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How to differentiate ‘actual surveyor points’ and ‘interpolated surveyor points’

 

1.To check your ‘actual surveyor points’, you may go to Surveyor Points >> Spread Input (as you can see as per image below):

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2. The actual surveyor points is listed in  Z  (mm) column.

3. As for the interpolated height (Z,mm). As the cursor moving around the surveyor points

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4. For further understanding, you may refer image below. You can see that the real Z value is 10.80m, but however at the ‘status bar’ the value of Z is 10.289m.

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This applies to EW3D and Road as well.
Note: Starting from MiTS 1.8 onwards, the default meshing method is TIN. If you are using previous version, make sure that you set the meshing method to TIN for the most exact surveyor point generation.

MiTS employs DTM ( or its variants), whereby it collects a group of surveyor points and interpolates the whole terrain based on those points. To ensure a faithful representation of the Ground Level, here are the few options you can adjust.

The Controlled by Size Method.

The way this works is like this.

  1. The program will generate a mesh of the whole terrain,bounded by the surveyor points.
  2. The program will divide the mesh into different elements, based on the “Mesh Element size”. The higher this number is, the more elements will be created, and thus more accurate
  3. For the points on the elements, the program will interpolate the z value based on the “interpolation technique.”

So, usually, the mesh produced by this method is always smoother, even when the surveyor point is scarce. However, because the program takes the whole surveyor points and do the interpolation, and the mesh point may not be the same as the surveyor points, so there might be some height difference between the surveyor points and mesh points.
To increase the accuracy of this method, you can do the following:

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  1. You can increase the Grid Cluster to a bigger value ( such as 5,000,000). Grid Cluster determines how many surveyor points to be used in the program. The higher the grid cluster value is, the more surveyor points used.
  2. You can also increase the Mesh Element Size to a bigger value ( such as 120). The higher this value is, the finer the mesh is.
  3. You can also change the Meshing Method to “TIN” to ensure the most exact interpolation. See below for more information.

Increasing both should allow you more accurate representation of the terrain, at the expense of computing time

What is the most exact interpolation technique? Answer: TIN Method!

The most exact interpolation technique is always TIN method (Meshing method=”TIN”), this is because TIN method is using the surveyor point itself as the element point to construct the mesh, unlike “Controlled By Size” method.

This means at the surveyor points, the interpolated and the actual height value will always the same for TIN method.

You can also increase the grid cluster, when using TIN method in order to increase the accuracy of the interpolation. The bigger this grid cluster value is, the more surveyor points will be used in TIN, so, more accurate!

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The drawback of TIN method is that,

  1. For surveyor points that are sparse, the interpolated surface may not be smooth, because all the points on the mesh are coming from the surveyor points.
  2. If there are a lot of surveyor points, TIN method can be slow, as the mesh can be very, very fine. Yes, it is more accurate, but it can be slow.
  3. If some surveyor points are wrong ( ie: the points are very different from other nearby surveyor points), then the cross section may look discontinuous and very ugly and also downright wrong ( other method will still give wrong cross section, but the error is not so glaring). It is the user’s responsibility to seek out the offending surveyor points and delete them. To verify that there is no wrong surveyor point, or wrong surveyor point doesn’t matter, please make sure that all the cross section existing level looks smooth.

How to check/increase the accuracy of the interpolation?

Option1: Compare against tracing contour

If your tracing file has contour, then you can compare the contour generated by the program with the tracing file contour, by clicking on the “string/contour” icon, change the Show Contour to Yes, and compare the superimposing of the tracing file contour and the Program contour.

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Bear in mind that these are all estimation, so it is not possible to get 100% match of the original contour and the generated contour.

However, with TIN method and big enough grid cluster value, you can expect to get 100% match of z value at most of the surveyor points.

Option 2: Visually inspect the generated terrain mesh. 

One can manually inspect the generated terrain mesh by clicking on “Strings/Contours”, and then set the “Show String lines=Yes”, as shown below, to visually inspect the generated terrain mesh.

string lines

 

As a rule, the more the element meshes, the more faithfully the terrain ground levels are being represented. You can always increase the “Grid Cluster” or the “Mesh Element size” and observe how it affects the generated terrain ground level.

Tips to increase the accuracy:

To verify that there is no wrong surveyor point, or wrong surveyor point doesn’t matter, please make sure that all the cross section existing level looks smooth in TIN method

How to know which value of “Mesh Element Size” and “Grid Cluster” and which Meshing Method is good enough for my project?

This is very project dependent, the only way is to experiment; if you increase the parameters, to a stage whereby the cut/fill volume doesn’t change much, then the value you pick is good enough. Increasing them further will only increase computing time with very little improvement in accuracy.