here sir..the default scenery at the region...Now i can see tht the pattern follow exactly like the default terrain.But i already made a new landclass with sbuilder x
View attachment 33890
not sure about your terminology:
LandClass is just raster based data used to map certain textures and autogen to the surface. You don't need that, you have photoreal.
Vector data draws line and polygon shapes
on top of the LandClass, even if the LandClass is in the higher scenery set. Vector data can also flatten the terrain, for example at airport backgrounds and water polygons.
So far about the theoretical side,
your problem indeed seems to be either the shoreline or there is water made of vector polygons in that area.
My tip is still to make an exclude
vector polygon. to suppress the shoreline. Also otherwise there might be a wake effect inside your landscape.