Geometric Control of Triangle Network with Point Measurements

Introduction

Geometric control against a triangle network/polygon is essentially a surface control. This means that we check a measured point against a surface in the triangle network/polygon. When it comes to polygons, all points in the polygon must lie in the same plane, meaning we cannot have twisted polygons.

The control can be performed vertically or normal to the surface in the triangle/polygon. This is shown in the figure below:

Figure: A = Vertical, B = Normal to

We see that this is an indirect deviation calculation between two coordinates. Based on the criteria vertical or normal to, the program calculates the theoretical point on the surface. It is then easy to find the vector deviation with its components in D_EAST, D_NORTH and D_HEIGHT.

Info

Regarding the sign of the deviation, this depends on whether we have positive or negative contours. Positive contours are drawn in the normal drawing direction (counterclockwise).

The figures below show the sign of the deviation against positive contour and negative contour. Positive deviations are drawn with green arrows and negative deviations are drawn with red arrows.

Figure: Positive contour with deviation

Figure: Negative contour with deviation

Tip

If we want to change the sign of the result, there is a specific option for this in the geometric control.

In this control, we may encounter situations where the program cannot calculate a point on a surface. The program will first try to go normally onto the nearest line, then onto the nearest point. Three different foot points can therefore be reported from the control:

1. Point
2. Line
3. Surface

We can also add a vertical offset and check against an "imagined" layer.

Figure: Example with vertical offset. A = Vertical, B = Normal to, C = Layer with vertical offset

Note

We specify the tolerance requirement in the control, from Min. tolerance to Max. tolerance.

Control Results

Horizontal List Field

After the control is performed, the program outputs the result of the calculation in the layer with measured control points.

Figure: Example of result in horizontal list field

The following properties are created in the horizontal list field:

• DIFF_ERROR - the total vector deviation
• D_EAST - the East component of the vector deviation
• D_NORTH - the North component of the vector deviation
• D_HEIGHT - the Height component of the vector deviation
• FOOTPOINT - indicates whether the theoretical point comes from a point (1), line (2) or surface (3) in the triangle network/polygon
• CONTROL_RESULT - the control result of the point. If the deviation is greater than the tolerance requirement, we get an X in the column

Report

We get a dedicated Excel report for the control.

Figure: Example of Excel report for geometric control