Background map support defaults on Hakunapi item and items HTML pages #198

This issue describes expected background map projection on collections/{collection}/items/ and collections/{collection}/items/{id} pages.

Related to #199 if refactoring HTML packages (see also comments below on this issue).

Basic principles

Based on the meeting 22th June 2026:

• Showing map user interface is NOT a core feature of Hakunapi collections resources.
• The map user interface is purely additional demo feature.
• Background maps are shown only for those coordinate systems to which an open tile map service is easily available without need to develop client side customization or hacks.
• When suitable background map is not available, the map user interface shows only feature geometries without background map.

These principles apply to both cases described below.

Generic services published using Hakunapi

OpenStreetMap currently used as default (for example https://tile.openstreetmap.org/13/4671/2305.png) and this should be the default behaviour.

NLS of Finland specific services published using Hakunapi

Choices on NLS of Finland specific services with sample tiles:

• avoin-karttakuva.maanmittauslaitos.fi/avoin/wmts/1.0.0/taustakartta/default/ETRS-TM35FIN/8/123/123.png
• avoin-karttakuva.maanmittauslaitos.fi/avoin/wmts/1.0.0/taustakartta/default/WGS84_Pseudo-Mercator/13/2305/4671.png

Background map projected as WGS84_Pseudo-Mercator could be used when selecting geographic coordinates (CRS:84, CRS:84h, EPSG:4326, EPSG:4258, EPSG:10690, EPSG:10689 or EPSG:10692) or web mercator (EPSG:3857) on the user interface.

Background map projected as ETRS-TM35FIN should be used when TM35FIN projection is used (at least EPSG:3067, EPSG:3903, EPSG:10774).

ETRS-GKnn background map tiles are not available as open service, so when ETRS-GKnn is selected, no background map is shown.

Other notes

Decision between current Leaflet and commented OpenLayers specific solutions is another question. Relates also to #199.