In 2019.2 it looks like we have two versions of that format supported:

As long as the version you use has a Writer (which one of them has) then it should be OK to export it back to the same format.

Thanks for your reply Mark. I have tried importing a small 5x5km chunk of Mastermap data and straight output again (i.e. no transformers used) using the "OS_MASTERMAP_TOPOGRAPHY_GML" Reader and Writer. But I only get an output GML file with a lot of "<!-- orphaned feature ..>" features.

If I tried either the GML or GML2 writer, making sure to set the schema to the latest "https://www.ordnancesurvey.co.uk/xml/schema/v9/OSDNFFeatures.xsd" from OS, I at least get meaningful output GML but unfortunately the syntax is not exactly the same as the source GML.

Hi @jaslee,

In order to avoid outputting a GML File with orphaned features you will need to define the Feature Collection Feature Type as well as the topography layer you are working with.

If you are working with existing Mastermap data you can simply read in the Feature Collection along with your Desired Layer:

Then write both these Feature Types out to the same writer and you should achieve the desired result.

Alternatively there are parameters to define the Feature Collection in the Advanced Parameters section of the Writer (http://docs.safe.com/fme/2020.0/html/FME_Desktop_Documentation/FME_ReadersWriters/os_mastermap_topography_gml/os_mastermap_topo_gml_writer.htm).

Many thanks Holly. Including the FeatureCollection layer, as you say, helps in outputting the GML correctly.