What if the user specifies an unmatching CRS

[cDanowski]

I was wondering about the follwing question. It's nothing time critical, just a thought about an improvement for the future.

Let's say the user wants to import data from WFS with actual CRS EPSG:25832, which would be transformed to EPSG:4326 during import.
But what if the user, who has to manually specifiy the CRS parameter in the import request, specifies some other CRS, e.g. EPSG:31466? Maybe the import will succeed, but definitely not with coordinates in EPSG:4326 as the transformation - should there happen any - will produce some other coordinates.

Is there any chance to detect the actually used coordinate system from the submitted dataset? Geotools might offer some method schema.getCoordinateReferenceSystem();, which gets CRS information from FeatureType, if present. Of course, NULL values might occur, if the input dataset does not specify its CRS information appropriately.

Two things might be done based on that extracted information:

1. One might use that automatically extracted CRS information - if present and NOT NULL - to compare it to the user-submitted CRS information to either:
   • throw an error, like Mismatch of CRS parameter. Request contained CRS parameter "EPSG:25832", but dataset inspection showed that geometries actually use "EPSG:4326"
   • override request parameter with the CRS information from the source dataset
2. if no valid CRS information can be extracted from input dataset, then assume that user definition in request must be correct

Maybe we can discuss that in more detail.

[SebaDro]

Yes, the described problem about automatically extratcing CRS information is a known problem. The most critical point is, that each data format handles the CRS information different. E.g. GeoJSON usually doesn't contain any CRS information since it will assuem WGS84 implicitly. Although, one could encode geometries in a different CRS within a GeoJSON dataset, so this would end up in misleading conversions.

Is there any chance to detect the actually used coordinate system from the submitted dataset? Geotools might offer some method schema.getCoordinateReferenceSystem();, which gets CRS information from FeatureType, if present. Of course, NULL values might occur, if the input dataset does not specify its CRS information appropriately.

As the KomMonitor Importer uses GeoTools under the hood, the WFSv1Converter has to know the CRS in beforehand because the GeoTools GML parser has to. The schema of a FeatureCollection might contain the CRS information but only if the GeoTools parser could determine it or it has been set manually to the parser. As far as I know, you first have to set the CRS parameter:

GML gml = new GML(GML.Version.WFS1_0)
gml.setCoordinateReferenceSystem(DefaultGeographicCRS.WGS84)

This ensures, that the schema of the FeatureCollection contains the CRS information. But again, you explicitly have to know the CRS before parsing the dataset.

Any user should be aware of the CRS problem, when trying to import geographic datasets into KomMonitor. Since the user should know his datasets, I don't hink it's a big deal to set the CRS parameter requesting the importer. Though, it would be helpful if the CRS could be determined automatically, I don't think there is an easy approach. Any suggestions?

[cDanowski]

okay, that is fine with me. Then I will just add a clear hint in the KomMonitor Web Client for the CRS parameter that the user must proceed with caution.

So this can be closed then.

[SebaDro]

I'd like to leave it open, because such a feature would be very helpful from a users point of view. Maybe, we could work on it for future versions.