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Z shapes - merging to existing z points

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Hi,

I'm using z shapes to model proposed ground level modifications. From what I gather the polygon is merged to existing z points if no points are snapped to the boundary. If any points are snapped to the boundary, then the entire boundary is set based on the manually specified elevation points.

Is there a way to use both options in one polygon? For example on one side of my polygon I want to merge to existing levels, but on the other side I want to snap elevation points to the polygon to set elevations (there will be a vertical wall to existing levels here). I guess problems may arise when transitioning from "merge" to "no merge" points on the polygon?

Cheers

Richard

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Gday Rusty,

What you want to do is certainly possible. There's a good illustrated example of how to do it on Pages 4-28 and 4-29 of the 2008 user manual. Basically you need to snap points in a _pts layer with elevations of -99999 at locations where you don't want to merge with the grid. You need to have snapped points on either side of the -99999 points with real elevations assigned (points A and B in the user manual diagram).

As for this:

From what I gather the polygon is merged to existing z points if no points are snapped to the boundary. If any points are snapped to the boundary, then the entire boundary is set based on the manually specified elevation points.

I think only the vertices that have points snapped to them are changed. Other vertices are interpolated from the existing z-values, and then any additional vertices due to the Shape_Width_or_dMax attribute are interpolated from these mixed elevations.

I think what you say would only be true if NO MERGE is used in shape options. IF MERGE ALL is used, any snapped points will be ignored.

I'm not sure I've been all that clear, but most options should be available to you. Table 4.8 in the user manual may be of help.

Kind Regards

Rhys

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Thanks for your reply Rhys,

It sounds like what I'm trying to do is pretty straightforward. I was a bit confused by comment :) on page 4-21 which says "If there are one or more points snapped to the perimeter vertices, the perimeter is not merged with the zpt values...".

However as you suggest, Teble 4.8 indicates that as long as I don't specify MERGE ALL or NO MERGE, then I can use a combination of snapped points to set elevations, vertices with no points to merge to existing values, and points with elevation -99,999 to ignore existing zpts and interpolate to the nearest vertices.

Cheers for your help

Richard

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Just to clarify a few points:

1. p4-21 of the manual is referring to the "Create TIN Zpts" command which is treated slightly differently to the "Read MI Z Shape" command which Table 4.8 refers to. One of the differences is how snapped points are treated. To do what you want to do Richard, you should use the "Read MI Z Shape" command.

2. The vertices automatically inserted by the Shape_Width_or_dMax attribute are (if being merged) based on the current surrounding Zpt values (ie. their not interpolated from the elevations at the digitised vertices) - this gives a smoother merging.

Hope this helps.

Time for those Easter beers!

Cheers

Bill

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