Mosaic dataset design
It is recommended to follow the source/derived/referenced model for mosaic dataset design, as described in the Standard Workflow. The sections below include specific details regarding this design when applied to elevation data.
For most elevation applications, it is assumed the fundamental data is a bare earth digital elevation model (DEM). As a result, all data can be managed in a single derived mosaic dataset, and the data management issues here focus on merging different geographic areas and different resolutions.
There are applications where multiple surfaces may need to be defined, such as the following:
- A digital surface model (DSM, showing tree canopies, buildings, and so on)
- A hydrologically enforced DEM
- Ellipsoidal height
- Subsurface bathymetry in oceans and major seas
If multiple such surface models are required, it is recommended that separate derived mosaic datasets be created for each. In many cases, the same source data may be used but different functions and properties defined. By using a single set of source mosaic datasets and creating specific derived mosaic datasets for each surface type, the data management is simplified.
See the Appendix for a discussion of Design Considerations Regarding Multiple Elevation Surfaces.
Mosaic datasets of elevation data often contain multiple overlapping datasets. One important consideration for mosaic dataset design is how to control the default ordering of the data so that it is dependent on both scale and suitability. At small scales the lower-resolution datasets can be accessed, but at larger scales the most appropriate elevation should be used. This is best handled by creating a Best field and defining it as an attribute that indicates the priority of the dataset. Typically, this can be set as a value computed based on metadata values such as cell size, accuracy, or time. Details on how to compute the Best value are given in the metadata section below. When using the By Attribute mosaic method and setting the Ordering field to Best with a base value of 0, the data with the highest priority (Best Value Closest to 0) is displayed on top. It is also important to take into careful consideration the MinPS and MaxPS values for each dataset so that the higher priority datasets are not necessarily used when more suitable small-scale datasets are available. Even if overviews are created for source mosaic datasets, the MaxPS value should not be set too high, or these higher priority datasets will be used even at global scales, which could affect performance.
Note that users of the mosaic dataset or image services can also change the display order by changing the By Attribute method field and value used, using the Lock Raster mosaic method, and using WHERE clauses to restrict the system to use specific datasets.