How do I use a geoid model in a one-point one-step transformation using System 1200?

Creating a One-Step transformation using a Geoid model in Datum & Map

2-step_transformations with a geoid model

Flash Movies

If you are not familiar with local side geoids, read the first part of the section below on how to create a local side geoid before watching the first movie below.

• One-step with 1 Horz point, 1 Benchmark, a known azimuth, using a geoid model (8 mins)
• What a one-step does, with comments on controlling a normal one-step (5 mins).

How do I use a geoid model in a one-point one-step transformation using System 1200?

Example files with local coordinates, WGS84 positions, and a geoid model field file are available for download in a zip file below. With these files you can use the simulator Determine Coordinate System program to practice this routine. My thanks to Henri Ayers for providing these files and the insight into the process.

Right-click here and select Save Target (or Link) As...

This process only works with a geoid model that is referenced to the WGS84 ellipsoid and has been created with the Apply on the local side option selected.

If you do not have such a model, you need to create this geoid model using the WGS84 ellipsoid in Leica Geo Office (LGO) and check the "Apply on the local side" box as shown below: The name of your model should describe its usage.

Next create a geoid model field file in LGO using this geoid model

and transfer it to the receiver System RAM.

Create a job on the receiver which contains the local point's known local coordinate.

In the example files, our known point is NBS1:

Occupy the known local point with your RTK rover.

Our NBS1 example was observed as:

From the Main menu, select Programs and then select Determine Coordinate System from the Programs menu:

Press F1[CONT]

Give your coordinate system a name, select the appropriate files, and select One Pt Localistn as the method:

Press F1[CONT]

The Transformation Name will be the same as the coordinate system. Make sure the Transormation type is Onestep, and the Height Mode is Ellipsoidal as shown:

Press F1[CONT]

Select the geoid model field file you created above (the example provides the model shown):

Press F1[CONT]

Select the point to match in Pos & Height. In the example, select NBS1:

Press F1[CONT]

Next determine your rotation. Normally we will just use WGS84 North as shown:

Press F1[CONT]

Select how you want to compute the ground scale. Normally we will just use the ellipsoidal height of the measured known point as shown:

Press F1[CONT]

View your results:

Press F1[STORE] to save the coordinate system. You will be returned to the main menu. If you select Survey, you see that the WGS84 job is the active job, and the coordinate system you just created is the active coordinate system.

You are now done and can survey and view your data in local ground coordinates with elevations relative to the known point, and with the geoid model taken into account.

Using the example file, we can view the position of ORM1 and see that its ortho height is 115.499.

If we had done a standard one point localization by selecting orthometric as our height mode, and not using a geoid model, the height of ORM1 would be 115.505.

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  • Creating a One-Step transformation using a Geoid model in Datum & Map

    In Datum & Map, you need to have two projects. You cannot work with only one project like you can in the receiver. Project A will be the project with WGS 1984 positions and project B will be the project with the local coordinates.

    1. Open Datum & Map.
    2. Select project A in the top-right window.
    3. Select project B in the bottom-right window.
    4. Click on the Match tab at the bottom of the program window.
    5. Right-click in the upper-right window and select Configuration.
    6. Set the configuration screen as shown below: Note the height mode.
       
    7. Match the points you want to use in the transformation. Make sure that all points are matched in position only. If you match any of the points in height, the geoid model will be ignored and the heights will simply be shifted from WGS 1874 to the local height.
    
    
    
    
    1. Click on the Results tab at the bottom of the program window.
    2. Right-click on the blank portion of the screen and select Store...
    3. Give the transformation a name, select to distribute residual by 1/distance and click okay.
        
    4. Go to the Coordinate System Management screen in SKI-Pro or LGO and right click on the one-step coordinate system you just created and select Properties from the pop-up menu. In this case, the ellipsoid used to define the geoid model that you enter here must be WGS 1984.
       
       
       Your coordinate system is now ready for use in SKI-Pro or LGO.
    5. Make sure that the new coordinate system is attached to project A, open that project, and go to the Points tab.
    6. Select the Local Grid view and then from the top pull-down menu select Tools | Compute Geoid Separations. The ortho heights will now be shown. Remember that SKI-Pro does not compute the geoid separations automatically.
    7. Transfer the coordinate system to the card (The geoid is not transferred) by right-clicking on the coordinate system and selecting Send To | PC Card. This will put the the coordinate system in \GeoDB\GPSTRF.DAT for System 500, or \DBX\TRFSET.DAT for System 1200.
    8. Create a geoid model field file using the same geoid model attached to your coordnate system and put it in the Data\GPS\Geoid directory on the card.
    9. Put the card into the receiver and transfer the coordinate system to the receiver RAM.
    10. You can either transfer the geoid model to the system RAM, or leave it on the PC Card.
    11. Edit the one-step coordinate system you just transferred and select the geoid model field you want to use in the Geoid field.
        
        Your one-step coordinate system is now ready to give you orthometric heights by adding the geoid separation from the model to the ellispoid height the rover has computed relative to the base station. This method relies on the Base having a correct ellipsoidal height. If you wish to adjust the system to a known ortho height at a point, you will need to edit the base's ellipsoidal height appropriately.