Outlook: WFS 3.0

Jay: Did you ever flashy-thing me? Kay: No. (M.I.B.)

If you want/have to deal with WFS version 3.0 also known as OGC API – Features, please forget eveything you ever learned about XML Filter Expressions!

Important note: You can NOT use XML Filter Expressions in order to query a WFS 3.0! That's technically impossible.

Normally, this chapter should end here, because filtering an API Features WFS needs a different approach.

But as WFS 3.0 is becoming more and more popular, I'll give you some hints, how to use WFS 3.0 with FME.

First of all, you'll need a service URL like this:

https://ogc-api.nrw.de/lika

Please copy and paste this URL into a web browser!

The first difference is, that you'll see a GetCapabilities-like document, which is human readable (by default) and not an XML.

"Capabilities" of WFS 3.0

Beneath "Ressources" please click the first link in order to show the Collections.

Please note: Collections are the most important concept of WFS 3.0.

According to the specification (chpt. 4.1.4) a feature collection or collection is a set of features from a dataset.

Collections-Link

Collections are presented the same way.

You'll see a (human readable) list of feature types (sets), the service provides, e.g.

• Flurstück

• Flurstückspunkt

• Gebäude, Bauwerk

• Katasterbezirk

• Nutzung

• Nutzung-Flurstück

• Verwaltungseinheit

Next, you may click on a feature type (set), let's say "Flurstück", which opens a graphical preview of both

1. Geometry

2. Attributes

Is this relevant for working with WFS 3.0 in FME?

To be honest: not really!

But another thing is really important working with FME: the URL

https://ogc-api.nrw.de/lika/v1/collections/flurstueck/items

If you modify the URL the following way

https://ogc-api.nrw.de/lika/v1/collections/flurstueck/queryables

i.e. by replacing "items" with "queryables", you'll get a list of attributes you can use for filtering.

So, let's try it!

Key	Value
OGC API URL	https://ogc-api.nrw.de/lika
Version	3.0
Collections	flurstueck (cadastral parcel)
Queryables	flstkennz

First, we need a different reader: OGC API - Features (WFS 3.0)

OGC API - Features (WFS 3.0) Reader

Next open [Parameters ...] as usual.

Expand [Constraints] and click [...] to select a collection.

Please note: if you just want to see a few features for inspecting, you can limit to number of features to read in reader parameters.

Run the reader and examine the features.

Comparison Operators

As mentioned before, it is not necessary/possible to write XML Filter Expressions to do attributive queries.

You can define queries via the user interface. Just expand [Constraints], [Collection Query Parameters].

Collection Query Parameters

In [Collection(s)] you can choose the collections, you've choosen above.

In [Parameters] you'll see default parameters predefined by the OGC specification, but unfortunately no service specific parameters, i.e. attributes.

OGC default parameters

But remember the Queryables-URL, which shows a list of attributes you can use for filtering.

You can use any attribute from the Queryables-List as Parameter Name, in this case "flstkennz".

Tadaa! You know where we are?

And of course, you can process a list as discussed in Bulk Processing by using the FeatureReader-Transformer.

Spatial Operator: BBOX

As discussed in BBOX, the bounding box is the swiss army knife of all spatial filters, because you can request a BBOX from the server and do the spatial operation locally in FME subsequently.

Also remember the predefined [Collection Query Parameters] we have seen above, especially

• bbox

• bbox-crs (Coordinate Reference System)

If your service comes in WGS84 (EPSG:4326) as default coordinate system, it is sufficient to pass a comma seperated coordinate list to the service (see spec):

1. lower left X,

2. lower left Y,

3. upper right X,

4. upper right Y

If the default coordinate system ist not WGS84, you have to define the coordinate system of your bbox coordinates in [bbox-crs].

Important note: you have to use the coordinate string provided by your service!

I.e. you can not use the string, you normally use in FME, e.g. "EPSG:25832".

bbox-crs	result
http://www.opengis.net/def/crs/EPSG/0/25832	works
EPSG:25832	will not work

To summarize:

Parameter Name	Parameter Value
bbox-crs	http://www.opengis.net/def/crs/EPSG/0/25832
bbox	367441,5620100,367619,5620254

BBOX Filter in WFS 3.0

Please compare the corresponding result shown in BBOX!

Ok, so far the detour to WFS 3.0.

If you want to, you can play around with this:

https://ogc-api.nrw.de/gebref/v1/collections/gebref/

id: DENW36HK100076NN

Caution: please check the coordinates in Inspector ...