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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

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

 
<cov-mat dim="6" band="2">
   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
</cov-mat>

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

 
<obs from="418">
   <direction  to=  "2" val="0.0000"   stdev="10.0" />
   <direction  to="416" val="63.9347"  stdev="10.0" />
   <direction  to="420" val="336.3190" stdev="10.0" />
   <distance   to="420" val="246.594"  stdev="5.0"  />
</obs>

<obs from="418">
   <direction  to=  "2" val="0.0000"   />
   <direction  to="416" val="63.9347"  />
   <direction  to="420" val="336.3190" />
   <distance   to="420" val="246.594"  />

   <cov-mat dim="4" band="0">
      100.00 100.00 100.00 25.00
   </cov-mat>
</obs>

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