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## 2.8 Set of observations

The pair tag `<obs>` groups together a set of observations which are somehow related. A typical example is a set of directions and distances observed from one stand-point. An observation section contains a set of

• horizontal directions `<direction … />`
• horizontal distances `<distance … />`
• horizontal angles `<angle … />`
• slope distances `<s-distance … />`
• zenith angles `<z-angle … />`
• azimuths `<azimuth … />`

The band variance-covariance matrix of directions, distances, angles or other observations listed in one `<obs>` section may be supplied using a `<cov-mat>` pair tag with attributes `dim` (dimension) and `band` (bandwidth). The band-width of the diagonal matrix is equal to 0 and a fully-populated variance-covariance matrix has a bandwidth of `dim-1`.

Observation variances and covariances (i.e. an upper-symmetric part of the band-matrix) are written row by row between `<cov-mat>` and `</cov-mat>` tags. If present, the dimension of the variance-covariance matrix must agree with the number of observations.

The following example of variance-covariance matrix with dimension 6 and bandwidth 2 (two nonzero codiagonals and three zero codiagonals)

 ```[ 1.1 0.1 0.2 0 0 0 0.1 1.2 0.3 0.4 0 0 0.2 0.3 1.3 0.5 0.6 0 0 0.4 0.5 1.4 0.7 0.8 0 0 0.6 0.7 1.5 0.9 0 0 0 0.8 0.9 1.6 ] ```

is coded in XML as

 ``` 1.1 0.1 0.2 1.2 0.3 0.4 1.3 0.5 0.6 1.4 0.7 0.8 1.5 0.9 1.6 ```

If two or more sets of directions with different orientations are observed from a stand-point, they must be placed in different `<obs>` sections. The value of an orientation angle can be explicitly stated with an attribute `orientation="…"`. Normally, it is more convenient to let the program calculate approximate values of orientations needed for the adjustment. If directions are present, then the attribute `station` must be defined.

Optional attribute `from_dh="…"` enables to enter implicit height of instrument for all observations within the `<obs>` pair tag.

Observed distances are expressed in meters, their standard deviations in millimeters. Observed directions and angles are expressed in centigrades (400) and their standard deviations in centigrade seconds.

## Example

 ``` 100.00 100.00 100.00 25.00 ```

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