Install OpenSnowMap in BaseCamp

You could try using the nakarte.me map viewer and add an overlay layer for opensnowmap http://tiles.opensnowmap.org/pistes/{z}/{x}/{y}.png .  nakarte allows you to write Garmin Birdseye .jnx files which Garmin will load into BaseCamp, if you first copy the jnx into the \Garmin\Birdseye folder of a usb stick (see JaVaWa website for details). If you intend to use the jnx files in a Garmin device for navigation, you will either have to purchase a Birdseye satellite image subscription or patch your firmware.  Alternatively you will have to learn how to convert .jnx to .img files.

Thank you for your tip. I found out that nakarte.me has even better maps for winter from mapy.cz so I downloaded jnx files with images from this map, but now I am struggling with showing them in Basecamp.

I tried copying them to various folders which were suggested on many threads with this kind of issue:

removable device/Garmin/BirdsEye

removable device/Birdseye

%APPDATA%/Local/Garmin/BaseCamp/JnxFiles

%APPDATA%/Roaming/Garmin/BaseCamp/JnxFiles

But none of them resulted in showing files in BaseCamp (My collection or Internal device memory)

The first one is the only correct option. You also need a copy of the file GarminDevice.xml in the /Garmin folder on the usb stick.

However, there appears to be a problem with one of the parameters in the JNX file format being used by nakarte.me, so  download MAPC2MAPC and use the utility Convert JNX to KMZ to to convert your JNX file. You can then check the correct georeferencing of the downloaded area by opening the converted file in Google Earth.   If you are happy with the map overlay in GE, you should then be able to import the KMZ file directly into BaseCamp, since KMZ is the Garmin Custom Maps file format.

You could also take a look at the Garmin IMG file format ski maps available from Skidea.com and copy them into the /Garmin folder of the usb stick.

Ok, I used Convert JNX to KMZ, but it threw an error during the process

System.ArgumentOutOfRangeException: Hodnota 2055701619 není platnou hodnotou argumentu Value. Hodnota Value musí být v rozsahu od 'minimum' do 'maximum'.
Název parametru: Value
v System.Windows.Forms.ProgressBar.set_Value(Int32 value)

and it stops at Level 2

16:42:02 draw order= 100
16:42:02 ====================================
16:42:02 level = 1
16:42:02 Tiles and doc.kml file written
16:42:02 DeletingD:\Programy\Garmin\Garmin\Sněhové mapy\nakarte.me_czech_winter_z15 (2)_l1_tiles
16:42:02 ====================================
16:42:02 level = 2

It creates a .kmz file, but it is much smaller than the original .jnx. I tried importing it into BaseCamp and it seemed to work fine - it showed the snow map layer on the right place, but it doesnt repaint with closer zoom. I exported nakarte with Zoom 15 but the kmz file is not possible to be zoomed in that close. I guess that the conversion didn't copy all the zoom layers as the filesize is really small. .jnx is 1,4 GB .kmz is 5,8 MB

Have you ever come accross this issue?

This is probably MAPC2MAPC software issue with unhandled expection of Scroll bar which might have nothing to do with maps. I tried contacting the developer and will see.

I haven't come across it before since I usually build my JNXs using a different process. However I agree here it seems to be a zoom levels issue and the small file size for the KMZ is because it is only converting one of the zoom levels, probably just level 12. MAPC2MAPC appears to be able to convert the JNX to Mobile Atlas and extract the png tiles from all levels of zoom.

The developer of MAPC2MAPC is usually very responsive and helpful. 

Meanwhile I will do some further testing.

mapy.cz is a vector map with more than five zoom levels available. When you select Zoom 15, nakarte indicates that it will download the tiles for zoom levels with a lower number. i.e. 14, 13 and 12. 

Have a look at the zoom levels of your nakarte JNX using the JNX Customizer v1.0.28 tool and hover over each zoom level in turn to see an example tile, the number of tiles, the zoom level and JNX scale.

In my example download at Level 15, there were in fact five levels downloaded which included Level 11.  

In JNX Customizer ensure the checkboxes are ticked for all five zoom levels and then save. 

Load this updated file onto your usb \Garmin\BirdsEye folder (I assume you now have the GarminDevice.xml in the \Garmin folder). Start BaseCamp and the JNX file should now be visible in the Internal Storage of the usb Garmin FlashDrive. You can then send it from the usb to a folder in BaseCamp, then eject the usb.

This now works for me for a test mapy.cz map of the centre of Prague and allows me to see all 5 zoom levels. If the zoom levels are not ticked then the JNX is not visible in BaseCamp.

I tried this again for a Zoom 16 download. Initially I could only see 4 zoom levels in Basecamp as the deepest zoom level was hidden but when I increased the distance values by one level for each zoom level in JNX Customizer and resaved, all five zoom levels became visible in BaseCamp. 

I had no luck with JNX Customizer - I couldn't find the file to download it and after I got it from the developer, it threw an error immediately and I couldn't start it.

On the other hand, as you said, the developer of MAPC2MAPC was very quick and helpful and he sent me test release with fixed bug in the coversion. Now it works fine and I get kmz file for each zoom level and I was able to display it in BaseCamp in CustomMaps folder

Thank you very much for your help and effort.

You're welcome - I'm glad you found a solution.