The Offset Z tool in Virtual Surveyor allows you to adjust the elevation of one or multiple Elevation Terrain items, in just a few steps.



The Offset Within tool appears in the TOOLS for Elevation Terrain tab when an Elevation Terrain is selected.


Overview


Usage Guide

  1. Select the Elevation Terrain item in the Project View.
  2. Go to the TOOLS tab.
  3. Set the offset value in the Offset Z box.
    1. Positive values move the terrain up.
    2. Negative values move the terrain down.
  4. Press Enter.
    1. The Elevation Terrain item is moved up or down according to the offset value setting.


Offset Value Information

You can find your Offset Z value in the Selection Box while the Elevation Terrain item is selected.


Why the Offset Z Tool is Necessary

If your photogrammetry was processed with an improper vertical datum, we know it takes a long time to go back and reprocess the photogrammetry. Obviously it is better to process the photogrammetry properly the first time, but sometimes different measuring standards are used. In the event where the vertical datum is processed improperly on your Elevation Terrain, Virtual Surveyor provides a quick fix with the Offset Z tool to adjust the offset datum. Here are a few situations where you can apply the Offset Z tool as a workaround solution:

  • You receive drone photogrammetry data and need to adjust the Elevation Terrain to independently measured check points in order to make vertical elevation datum changes.
  • You need to move the Elevation Terrain item from one elevation datum to another datum.
  • A specific scenario typically found in European countries where cross-border measurements are needed to correct the elevation datum from one country's vertical datum to another.

Read the following section to see examples of when these situations apply and how you can use them with your own data.


When to Use Offset Z

Adjust the Elevation Terrain to Independently Measured Check Points

If you find that your Elevation Terrain isn't matching your own independently measured check points, this method is a good workaround to adjust the average error and get your Root Mean Square Deviation (RMSE) to a more accurate measurement. 


Use the Check Points function to see the before and after results of using Offset Z:

  1. Import independently measured check points into Virtual Surveyor.
  2. Select the DSM in the Project View and click on the EXPORT tab.
  3. In the Survey section, click Check Points.
  4. In the Check Points Statistics window, see if the Average Error shows as a positive or a negative value.
    1. If it is "0.00", no further action is needed.
  5. If it shows as either a negative or positive value, you may want to apply the Offset Z value.
  6. In the example image below, the Average Error shows as 0.22 ft. and your RMSE is .25 ft.
  7. Click OK.
  8. Ensure the DSM is still selected and click on the TOOLS tab.
  9. Set your Offset Z to 0.22 ft and press Enter to move the elevation up.
    1. You will see the terrain update in the Viewport.
  10. Go back to the EXPORT tab and click Check Points.
  11. Your Average Error now shows 0.00 ft and your RMSE is more accurate at only .11 ft.


Go From One Elevation Datum to Another Datum

In situations where your Elevation Terrain's vertical datum doesn't match a universal or a country's datum, you don't need to go back and reprocess your data. A quick fix is to use the Offset Z tool to move your elevation from the drone photogrammetric ellipsoidal height to a universal or country's geoidal height. In Virtual Surveyor, it only takes a couple of steps to make this adjustment.


Note: This situation only applies for small areas. Changing from an ellipsoidal elevation to a geoid elevation is not a constant, so for larger areas this workaround would not be ideal. But for small areas, which is often the case in surveying, this change can be seen as a constant.


In the example below, the Elevation Terrain is set to the drone's photogrammetric ellipsoidal height. By comparing the drone ellipsoidal height information with the vertical datum we're currently using, you can see that it is 13.37 feet below the vertical geoid Z height. 


To adjust the drone's ellipsoidal height to the vertical geoid Z height:

  1. Select your Elevation Terrain item and click on the TOOLS tab.
  2. Set your Offset Z value to 13.37 ft.
    1. This moves the Elevation Terrain item up 13.37 ft, and the points now fit on the surface of the terrain.

  


Match Elevation Terrain Items with Cross Border Elevation Measurements

This use case is just a specific example of going from one elevation datum to another. This situation most often occurs in Europe where countries share borders with each other and use their own elevation datums


As an example, lets say you have a project in Belgium that crosses into the Netherlands. You've received some photogrammetric data that was processed using the Netherland's vertical datum of Normaal Amsterdams Peil (NAP) and you need the Elevation Terrain's vertical datum to be in Belgium's Tweede Algemene Waterpassing (TAW). Rather than trying to get the photogrammetric elevation datum reprocessed so that it's using TAW, you can quickly use the Offset Z tool as a workaround to match your Elevation Terrain item to Belgium's elevation datum. 


Thanks to available data, we know that the NAP is 2.33 meters higher than the TAW (Tweede Algemene Waterpassing - Wikipedia article). With knowledge of the difference between the two countries' elevation datums, we can now use the Offset Z tool to make the adjustment.


How to match your elevation datum to another, with the Offset Z function:

  1. Know what elevation datum you are using and learn what elevation datum you need to match it to.
    1. In this example we are using NAP and matching it to TAW.
  2. Open your project in Virtual Surveyor.
  3. Select your Elevation Terrain item in the Project View and click on the TOOLS tab.
  4. Set your Offset Z value to -2.33 and press Enter.
    1. The Elevation Terrain item, which was set up using the NAP datum, updates in the Viewport as it drops down 2.33 meters to match the the TAW datum.
  5. With the Elevation Terrain item still selected, look in the SELECTION box for the Offset Z value.
    1. It should show as -2.33 m and your vertical datum is now matched with TAW.


Tips and Tricks

  • You can select multiple Elevation Terrain items in the Project View and apply the offset Z settings to all of them simultaneously.