Extrapolating a TIN surface

[Anonymous]

Hello,

I have a surface that is defined by a few points with a lat, long, and elevation (7 of them to be exact).  The surface has a funny shape, and does not encompass the entirety of my project boundary.  What I would like to do is extrapolate this surface so that it extends to the edges of my project boundary while retaining the slopes of the edges as they extend outwards.  Is this possible?  I have been banging my head against the wall trying to get it to work.

I had a little success by extracting a 3D object from the TIN surface and extruding the edges outwards, but ran into a roadblock because one of the edges is concave and it gets really ugly if I try to extrude that edge.

Thank you in advance!

[jeff_rivers]

Extract the boundary of your surface, and make a featureline out of it.  Edit the FL to remove the concave section, replacing it with a single straight segment.  

Create another featureline from your project boundary.  

Create a new surface from these two featurelines, it should be what you want.  

[sboon]

You may not have enough data for this to work properly, but what you're looking for is surface Kriging.  It's available through the surface smoothing command.  Basically what you do is select existing TIN points in a region reasonably close to the area you want to expand the surface into.  Try to limit this region to points that make sense for predicting the elevations of the new area.

The second step is to pick an area for the new TIN points. Usually I create a grid with appropriate spacing.  For each new point the software does some statistical magic to calculate the elevation, based on the distance from each source point.

Steve
Please use the Accept as Solution or Kudo buttons when appropriate

[Anonymous]

Thank you, kriging may be what I am looking for.  However, I am not sure what you mean when you say "create a grid."  Could you elaborate on this a bit more?

Unfortunately, in response to the previous answer, I don't think that adding feature lines created from the project boundary is the answer.  I'll give some more info on what I am trying to do and hopefully that might explain why.

I have a project boundary, some LIDAR surface data, and a few GPS located points that represent drill holes which are associated with a depth from the surface to a mineral layer.  I can bring the LIDAR data in to visualize the terrain surface, get contours, and all sorts of stuff.  I can bring in the drill hole data and create a surface from those to represent the layer of mineral beneath the terrain surface.  However, the mineral layer surface created from the drill hole points only extends to the outermost drill hole points which are well within the project boundary, but I want to estimate the location of the mineral beneath the surface at any point within the project boundary based on the strike and dip of the surface from the drill hole data.  So I can't just slope the edges of the surface down to 0,0 or anything, the mineral layer surface needs to be "stretched" out to the edges of the project boundary but it needs to maintain its existing slope.  So if the surface is tilted such that the west side is lower in elevation than the east side, when I stretch out the surface the west edge (of the surface boundary) should continue down at the same slope so the new west edge is even lower and the east edge is even higher than the original edges.

[jeff_rivers]

Ah, I misunderstood what you meant by 'project boundary'.  I assumed you had elevation data for the line defining the outer boundary of your project.  

Now if I understand correctly, you want to get the radial slope along your surface, and project that slope outwards till reaching your project boundary. So you are assuming that the slope of the mineral surface is approximately constant radially, although it varies at different points around the surface.  

One thought now is to use a corridor to create the projected surface.  Make a featureline along the outer boundary of the mineral surface, offset it inwards an appropriate distance, then create an alignment & profile from that line.  Then use an assembly with two link width & slope subassemblies.  The first sub goes from that inner alignment to the boundary featureline, and it gets the instant slope of the surface.  It then passes the slope along to the second sub.  The second sub goes from the boundary FL and extends to the project limits, and it gets its slope from the first sub.  

Can you post your drawing, or is it client/confidential information?  

[Anonymous]

I'm not super comfortable with posting the entire drawing, but I can at least put up a screenshot so you can see visually what I am trying to do.

In the screenshot, you can see two things.  

The first is a thick yellow boundary.  This is just a polyline representing the boundary of an existing permit area that may be used.  The polyline is not associated with any elevation data, think of it as a study area.  You can also see a smaller surface, most of which lies within the polyline boundary.  You can see the 7 points that were used to define the surface, and it has orange and green contours.  As you can see, it is reasonably smooth, though it does have a bit of slope to it, and it gets funky on the west side of the surface.  

What I am trying to do is estimate the elevation of the mineral at a point that lies on the XY plane within the yellow boundary but outside of the orange and green contoured surface.  

[jeff_rivers]

Got it, seeing a picture helps.  I'd just create a featureline along the surface created from your point data, then offset it to make a couple of new featurelines that extend into the area within the yellow boundary.

Then use the Modify - Elevation by Reference tools to set the elevations of those featurelines to match the grade of the surface.  Create a few slope labels and place them within the surface to see what the slope is doing perpendicular to the surface boundary.  The Featureline editing tools will allow you to adjust the points along the new featurelines to continue those slopes out into the area bounded in yellow.  

Last, make a new surface from those featurelines.  

The featurelines are in red, and the orange dashes show the slopes extrapolated from reference points on the existing surface.