sted
ıly the
racy of
res in-
nainly
f GPS
nbina-
datum
lon we
oordi-
30 cm
. Also
maller
is also
oordi-
Fig.5 Influence of ground control accuracy (oc) and of
drift parameters on the accuracy of adjusted GPS blocks
Be 1:30000
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10 30 50 100 [em] PS cp
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control case c no drift par. (1)
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nates of ground control image points is the same (cp: = 00)
as for image tie-points. A few cases have been run in which .
the image coordinate accuracy of ground control points has
been assumed to be oc p; — 3ym, as could be with signalized
control points. 'The effects have turned out to be very minor,
and that case is not pursued here any further.
In fig. 5 the results are displayed which demonstrate the ef-
fects of variation of ground control accuracy on the adjusted
blocks, for the described specifications where especially the
GPS camera position accuracy is kept fixed to ogps = do.
Four different cases of ground control and datum transfor-
mations resp. drift corrections are distinguished. The cases
are depicted in fig. 7 and 8.
The graphs show directly that the curves (1) behave unlike
the others. That case refers to blocks with 4 control points
only and no datum transformation applied. The result is evi-
dently determined by the internal block accuracy and by the
absolute GPS postioning. The errors of the 4 ground control
points have practically no additional effect, even if they are
large, if no free drift or datum parameters are applied.
In all other cases, where the block is fitted via free param-
eters onto the control points, the error level is considerably
higher, even if the control points have no errors at all. It is
the GPS errors and the internal block accuracy which raise
the error level immediately, although the different control
scenarios (c, a, b) react differently. The influence of ground
control errors is superimposed on that basic behaviour of
blocks. They represent an additional error component which
can gain considerable effect if the errors of ground control
points increase to large values. The relations of fig. 5 indi-
cate, however, that the effects of ground control errors are
very minor (in most cases < 5% ) as long as the control
points are adequately precise, i.e. as long as oc p < 00.
697