Copyright © 2014 Open Geospatial Consortium
5.1.2 Data
There are two core ways to use the TIXM schema: As a metadata wrapper around existent Terrain Data
As a self-containing set of GML 3.2 points, next to its metadata
5.1.2.1 External Reference
In the first case, common Terrain Data files are linked through with the gml:fileName element. This element can be any URI. The MIME-type of the resulting binary data
block can also be specified. Example of a link to an external DTED file:
gml:rangeSet gml:fileNamehttp:www.somedata.orgterrain.dt0gml:fileName
gml:fileStructureRecord Interleavedgml:fileStructure gml:mimeTypeapplicationx-dtedgml:mimeType
gml:rangeSet
The external link has to be resolved separately from the TIXM file. This could be done asynchronously by software applications i.e., in a background process and only
when needed in order to reduce bandwidth. The TIXM file contains metadata regarding the geographical extent and data density, so this decision can be made
without the external data.
Using a TIXM file to refer to external binary data files means that readily available Terrain Data formats can be re-used by clients that support them. It also means that on
top of GML 3.2 and XML support, clients will have to implement support for a set of well-known raster formats.
5.1.2.2 Inline Data
TIXM also allows you to inline elevation data straight into the file, making it a stand- alone terrain format. For instance, elevation data can be encoded using a string of
tuples:
gml:rangeSet gml:DataBlock
gml:rangeParameters gml:tupleList10.0 10.0 130.0, 10.0 11.0 135.0, 10.0 12.0
132.0gml:tupleList gml:DataBlock
gml:rangeSet
Alternatively, more descriptive geometry can also be used instead of a simple list of tuples. The TIXM schema defines tixm:ElevatedPoint geometry type. This type
contains apart from an elevation some metadata to describe the elevated point itself:
complexType name=ElevatedPointType sequence
element name=elevation type=tixm:ValDistanceVerticalTypePropertyType
element name=height type=tixm:ValDistanceVerticalTypePropertyType
element name=source type=tixm:AcquistionMethodType
element name=surfaceType type=tixm:SurfaceType
element name=surfaceTypeDescription type=string
element name=recordedSurface type=string
element name=penetrationLevel type=tixm:ValDistanceVerticalTypePropertyType minOccurs=0
element name=knownVariations type=string minOccurs=0
element name=maximumHeight type=tixm:ValDistanceVerticalTypePropertyType minOccurs=0
element name=survey type=tixm:SurveySetPropertyType
sequence complexType
The use of inline XML to define points has the advantage that is easier for a client to implement no additional Terrain Data Format support is required. The disadvantage
is that the entire block of elevation data has to be downloaded together with the surrounding metadata. This could potentially be performance problem when applied
on a large scale.
5.1.3 Metadata