# Converting from orthometric to ellipsoidal heights

When imagery is collected via a satellite or aerial sensor, the location information (such as height) is typically measured above an ellipsoidal model of the earth, whereas many available digital elevation models (DEM) are modeled using an orthometric model. Basically, this results in two different elevation values for the same location because they are not measured from the same base location. Using a geoid correction, you can convert from one to another.

Using the Arithmetic function within a mosaic dataset, you can convert the orthometric height using a geoid before applying an orthorectification to your imagery. To do this, apply the following equation:

``h = H + N``
, where
• h = ellipsoidal height
• H = orthometric (geoid) height
• N = geoidal separation

The steps below guide you through creating a mosaic dataset containing elevation data. The Arithmetic function is applied to the entire mosaic dataset according to the equation above to generate a DEM with ellipsoidal heights.

#### Set up the mosaic dataset

1. Use the Create Mosaic Dataset tool to create an mosaic dataset.
2. Use the Add Rasters To Mosaic Dataset tool to add your elevation raster data to the mosaic dataset.
1. It is recommended to build the overviews for the mosaic dataset.

1. Open the Catalog pane and navigate to the mosaic dataset.
2. Right-click the mosaic dataset and click Edit Mosaic Dataset Functions.

The Function Editor view opens with the mosaic dataset in it.

3. Click the Raster Functions button . The Raster Functions pane appears.
4. In the Raster Functions pane, click in the Search box and type Arithmetic.
5. In the Raster Functions pane, drag the Arithmetic function into the Function Editor.
6. Place you cursor on top of the yellow Mosaic function box. On the right side of the box, you will see the word Out. Click on it, and drag the mouse to the Raster input on the Arithmetic function.

This represents H in the equation above.

7. Double-click the Arithmetic function. The Arithmetic Properties dialog box appears.
8. For Raster2, click the Browse button and navigate to the \Pro\Resources\pedata\geoid folder in the ArcGIS Pro install location.
9. Choose the WGS84.img and click OK.

This represents N in the equation above.

10. Click the Operation drop-down arrow and choose Plus.
11. Click OK, to close the Arithmetic Properties dialog box.
12. Click Apply to persist your edits.

These steps allow you to convert orthometric height to ellipsoidal. If you need to convert ellipsoidal height to orthometric, you can use the Arithmetic function to apply the

``H = h - N``
equation.

Once completed, the mosaic dataset can be used as the DEM input to orthorectify the imagery in another mosaic dataset.