Developer’s Corner: Customizable arrow in GeoServer

Today we want to share a recent development in  GeoServer: an arrow mark whose proportions can be customized to get the type of arrow you’re looking for.

The general syntax for this arrow mark is the following:

<WellKnownName>extshape://arrow?hr=[hrValue]&t=[tValue]&ab=[abValue]</WellKnownName>

All parameters are ratios, and they are all optional. By default the arrow is the usual 1 by 1 mark, whose actual size you can control in SLD via the Mark size attribute. In particular, the parameters and their default values are:

• t: thickness of the arrow base, between 0 and 1, default is 0.2
• hr: height over width ratio, a positive number, default is 2
• ab: arrow head base ratio, between 0 and 1, default is 0.5

The values can be modified to change the proportions of the arrow, for example, varying t between 0 and 1 results in the following arrows:

Changing the height to width ratio will generate arrows that are wider or thinner:

Finally, changing the arrow head base will move the position where the head of the arrow starts:

Playing with all the three parameters will allow you to get the arrow of the desired proportions. Now, what if your desired characteristic is just having a long arrow? Since size is controlled by the mark size attribute, you’ll have to make a big arrow whose height ratio makes for a thin one, and maybe push up a bit the arrow base, like in this example:

Which will result in the following arrows: