Delta Watch: How to set up and use local coordinate systems

New construction activity may affect surrounding infrastructure. The engineer has specified maximum allowed displacements/settlements/tilting of each structure. As the survey grid is not normal to the monitored structure, one needs to create a series of local transformations that calculate deformation data in the longitudinal and transversal directions for each structure.

The site is using a state-coordinate system, so all the observation data relate to this system.

Delta Watch provides different local coordinate transformations:

• Identity
• Rotation
• LQS - Two points

When configuring point groups, there is always a transformation visible called Identity transformation. This is the default setting within Delta Watch and means that no transformation will be applied to a coordinate.

All other types of transformations have to be configured in the module via Administration > Coordinate transformations. Regardless of the method used, if configured correctly, the delta values will be the same. However, it is critical to understand the fundamental differences between a Rotation transformation and an LQS - Two points transformation.

The following paragraphs will clarify both methods. Make sure you understand the behavior of each method before using them.

This is the default setting for any point group or limit definition. The user does not have to configure anything. Any data visualized or exported is on your global project grid as defined in Delta Watch in the module Administration > System configuration > Coordinate systems.

Transformation type: No transformation

A rotation transformation is a geodetic transformation and uses the so-called right hand rule. A rotation transformation is applied to all points in the point group(s) using it, regardless of their location in the coordinate system.

Delta Watch accepts values ranging from -360° to 360° as input value.

When Delta Watch rotates the complete coordinate system, the vertical axis becomes the New Northing or X. For virtual sensors like convergence and tilt, it is called the longitudinal. We strongly suggest that if using this method, you also use the vertical axis as your longitudinal.

Transformation type: Geodetic

*This field needs to be configured when defining the transformation.

A two-point rotation is configured using any two points defined in the point and limit management table. Points can be real monitoring points or manually created points for this specific purpose.

When you use a two-point rotation, there are some restrictions and differences you need to be aware of. As this type of transformation is mathematical, it shifts and rotates all points and also calls the horizontal axis the X-axis. The selected start point will get a coordinate of {E,N}={0,0}, and then Delta Watch will calculate the vertical axis first. Then it only calculates the points in the point group of which the Y-axis value falls in between the start and end point.

If you are using existing observations but also need to include observations that would be disregarded by doing so, as in, Y-value does not fall in between start and end points, then you need to calculate your own start and end points and enter them in the point and limit management table for this purpose.

Refer to Figure B for an illustration of which points will be included. Figure C shows only the points that will be calculated, as all these points fall in between the start and end point Y-value.

Transformation type: Mathematical

*This field needs to be configured when defining the transformation.

The LQS - Two points transformation has a special feature which allows one to calculate transversal delta data that is always positive toward the line of the excavation. The start and end points in this instance are typically not active monitoring points but rather dummy points provided by the surveyor. This is useful in situations where the center line is the middle of a road or above a tunnel. A positive delta value in the X-direction (transversal) means it is moving toward your center line.

For the purposes of this document, it is assumed that you have

• An active project with several points in the point and limit management table
• Defined one or more point groups

For setting up the coordinate transformations, it is not required to have a so-called automatic network setup. However, to check your configuration, it does help if there is data from several epochs available that can be viewed in charts or reports.

General workflow

The configuration process has several steps in different modules of Delta Watch, and depending on the chosen method, you might also need to use an external tool. For the purposes of this guide, each step will be explained in detail later, but for reference purposes, this is the general workflow.

1. Create your local coordinate system. Depending on the method, do one of the following:
   
   1. Rotation:
      
      1. Work out the angle that you need.
   2. LQS - Two points:
      
      1. Use existing observations (make sure you know the names).
      2. Create a center line. (first create dummy points in the point and limit management table)
2. Create specific point groups.
   
   1. Each group should only contain those observations to which you want to apply the transformation.
3. Assign the transformation to the point group.
4. Configure or adjust alarm limits.
   
