Understanding Elevation scale and offset

The Elevation scale is used to convert the pixel values in a digital elevation model (DEM) into their correct elevation value. For example: since an 8-bit channel can only contain integers between 0 and 255, you may have a DEM that was multiplied by 10 to maintain the decimal precision of its elevation values. A DEM pixel may have a value of 102, but the actual elevation that it represents is 10.2. To convert the DEM pixel value from 102 to 10.2 you must multiply it by 0.1. Therefore, you type 0.1 in the Elevation scale box to convert the DEM pixels back to their true values.

The Elevation offset is used to add a value to the pixel values in a DEM to obtain their actual elevation value. Using the same example, perhaps the DEM pixel with a value of 102 actually represents an elevation value of 1,102. To store the elevation values in an 8-bit channel, 1,000 was subtracted from all the pixel values when the DEM was created. Therefore, you must type 1,000 in the Elevation offset box to restore the true values.

You can also use Elevation offset to adjust the elevation reference of a DEM. The elevations in a DEM can be calculated above Mean Sea Level or an ellipsoid. The elevation reference in the DEM must match the elevation reference of the imagery that you want to orthorectify. To compensate for a discrepancy, you can type the difference between the two elevation references in the Elevation offset box. You can also convert the DEM.

The Elevation scale and Elevation offset can be used together to convert the DEM pixel values to their actual elevation values. The equation for the conversion is:

elevation = Scale * (DEM pixel value + Offset)

Continuing the same example, if you add the DEM pixel value to the Elevation offset (102 + 1,000) and multiply the result by the Elevation scale (1,102 * 0.1), then the DEM pixel value actually represents an elevation value of 110.2.

Tip: If you do not have a DEM, you can use the average elevation of an area to orthorectify the image. However, this will not produce results as accurate as using a DEM. Type the average elevation in the Elevation offset box.