Interpolation 2 points

Question

I have point shapefile. Some points have Z value and others not. I have to determin somehow missing values.
I want to do next:
0.) find a point(1) without Z value
1.) select two points(2,3) with Z value on the opposit side of first point(1)
2.) calculate average Z of points(2,3)
3.) select point(1) and write calculated average Z in Z column of point(1)

I had once code for this in Avenue (AV 3.3), but now is different.

Can you give me some advice?

FranciNovak · Answer

If I simplify whole procedure.

I have point shapefile (manholes).
Each manhole should have data about invert elevation and terrain elevation.
But some manholes don't have invert elevation data.

Is possible in Python to do next task in one move (click):
1.) Selecting 3 points.
2.) Calculating (z1+z3)/2 from point which have data and assign this value to point where is no data (0)

(see Picture belowe).

kind regards

FranciNovak · Answer

It would be only approximately interpolation for rare single points without Z.
There is not enough points (digital elevation model) for using Spatial Analyst.
I would like only to obtain constant fall of elevation (Z) in one direction (manholes invert elevation - sewage system).

In AV 3.3 I had 2 icons:
I selected two points(2,3) on the opposit side of first point(1). Pressed 1 icon and got (x2, y2, z2) and (x3, y3, z3) in the memory.
I selected points(1) without Z. Pressed 2 icon and got (x1, y1). It was calculated z1 depends on distance from point x2 and x3 and Z value was written in table (linear interpolation).

XanderBakker · Answer

I have point shapefile. Some points have Z value and others not. I have to determin somehow missing values.
I want to do next:
0.) find a point(1) without Z value
1.) select two points(2,3) with Z value on the opposit side of first point(1)
2.) calculate average Z of points(2,3)
3.) select point(1) and write calculated average Z in Z column of point(1)

I had once code for this in Avenue (AV 3.3), but now is different.

Can you give me some advice?

H Franci,

I think it is necessary to provide some more information in order to solve it:* how do you want to determine the points on opposite sides of the current point without Z?
* what to do if one of those point doesn't have a Z value itself?
* wouldn't it be more accurate to use a distance weighted average value?
* what if you can't interpolate between points and have to extrapolate?
* do the points represent vertices on a line or are they randomly distributed?

In this case maybe the easiest way would be to create a surface or TIN based of the known Z values and then use "Add Surface Information (3D Analyst)" to obtain the Z values in the point feature class.

Kind regards,

Xander