MapTileDownloader: Empty tiles from WMS-servers due to excessive stretching

Description

There is a problem with the aspect ratio of the {bbox}-variable. They are not square (in EPSG-4326) which can lead to problems with WMS servers using raster maps. Probably due to excessive stretching (by a factor of 1.49 in the example below), it can create an empty tile. Many WMS servers enforce a maximum stretching factor (e.g. 1.2).

Possible fix: an additional EPSG-3857 {bbox}, based on TMS (like https://github.com/timwaters/whoots/blob/master/whoots_app.rb). Those should be always square and won't require stretching.

How to reproduce?

For example this is an request generated by the app which returns an empty tile: https://sgx.geodatenzentrum.de/wms_dtk_bkg?FORMAT=image/png&LAYERS=DTK500&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG:4326&BBOX=8.43750000,47.87214397,8.61328125,47.98992167&WIDTH=256&HEIGHT=256

If I change the link so the latitude and longitude have the same distances, it works again: https://sgx.geodatenzentrum.de/wms_dtk_bkg?FORMAT=image/png&LAYERS=DTK500&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG:4326&BBOX=8.43750000,47.87214397,8.61328125,48.04792522&WIDTH=256&HEIGHT=256

The same is true if I reduce the pixel height of the tile a bit: https://sgx.geodatenzentrum.de/wms_dtk_bkg?FORMAT=image/png&LAYERS=DTK500&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG:4326&BBOX=8.43750000,47.87214397,8.61328125,47.98992167&WIDTH=256&HEIGHT=247

Your Environment

OsmAnd Version: 4.3.5 Android/iOS version: Android 11 Device model: Oneplus 7 Pro

Maps used (online or offline):
https://sgx.geodatenzentrum.de/wms_dtk_bkg?FORMAT=image/png&LAYERS=DTK500&SERVICE=WMS&VERSION=1.1.1&REQUEST=GetMap&STYLES=&SRS=EPSG:4326&BBOX={bbox}&WIDTH=256&HEIGHT=256

osmandapp / OsmAnd

MapTileDownloader: Empty tiles from WMS-servers due to excessive stretching #16154

Description

How to reproduce?

Your Environment