Add SLD tag to define arrows direction convention

[geojs]

I added a new SLD tag to handle arrows direction according to meteorological convention or oceanographic convention: https://blogs.esri.com/esri/arcgis/2013/07/17/displaying-speed-and-direction-symbology-from-u-and-v-vectors/ I need this feature to display wave direction from WW3 output: http://polar.ncep.noaa.gov/waves/products.shtml

[guygriffiths]

I'm happy to include this distinction. A few things, however:

• We don't merge any pull requests into master, except for urgent bugfixes. This should go into develop
• You have implemented the conventions the wrong way around. The current behaviour of EDAL is that arrows are plotted in the direction of the vector (i.e. what is labelled as oceanographic in your link), and your implementation reverses this for OCEANOGRAPHIC instead of METEOROLOGICAL.
• I think the labels DEFAULT and METEOROLOGICAL would be better, since what you've labelled as OCEANOGRAPHIC is the actual vector direction. After which DEFAULT should be the default, if no value is specified.
• With these points in mind, we don't need an oceanographic_arrows.xml style (since it is already the default behaviour). Instead you should implement a meteorological_arrows.xml style, as well as corresponding ones for the other arrow styles.
• Please do not remove the existing constructors on ArrowLayer, SizedArrowLayer, and ColouredSizedArrowLayer. Create additional ones instead.
• Wind barbs and upstream dots are a particular styles which already have an implicit meaning. These styles should not be modified, nor should they accept the <resc:ArrowDirectionConvention> tag.

However, bearing all this in mind, along with the fact that you were trying to display data from WW3, I wonder if there is not some other issue at play here? ncWMS should display WW3 data as you would expect, since we have always used the oceanographic convention for vectors (with the exception of wind barbs, which are their own specific style). If that's the case, I'm more than happy to work with you to debug the problem - I've not had anyone requesting that ncWMS plot arrows according to the meterological convention, and I've got no pressing reason to do so.

[geojs]

The modifications:

• Change merge to develop
• I reversed the implementation.
• Use DEFAULT instead of OCEANOGRAPHIC.
• Remove oceanographic_arrows.xml, add 3 styles meteorological_arrows.xml, meteorological_triangle_arrows, meteorological_fat_arrows.xml.
• Add news constructors.
• Don't change the code for wind barbs and upstream dots styles. Do I need to return an error (SLDException) if <resc:ArrowDirectionConvention> is set for these styles ?

It is to display this WW3 data: ftp://ftp.ifremer.fr/ifremer/ww3/HINDCAST/NORGAS/2013/dp/ww3.201305_dp.nc Example with "default-arrows" style: I guess the arrows direction are in the "wrong way", maybe it use meterological convention ? Thanks for your help.

[guygriffiths]

The problem is that WW3 are using a non-standard convention: https://forge.ifremer.fr/plugins/mediawiki/wiki/ww3/index.php/En:wave_direction_conventions which is the opposite of what your link (and my experience of ocean data) suggests.

I'm happy to add this change - if you could implement vector_meterological_arrows.xml, sized_meteorological_arrows.xml, and colored_sized_meteorological_arrows.xml in your branch then I'll merge it in. Apologies - looking back it wasn't at all clear, but this is what I meant by "the other arrow styles".

[geojs]

Thanks you for the link about the non-standard convention with WW3 (my experience with ncwms is limited to display WW3 outputs), I have now a better understanding of direction conventions.

I implemented the styles vector_meterological_arrows.xml, sized_meteorological_arrows.xml, and colored_sized_meteorological_arrows.xml in my branch.

[guygriffiths]

Great, thanks for that. I've merged into develop, so that'll make the next release.