down in the order, added or removed with the four buttons placed below the window. An example of a composite line can be seen below:
Colour – controls the colour and opacity of the line.
Units – this controls how the symbol will scale with the map. First three choices are screen units dependant. It means that these will not scale with the map and symbols will always be displayed at the same size. The last choice – meters – is in ground units what means that it will scale with the map.
Pattern – this is a numeric representation of filled and clear segments in the line. Patterns are usually defined by 2 to 4 numbers separated by a empty space. The pattern starts with a filled segment and then alternates between clear and filled.

Patterns are pretty straightforward to understand when you use even number of numbers.
It can be best explained with an example.
The line that you can see below has 5 filled (F) segments followed by 2 clear (C) segments, so the pattern is 5 2 and repeats itself every 5+2 units. Effectively it looks something like this: 5F 2C 5F 2C 5F 2C 5F 2C 5F 2C …

It is slightly different when you use an odd number of numbers.
Lets define a pattern as 5 2 3.
In this case we will have 5F 2C 3F 5C 2F 3C 5F 2C 3F 5C 2F 3C 5F 2C 3F …
When odd number of numbers is used the pattern inverts on itself on every even pass through (see the boldfaced sections above). We have effectively created a pattern that will repeat itself every 5+2+3+5+2+3 (20) units.

Pattern offset - defines how far from the start, the pattern should be moved

Joins, Caps & Offset

The second tab allows you to configure:

Offset – this box controls the location of the component line in relation to the line centre. Below is an example of a composite line similar to the one above but created from 3 components and utilizing offset feature.
Geometry offset - it is similar option to normal offset, but the only difference is the style is being applied in context of whole polygon (not in context of each polygon line like in 'Offset') .
Line join – you have four choices of possible line join styling. Red, dash-dot line is an actual geometry, grey part is a brush and the red dot indicates a node.
Line cap – you have three choices of possible line cap styling. Red circles represent start and end point and gray area is a brush.

Markers

Last but not least, on the third tab you can define elements on entire line length:

The markers can be set separately for each part of the line: Mid, Start, Node and End.

Mid point term defines a entire length a the line.

With Edit… menu you can:

Load / Clear – sets or removes an image for the marker.
Size – controls the size of the marker according to the selected Unit type.
Colormode – enables override colour (controlled by Override black with and Override white with menus).

Additionally marker can be override with four options:

Size: overrides the size of the marker
White: overrides the white colour of the marker
Black: overrides the black colour of the marker
Angle: sets the marker to follow direction of then line
Trimming ratio: sets the distance between each marker and each part of the line
Gap ratio: sets the density of markers

How to create a Line style

In this example we will create a line style which has directional arrows along the length of it but no big arrow at the start of the line. To create this line style please follow the steps below.

Please click on the Resource Editor tool from Administration tab.
Click on Line styles
Choose the style that you want to edit (the easiest style to edit will be the Black Line) and click Edit in the Line Style Picker.
On the next screen, click Components to be sure that there is only one element – the black line. If there are more elements you can simply delete them by clicking Delete button. Next set the width of the line. In this example we are using a line that is 2 px wide because it nicely fits the arrow symbol that we will be configuring later.
Please click on Markers tab and then click Edit on the Mid marker.
Select any style and click Edit button. Then click on the image and select an image of an arrow (preferably a .png with transparent background) from your local computer.

Depending on the width of the line please set the size of the symbol accordingly (in this example we set the Size to 10px) and click Apply.
Depending on whether you want the arrow symbol to adjust its colour to the colour of the line or not, you should check the Override Black / White: for the former and leave it unchecked for the latter. In our example the symbol only uses black colour so there is no need to check the White: box. We also want the symbol to change angle in accordance to the angle of line segment that it describes so we need to check the Angle: box. We set the Gap ratio: to 5. This means that there will be 5 symbol size lengths gap between each symbol. If you like the new Line Style that you have created please accept this settings by clicking Apply.
Please click Save as… on the Line Style Picker and select the name and location where the new Line Style should be saved.

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