# Geometric function

## Overview

The Geometric function produces an orthorectified image based on a sensor definition and a terrain model. It increases the positional accuracy of a dataset by accounting for elevation.

To learn about orthorectification, see Fundamentals of orthorectifying a raster dataset.

To learn about orthorectification, see Introduction to ortho mapping.

## Notes

You can use this function when your raster data has rational polynomial coefficients. If you don't have a digital elevation model (DEM), you can specify the Constant Z value, but this generates an inaccurate result; even a low-resolution DEM tends to have better accuracy than a Constant Z value. If you have a DEM, check the Use DEM check box.

Computation time for increasing positional accuracy can be higher for some sensor models. To improve performance, set a Tolerance. This allows for a less rigorous solution, using an approximation to reduce the number of computed transformations. The specified tolerance threshold is the maximum distance from the source point. The default for the NITF (NCDRD) raster type is 2. If left blank or 0 is specified, the approximation is not used.

If you click the Geodata Transform button, a window appears displaying the information specific to the geometric transformation that will be applied to the raster data.

## Parameters

Parameter nameDescription

Input Raster

The input raster.

Constant Z

Specify a constant elevation to use for the Geometric function.

Use DEM

Check this box if you want to specify the DEM.

DEM

Browse to the DEM to use in the Geometric function. You can use a DEM contained in the mosaic dataset as a raster dataset or as a mosaic dataset whose output is a DEM.

Z Factor

Satellite rational polynomial coefficients (RPCs) are scaled for elevation datasets with vertical units in meters. If your elevation uses other vertical units, enter a Z Factor to rescale to meters. For example, if your elevation units are in feet, you would use a value of 0.3048 to convert your elevation units from feet to meters.

Z Offset

The base value to be added to the elevation value in the DEM. This could be used to offset elevation values that do not start at sea level.

Geoid

Most elevation datasets, such as USGS NED or ArcGIS Online World elevation, are orthometric heights, so it is necessary to select the Geoid correction for compatibility with satellite RPCs, which require ellipsoidal heights. Check the Geoid check box to apply the geoid (EGM96) correction to the z-values, unless your DEM is already referenced to ellipsoidal heights.

Parameter nameDescription

Raster

The input raster.

Method

Choose the elevation method for the geometric function:

• Use Constant Z—Specify a constant elevation to perform the Geometric function.
• Use DEM—Specify a DEM to perform the Geometric function.

Constant Z

Specify a constant elevation to use for the Geometric function.

DEM

Specify the DEM to use for the Geometric function. You can use a DEM contained in the mosaic dataset as a raster dataset or as a mosaic dataset whose output is a DEM.

Z Factor

Satellite rational polynomial coefficients (RPCs) are scaled for elevation datasets with vertical units in meters. If your elevation uses other vertical units, enter a Z Factor to rescale to meters. For example, if your elevation units are in feet, you would use a value of 0.3048 to convert your elevation units from feet to meters.

Z Offset

The base value to be added to the elevation value in the DEM. This could be used to offset elevation values that do not start at sea level.

Geoid

Most elevation datasets, such as USGS NED or ArcGIS Online World elevation, are orthometric heights, so it is necessary to select the Geoid correction for compatibility with satellite RPCs, which require ellipsoidal heights. Check the Geoid check box to apply the geoid (EGM96) correction to the z-values, unless your DEM is already referenced to ellipsoidal heights.

Tolerance

Specify the maximum tolerable error in the geometric function, given in number of pixels. The default for the NITF (NCDRD) raster type is two.