Hi @Dmitry Boyko ,
We have the following answer for you from a member of the Microsoft Maps team:
Currently MapPolylines are drawn without antialiasing and there is no setting to change that behavior. Looking at your screenshot, you may be better served by using a custom tile layer. See CustoMapTileDatasource here: https://learn.microsoft.com/en-us/windows/uwp/maps-and-location/overlay-tiled-images You can draw each tile in a callback using whatever method you like including antialiasing. It will also tend to perform better for a large collection of static lines like the topographic contours in your example.
Sincerely,
IoTGirl