• Which the release of FS2020 we see an explosition of activity on the forun and of course we are very happy to see this. But having all questions about FS2020 in one forum becomes a bit messy. So therefore we would like to ask you all to use the following guidelines when posting your questions:

    • Tag FS2020 specific questions with the MSFS2020 tag.
    • Questions about making 3D assets can be posted in the 3D asset design forum. Either post them in the subforum of the modelling tool you use or in the general forum if they are general.
    • Questions about aircraft design can be posted in the Aircraft design forum
    • Questions about airport design can be posted in the FS2020 airport design forum. Once airport development tools have been updated for FS2020 you can post tool speciifc questions in the subforums of those tools as well of course.
    • Questions about terrain design can be posted in the FS2020 terrain design forum.
    • Questions about SimConnect can be posted in the SimConnect forum.

    Any other question that is not specific to an aspect of development or tool can be posted in the General chat forum.

    By following these guidelines we make sure that the forums remain easy to read for everybody and also that the right people can find your post to answer it.

FSX Would a hydro polygon out of its LOD9 flood the entire area?

My scenery was ok and then I added a few extra hydropolygos. One was a polygon style (river mouth) and the others were just polylines, both marked as Hydro Polygon Canal Perennial.

Now I see that a whole area has been flooded by the sea. Is this because of the hydropolygons? some (polyline hydros) may have gone out of the limits of their LOD9 but it is basically a great part of the LOD9 being reworked that got flooded.

As shown in the picture attachment, my area of work is the LOD9 marked as A. However, now after adding some rivers both LOD9s B & C are flooded. Strange is that part of A is showing above sea level (not totally flooded).

The only excludes I have are small and are within area A, in particular at the airfield, nowhere else. I understood that an exclude would affect an entire LOD9 but I never thought anything else would cause a LOD9 area to be flooded.


  • MPCE-LOD9.jpg
    72 KB · Views: 465
Last edited:
I have been experimenting with this issue. I had three things that could be the culprit:

a) The polygon (area) tagged Hydro River Perennial that is at the mouth of
one of the rivers.

b) The polyline tagged Hydro River Perennial that continues (a) for the rest
of the river

c) Another river implemented as polyline (rather than mixed) and also tagged
as Hydro River Perennial.

If I disabled all three everything went back to normal (no flooding). If I left only (a) it remains normal. As soon as I enabled EITHER (b) or (c) inland flooding ocurred in the adjacent LOD9.

Since I had used the Wonderful Extents Calculator to know my bounds (when doing the remake of the coastal area) I knew the boundaries (West and South) those rivers should have. I edited the polylines of both rivers to cut them short so that they remained WITHIN the LOD9 where they go out to see (same as the airport's LOD9).

Unfortunately, even when the polyline river(s) are cut short to remain within the LOD9 labelled A in the figure flooding would still ocurr.

I think there is something weird or perhaps not documented with regards to Polylines and Rivers. This is strange because in a previous project I also used a polyline hydro for a couple of rivers coming out to see and I did not experience any inland flooding.

I am at a loss here.