Preprocessing

Elevation data normally requires minimal pre-processing. The main pre-processing that may be necessary is the potential optimization of the file format or creation of overviews as defined above.

One consideration that should be reviewed for each collection before creating pyramids is the definition of NoData values.

Elevation data often contains NoData values. These are typically required for areas of the raster where no elevation data is available such as the edges of a project or holes in the project for example where no sufficient data collection samples were available. The edge of such NoData areas for elevation data is often very irregular, so the use of footprints to clip elevation data by the footprints often does not work well. Typically, elevation data is not compressed using lossy compression so NoData values can easily be defined. The recommended approach to handing NoData is therefore to ensure that NoData values are correctly defined for any dataset. This should be done before pyramids are generated, to ensure that pyramids do not have artifacts from incorrectly sampling NoData values.

As part of the pre-processing step it is advisable to review a representative sample of the datasets to ensure that NoData is correctly defined. If not then use the Set Raster Properties tool to define NoData values.

There are some cases where a mask (as a separate raster or footprint) may exist to define NoData. In such cases the Mask can be applied once the source mosaic dataset has been created by applying a mask function to the rasters or the footprint used to clip the data.

It is presumed all elevation data is orthometric. This should be verified, and the datum for each collection should be noted for entry in the Metadata section. If any data collections are referenced to an ellipsoid rather than geoid, the notes below for converting from orthometric to ellipsoidal height may be reversed.

Units must also be verified. If elevation data is received in units other than meters, the source mosaic dataset should be created in original units, and an arithmetic function should be applied when the source mosaic dataset is ingested into the derived mosaic dataset. For more information, see this Help topic.