the ratio of the covariance value of that cell to the unknown variance of the corresponding horizontal axis. The variance cell for the elevation would have the normal meaning. The other
diagonal cells in the covariance matrix, for the two horizontal coordinates, would be missing or have null values. The off-diagonal cells for the covariances between the horizontal coordinates
would also be missing or have null values.
Using this approach, elevation accuracy data types can be defined. Such a set defined using ISO standard IDL data types and structures is:
Type: Matrix Cell, of a covariance matrix struct MatrixCell {
string 2 axes; Axes of covariance matrix float value; Units: Meters squared
}; Type: Elevation Covariances
typedef sequence MatrixCell, 3 ElevationCovariances; Covariance matrix cells included only when correct
value is known Required values of ìaxesî string: ZZ
Optional values of ìaxesî string: XZ, YZ Where X, Y, and Z stand for three ground coordinates
Matrix cell XZ contains the ratio of XZ to XX Matrix cell YZ contains the ratio of YZ to YY
4.8. Image SRS Definition
The SRS of an image is often specified by a ground position SRS definition plus an image geometry model, that together relate image positions to ground positions in that ground SRS. The
ground position SRS can be specified as in the draft ISO standard for geospatial metadata, including the “Spatial reference by coordinates” and lower level metadata entities as discussed in
Table 1 of Section 3.1. The image geometry model can be specified by the values of the set of parameters used by a specified mathematical model of the image geometry. These parameters are
considered metadata for the image.
Image geometry model metadata is already partially discussed or implied in Abstract Specification Topic 7: The Earth Imagery Case, and in proposal document 98-033: Alternatives for Transferring
Orientation Data of Digital Aerial Images. These documents describe a number of possible forms of image geometry model metadata:
1. Values of image geometry model parameters: 1.1. For rigorous geometry models there are many existing rigorous geometry models
1.2. For real-time geometry models, including: 1.2.1. Polynomial models Section 3.2 of Topic 7
1.2.2. Ratios of Polynomials Section 3.4 of Topic 7 1.2.3. Universal Real-Time Model Section 3.5 of Topic 7
2. Positions of points in both ground and image coordinates: 2.1. Grid of Points with Interpolation Section 3.3 of Topic 7
2.2. Set of reference points, used by client for fitting parameters of image geometry model Sections 2.3 and 3.2 of 98-033
The Open GIS Abstract Specification Page 47
Volume 15: Image Exploitation Services 00-115 corrigendum
Technical Note: The OGC Technical Committee TC must answer several questions on the possible forms of image geometry model metadata, including:
1. Are there other possible forms of image geometry model metadata? 2. To what degree can or should the TC leave the selection of one or more forms of image
geometry model metadata up to organizations that propose implementation specifications in response to an RFP?
3. For which future RFP should the forms of image geometry model metadata be selected whether selected by the TC or by the RFP responders?
4. Which one or more forms of image geometry model metadata should the TC select or prefer?
4.9. Features With Geometry
