coastline data - vertical datum

[dfhill]

Hello,

Quick question. Typically, one will download bathy data and coastline data (the latter, say, from coastline extractor). One then loads the coastline and the bathy into SMS and generates a grid (say, for use in ADCIRC). The coastline becomes part of the grid, and presumably (?) the nodes on the coastline boundary are assigned a depth of zero. Is this correct?

If it is correct, is this not in error, since most coastline data describe the MHHW mark?

Thank you for any replies.

Dave.

[Rusty Jones]

Hi,

When we generate a mesh we don't assign the nodes on the boundary of the mesh an elevation of 0.0. While this might make sense in your specific application, it isn't always the case that this would be desirable.

However, it isn't difficult to get the same effect using the tools for working with TINs in SMS. To do this:

1. Read in your map and elevation data (scatterset)

2. Click on the coverage to make it active.

3. Use the select feature point tool (the top one) and select the all of your nodes. Assign them an elevation of zero using the edit window above the graphics window.

4. Repeat #3 using the select vertex tool.

5. Choose feature objects | Map->Scatter - make sure you change the z values to arc node and vertex elevations. Also turn off the triangulate toggle. Click Ok.

6. This will create a scatterset with breaklines for your arcs but no triangles.

7. Click on the new scatterset to make it active and switch to the scatter module.

8. Choose Scatter | Merge scatter sets.

9. Select to merge the new scatter set and your original elevation scatter set. In the overlapping region options, choose delete lower priority scatter points and leave maintain triangulation checked. Select your new scatterset in the top and move it up so it is above your original scatter set. Click OK.

The created scatterset should have the same triangulation except for right around your breaklines which will have been forced in at its existing elevation (0.0).

I hope this helps.

Hello,

Quick question. Typically, one will download bathy data and coastline data (the latter, say, from coastline extractor). One then loads the coastline and the bathy into SMS and generates a grid (say, for use in ADCIRC). The coastline becomes part of the grid, and presumably (?) the nodes on the coastline boundary are assigned a depth of zero. Is this correct?

If it is correct, is this not in error, since most coastline data describe the MHHW mark?

Thank you for any replies.

Dave.

[dfhill]

Rusty,

So, can you clarify the following. If one loads in a coastline file, and a scatterset (bathy) file, and generates a mesh, how are the depth values at the boundary nodes determined? By simple extrapolation? For the most part, the boundary nodes will be 'outside' the polygon defined by the bathy nodes, so it would have to be extrapolation and not interpolation. Do you ever run into problems with the extrapolation leading to unrealistic values of depths at the boundary nodes?

Thanks

D.

Hi,

When we generate a mesh we don't assign the nodes on the boundary of the mesh an elevation of 0.0. While this might make sense in your specific application, it isn't always the case that this would be desirable.

However, it isn't difficult to get the same effect using the tools for working with TINs in SMS. To do this:

1. Read in your map and elevation data (scatterset)

2. Click on the coverage to make it active.

3. Use the select feature point tool (the top one) and select the all of your nodes. Assign them an elevation of zero using the edit window above the graphics window.

4. Repeat #3 using the select vertex tool.

5. Choose feature objects | Map->Scatter - make sure you change the z values to arc node and vertex elevations. Also turn off the triangulate toggle. Click Ok.

6. This will create a scatterset with breaklines for your arcs but no triangles.

7. Click on the new scatterset to make it active and switch to the scatter module.

8. Choose Scatter | Merge scatter sets.

9. Select to merge the new scatter set and your original elevation scatter set. In the overlapping region options, choose delete lower priority scatter points and leave maintain triangulation checked. Select your new scatterset in the top and move it up so it is above your original scatter set. Click OK.

The created scatterset should have the same triangulation except for right around your breaklines which will have been forced in at its existing elevation (0.0).

I hope this helps.

[Rusty Jones]

Hi Dave,

When you create a mesh, the boundary nodes elevations are interpolated/extrapolated like every elevation.

So yes, if the boundary node is outside the triangles in the scatterset extrapolation will be used. Your options for extrapolation are limited. You can assign all of them a specific value or you can use IDW extrapolation. IDW gets a number of vertices around your points (you can control this with the options button), computes a weight for each vertex value, and computes the weighted average of these values.

Extrapolation is definitely problematic. If you have a surface that is sloping up to the west, but you extrapolate to the West of the surface the extrapolated points will actually slope down. This is because as you move further from the surface the weights of the points equalize (all vertices are far away rather than some close and some far).

I try to encourage people to avoid extrapolation if at all possible. Generally an engineer can provide better estimates outside of the surface than would any algorithm inside SMS. An engineer may know more about the terrain from maps, images, or personal experience and can infer information that an algorithm cannot. Another option is to find more data like perhaps find DEM elevations along the coastline. If you do this you may want to try creating breaklines where your surfaces meet. If you want all the coastline points to be at elevation 0.0 and have all of the nearby elevations slope up to this value, the approach I explained yesterday would work well. If you do decide to give extrapolation a try, make sure you look at the extrapolation options button and set the # nearest points. Sometimes the defaults give far too many points and it can take a very long time.

Cheers,

Rusty

Rusty,

So, can you clarify the following. If one loads in a coastline file, and a scatterset (bathy) file, and generates a mesh, how are the depth values at the boundary nodes determined? By simple extrapolation? For the most part, the boundary nodes will be 'outside' the polygon defined by the bathy nodes, so it would have to be extrapolation and not interpolation. Do you ever run into problems with the extrapolation leading to unrealistic values of depths at the boundary nodes?

Thanks

D.