Calculating slopeX and SlopeY for Hydro Polys in ArcGIS

[Timmo]

Has anyone been able to calculate this?

Generating a 3D polygon isnt too hard in terms of workflow (simply drape the polygon over a DTM in ArcGIS/3D Analyst) although it might be difficult to do for a whole country (i.e. ArcGIS/3D Analyst isnt the most stable of programs with large datasets sometime)

...however when it comes to subdividing the polygons and assigning a slope I'm scratching my head a bit....I wonder how MS/Aces did it? Presumably, if you have each vertex at an assigned Z value (i.e. a Z enabled polygon) then the slope is inherent in it anyway?

Maybe: Union the water polygons and a Slope polygon file (Generated from a TIN) this will only give a slope so there would need to be another step (perhaps using an aspect polygon shapefile??) to calculate the x and y components?

...it seems quite difficult? I doubt even converting a country wide DTM into a TIN and then into a polygon slope file would be easy due to the sheer amount of data to be processed.

The other option which I havent explored is to simply drape river water polygons on the terrain by changing the terrain.cfg

Where for art thou Doug!

 

[rhumbaflappy]

The default terrain.cfg contains this:

// Legacy_LandWaterMask_Water_NoFlatten
[Texture.381]
Name=Legacy_LandWaterMask_Water_NoFlatten
Color=FF638AC2
guid={5835459A-4B8B-41F2-ADC1-DEE721573B28}
Textures=OCEAN_SEA_LARGE_LAKE
FlattenMode=none
LandClassRemapType=none
ExcludeAutogen=Yes
RenderToTexture=Yes
RenderPriority=80000
Water=Yes

This is mesh-clinging water ( FlattenMode=none ).

Dick

 

[DougMatthews]

Has anyone been able to calculate this?

Generating a 3D polygon isnt too hard in terms of workflow (simply drape the polygon over a DTM in ArcGIS/3D Analyst) although it might be difficult to do for a whole country (i.e. ArcGIS/3D Analyst isnt the most stable of programs with large datasets sometime)

...however when it comes to subdividing the polygons and assigning a slope I'm scratching my head a bit....I wonder how MS/Aces did it? Presumably, if you have each vertex at an assigned Z value (i.e. a Z enabled polygon) then the slope is inherent in it anyway?

Maybe: Union the water polygons and a Slope polygon file (Generated from a TIN) this will only give a slope so there would need to be another step (perhaps using an aspect polygon shapefile??) to calculate the x and y components?

...it seems quite difficult? I doubt even converting a country wide DTM into a TIN and then into a polygon slope file would be easy due to the sheer amount of data to be processed.

The other option which I havent explored is to simply drape river water polygons on the terrain by changing the terrain.cfg

Where for art thou Doug!

We compute those slopes within our water sloping code. We have broken the water polygons up into sloped triangles and dissolved adjacent triangles with very similar slopes into the shapes you can see examining the default CVX BGL files in TmfViewer. We determine the unit normal of those result polygons and convert to be meters per degree.

The first part (breaking the source polygon up) may be difficult to do with off-the-shelf application.

I recommend trying two things:

1) If you have achieved 3D polygonal water already, try just setting slope x and y to 0. See how it looks. These values are used for extrapolating flattening to nearest mesh points. At reasonable terrain setting it may turn out to be acceptable.

2) If you have NOT achieved 3D polygonal water already, try using the texture Dick pointed out in this thread to drape your water on the terrain. See how it looks.

-Doug

 

[Timmo]

Ok I will do a little test and see.

Aces/MS used the same LINZ Topo data I am so the reason I need to re-convert this not to change the xy accuracy but to change the Z alignment to fit the other main mesh in use in NZ (payware 20m mesh)

 

[Timmo]

Just thought I'd report my findings although I haven't found a solution yet.

Just using 3D polygons with values of '0' for SlopeX and SlopeY does work....but isn't satisfactory. It triggers the water slope-reclass to rock too often (especially in narrow gorges)....in my opinion this threshold is set too low, it is actually quite difficult to notice water with high slope when using the water effects. It would be better to have a sloping water polygon remain as water rather than reclassing to rock....ah well, can't change that now I guess.

The problem Im having is the 3D interpolation in ArcGIS wont subdivide the polygons, it will just simple assign each vertex a Z (height) value..this causes ridges down the river.

Im going to try subdividing the river polygons before 3D conversion and possibly adding a generic slope value and see what that does.

Otherwise, I may have to re-interpolate my source 3D mesh using the pre-split rivers and a function of the interpolation tool which classes those as 'lakes' (i.e. polygons with equal height)