   1. Make sure the correct coordinate transformation is selected.
   2. Assign the limit to one or more point groups.

Now the point group with the applied coordinate transformation can be used, for example, for data export or report creation. It is also accessible in the charts module to see delta XYZ displacements plotted over time.

If the intent is to use the Rotation method or you have to create a center line not based on actual observations, then you might need external tools to help you in the task. Topcon has a simple spreadsheet to assist you in calculating the following:

• the desired rotation angle
• dummy points shifted along a line based on known points

The spreadsheet can be found at the bottom of this article and should be used at your own risk.

Create your local coordinate system

1. In Delta Watch, navigate to the module Administration > Coordinate transformations.
2. Click on to add a new transformation definition, or select an existing one to edit ().
3. Select the type from the drop down menu.
4. Complete the mandatory fields. Make sure you give the transformation a meaningful name and label each axis appropriate to the method selected and project requirements.
5. Depending on the method you use, 
   
   1. LQS - Two points: Select the point and right-click to set it as either start or end point.
   2. Rotation: Enter the angle of rotation.
6. Only available when using LQS - Two points transformation: Enable if you want any transversal movement to be positive if moving toward your defined line.
7. Click Apply.

Assign the coordinate transformation to a point group

Now that you have defined one or more local coordinate transformations, you can use them.

1. Navigate to Administration > Point and limit management > Point group.
2. Either create a new point group and add some observations to it, or select an existing one.
3. Right-click on the point group definition and select Set transformation.
4. A new dialog box will show where you can select the transformation you want to use. When finished, the name appears in the column Transformation of coordinates for the selected point group.
5. Click Apply to save your settings.
6. Repeat the above steps for other point groups.

Use coordinate transformation with limits

If you are using Delta Watch to show limits in the charts or send an alarm when limits are exceeded, then you have to ensure that the limit is configured correctly for the point group(s) with which it is going to be used.

Follow the steps to achieve this.

1. Navigate to the tab Limit monitoring.
2. Select an existing definition or add a new one and give it a name.
3. Set the coordinate transformation you want to use.
4. You will see the names you have defined for your axis appear in the configuration table for the limit values. Configure the limit values.
5. Click Apply.
6. Repeat for more limit definitions.
7. Navigate back to the Point group tab.
8. Select the point group to which you want to assign the limit. Right-click and select Set limits.
9. Click Apply to save the changes.
10. If the selected limit has a different coordinate transformation than the point group, a warning sign  will appear to the left of the point group name.

Checking the setup can effectively only be done by introducing real movement on your monitoring site and processing the data in Delta Watch. If the introduced movement shows in the expected direction on your map and charts, then all is well.

One important thing to keep in mind:

After you have made any changes to coordinate transformation settings or point groups, always close all active open modules. The changes will not be loaded until reopened.

Cannot see the new transformation in the point and limit management table

If you had just added a new coordinate transformation while the point and limit management module was open, then the new configuration will not yet show. Either click Refresh, or simply close all open modules and reopen the one you want to work with. This forces Delta Watch to read all configurations it needs for that module.

Chart data does not change after making the configuration

If you assigned a coordinate transformation to a point group or made a change to the actual configuration of the transformation, then these changes will not be used by any module that is open. You can simply click Refresh or close and reopen the module.

Map views

When in System center > Map > Views, and you look at an info field, or when using the chart function directly from the info field, data shown will use values based on the project global coordinate system.

When you want to examine deformation in the local coordinate system as assigned to a point group, then you will have to use Analysis > Charts.

Using both methods in one project

To avoid confusion when looking at data in Delta Watch, avoid using both transformation types in the same project unless for very good reasons. If you do use both methods, make sure you use appropriate names for both the transformation definition as well as for the axis labels.

Data Export

Currently the header of coordinate data cannot be changed in the exported files. Make sure you know which column is what based on the transformation method used on the specific point group.

The best advice is: When exporting data, create a single export definition per point group! Refer to the figure for clarification of the header when exporting a point group with an LQS - Two points transformation applied.