[GIS] Specifying distance between arrows in line SLD in GeoServer

geoserversld

I have designed an SLD for a line format shapefile in GeoServer, in which I am using an arrow symbol as shown in figure.

line format shapefile

The code is:

<?xml version="1.0" encoding="UTF-8"?>
<sld:StyledLayerDescriptor xmlns="http://www.opengis.net/sld" xmlns:sld="http://www.opengis.net/sld"
    xmlns:ogc="http://www.opengis.net/ogc" xmlns:gml="http://www.opengis.net/gml" version="1.0.0">
    <sld:NamedLayer>
        <sld:Name>Blue arrows</sld:Name>
        <sld:UserStyle>
            <sld:Name>Blue arrows</sld:Name>
            <sld:FeatureTypeStyle>
                <sld:Name>name</sld:Name>
                <sld:Rule>
                    <sld:Title>A blue line with end arrows</sld:Title>
                    <sld:LineSymbolizer>
                        <sld:Stroke>
                            <sld:CssParameter name="stroke">#73ed5d</sld:CssParameter>
                            <sld:CssParameter name="stroke-width">3.26</sld:CssParameter>
                            <sld:CssParameter name="stroke-linejoin">bevel</sld:CssParameter>
                            <sld:CssParameter name="stroke-linecap">square</sld:CssParameter>
                        </sld:Stroke>
                    </sld:LineSymbolizer>
                    <sld:LineSymbolizer>
                        <!--<sld:VendorOption name="placement">centralPoint</sld:VendorOption>-->
                        <sld:Stroke>
                            <sld:GraphicStroke>
                                <sld:Graphic>
                                    <sld:Mark>
                                        <sld:WellKnownName>shape://oarrow</sld:WellKnownName>
                                        <sld:Fill>
                                            <sld:CssParameter name="fill">#0000FF</sld:CssParameter>
                                            <sld:CssParameter name="fill-opacity"
                                                >0.5</sld:CssParameter>
                                        </sld:Fill>
                                        <sld:Stroke>
                                            <sld:CssParameter name="stroke"
                                                >#0000FF</sld:CssParameter>
                                            <sld:CssParameter name="stroke-width"
                                                >1</sld:CssParameter>
                                        </sld:Stroke>
                                    </sld:Mark>
                                    <Size>20</Size>
                                </sld:Graphic>
                            </sld:GraphicStroke>
                        </sld:Stroke>
                    </sld:LineSymbolizer>
                </sld:Rule>
            </sld:FeatureTypeStyle>
        </sld:UserStyle>
    </sld:NamedLayer>
</sld:StyledLayerDescriptor>

I have to specify distance between this arrows.

Best Answer

The <Size> setting of 20 in your XML specifies an arrow length of 10 pixels followed by a gap of 10 pixels before the next arrow starts being drawn. If you add a dasharray to the XML inside the <sld:Stroke> tag as follows:

<sld:Stroke>
    <sld:CssParameter name="stroke">#0000FF</sld:CssParameter>
    <sld:CssParameter name="stroke-width">1</sld:CssParameter>
    <sld:CssParameter name="stroke-dasharray">11 0</sld:CssParameter>
</sld:Stroke>

Then each arrow will be 10 pixels long with a 1 pixel gap between them.
If you then modify the XML inside the <sld:Stroke> tag as follows:

<sld:Stroke>
    <sld:CssParameter name="stroke">#0000FF</sld:CssParameter>
    <sld:CssParameter name="stroke-width">1</sld:CssParameter>
    <sld:CssParameter name="stroke-dasharray">11 29</sld:CssParameter>
</sld:Stroke>

Then each arrow will be 10 pixels long with a 30 pixel gap between them. The two values in the dash array indicate, respectively, the number of pixels pen-down, then number of pixels pen-up.

If you want to make the gap between arrows proportional to your map scale then you could try this:

<sld:Stroke>
    <sld:CssParameter name="stroke">#0000FF</sld:CssParameter>
    <sld:CssParameter name="stroke-width">1</sld:CssParameter>
    <sld:CssParameter name="stroke-dasharray">11 
        <ogc:Div>                
            <ogc:Literal>377953</ogc:Literal>
            <ogc:Function name="env">
                <ogc:Literal>wms_scale_denominator</ogc:Literal>
            </ogc:Function>    
        </ogc:Div>
    </sld:CssParameter>   
</sld:Stroke>

Which aims for approx 100m gaps.

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