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Figure 5-18: Availability of LODs for Elevation and MinMaxElevation Datasets
The CDB standard does not require that the entire LOD hierarchy be stored for the MinMaxElevation dataset. In fact, it is possible to omit some of the finest levels-of-detail from
the hierarchy. The CDB Standard recommends that the MinElevation and MaxElevation need only be stored to LOD = n - 4 and coarser where n is the finest available LOD of the Primary
Terrain Elevation component in a geocell. For example if Primary Terrain Elevation data is available for LOD = 15, then the MinMaxElevation hierarchy need only be provided for LOD = -
10 to LOD = 11. Note, that LOD = -10 to LOD = 0 are always required subject to the availability of Primary Terrain Elevation data these guidelines are explained in more detail in
section 5.6.1.6.4, Default Write Value.
Note that the presence of the MinMaxElevation dataset has a negligible effect on the size of the CDB. In fact, the dataset adds only 1 of additional storage over and above that required by the
Primary Terrain Elevation component. This is a small price to pay in order to provide the means to significantly speed-up line-of-sight computations in applications requiring the utmost in
determinism and real-time.
5.6.1.6.2 Data Type
Requirement 94
http:www.opengis.netspeccdb1.0core min-max-elevation-data-type
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The MinElevation and MaxElevation components SHALL be represented as floating-point or signed integer values. Integer values for tiles at LOD larger than 0 SHALL be scaled
according to the following formula: ��������� = ��������×2
Integer values can make use of TIFF’s 8-bit, 16-bit, or 32-bit representation.
5.6.1.6.3 Default Read Value
The Line-of-Sight algorithm is described in Section 8 Volume 0 - CDB Primer Formerly Appendix A. Note that the algorithm starts with the coarsest LOD of the MinMaxElevation
dataset; the algorithm recursively executes with progressively finer level-of-detail versions of the MinMaxElevation dataset until the algorithm decides it no longer needs to access finer levels or
until the algorithm no longer finds finer levels of the MinMaxElevation dataset.
If none of the LODs of the MinMaxElevation dataset are provided, then simulator client-devices SHOULD assume default MinElevation and MaxElevation values. The default values for these
datasets can be found in \CDB\Metadata\Defaults.xml and can be provided to the client-devices on demand. Handling of defaults falls under the following two cases:
● CASE I: In the case where the tile-LOD for the MinElevation and the Primary Terrain Elevation components are both missing, the CDB Specification recommends a default
setting of Default_MinElevation_CaseI = Default_Elevation-1. Similarly, where a tile- LOD for MaxElevation and the Primary Terrain Elevation components are both missing,
the CDB standard recommends a default setting of Default_MaxElevation_CaseI = Default_Elevation-1.
● CASE II: In the case where the tile-LOD for the MinElevation is missing and the
Primary Terrain Elevation is not missing, the CDB standard recommends a default setting of Default_MinElevation_CaseII = as supplied in Defaults.xml. In the event where this
default value is not supplied, the CDB standard recommends that client-devices use a default value of -400 m corresponding to the shore of the Dead Sea for MinElevation.
Similarly, when MaxElevation is missing and the Primary Terrain Elevation is not missing, the CDB standard recommends a default setting of Default_MaxElevation_CaseII = as supplied in
Defaults.xml. In the event this default value is not supplied, the CDB standard recommends that client-devices use a default value of 8846 m corresponding to the peak of Mount Everest for
MaxElevation.
5.6.1.6.4 Default Write Value
