User Instructions
and Technical Guide
OPUS Projects
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NOAA | National Geodetic Survey
Figure 1.22 - Session solution Constraints
1.3.5.2.4.7
Constraint Weights
Constraint weights are
applied only to project
marks or CORS selected to
be held as constrained
points in the processing.
Again, any value initially
shown here will only be a
default that can be
changed when the
processing is performed.
Please be aware that
currently there is no
differentiation between
the
weights applied to
horizontal and vertical
constraints.
Before defining the
available
Constraint
Weight choices, it may be
beneficial to preview
constraints being set for
session processing so that
the
implications of Constraint Weights default can be more fully appreciated. At the bottom of Figure
1.22 one sees that a
Constraint Weight of NORMAL was selected. This could be the default for this
project, or might have been chosen specifically for this session processing. The three choices for
Constraint Weights are:
LOOSE -allows up to one meter of float for the constrained points in the adjustment.
NORMAL -allows up to one centimeter * of float for the constrained points in the adjustment.
TIGHT -allows up to one-tenth of a millimeter of float for the constrained points in the adjustment
(effectively fixing and not allowing the constrained control points to move).
TIGHT (sometimes called fixed) is the default for new projects, but, as the name implies,
NORMAL should be considered a recommendation allowing up to 1 cm* of float for the constrained
control points to be adjusted to a true best-fit solution. *
The constraints are not barriers.
The NORMAL constraints instruct the program to limit the adjustment to less than about 1 cm, but if it
"wants" to be greater than 1 cm, it will, although the constraint will increasingly hinder taking on larger
and larger values. Think of it as a rubber band. For adjustments less
than a couple cm,
the rubberband is not stretched tightly and the adjustment can "easily" reach (take on) any of those
values if the solution demands. Beyond that, the rubber band is stretched tighter and tighter. The
adjustment can reach (take on) larger values, but its got to work harder to do so. Be aware that
the rubber band is never slack. It is always "pulling" towards the specified constraint value to some
extent.
Continuing with the preview, at the center-right of Figure 1.22 example, we see that project marks and
CORS have been chosen to be constrained as either 3-D, HOR-ONLY, VER-ONLY, or NONE (i.e. completely
unconstrained). The default settings for new processing is 3-D constraints on all CORS; NONE for all
project marks. The session processing should always follow the intent of the field survey plan.
OPUS Projects
User Instructions and Technical Guide
NOAA | National Geodetic Survey
P a g e
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37
Figure 1.23 - 'USER' network design
Figure 1.24 - 'CORS' network design
1.3.5.2.4.8
Session Network Baseline Design
The
Session Network Baseline Design provides a basic network design strategy for the baselines created
during session processing. These network baseline design options for processing should be kept
in mind
during the survey design and preparation stage so that simultaneous observations of project marks and
CORS, constrained and unconstrained marks will form baselines that are purposely occupied and
processed. The
Session Network Design default can be changed at processing and, to a limited extent,
modified by selecting project marks to be excluded from the processing or to be assigned as
HUBs. You
can only process baselines for marks or hubs that have been simultaneously occupied. The term “hub”
is NGS jargon meaning a mark that is preferentially selected for inclusion in baselines. In other words,
an included project mark that is a hub is more likely to be connected to other project marks. The four
Session Network Baseline Designs are:
USER: Any or all included
project marks or
CORS can be selected as hubs offering a
limited opportunity to customize the
network design for the session. However,
it is recommended that a hub(s) selected
be CORS or other active stations with 24
hour files. While any selection of one or
more hubs is possible it is recommended
to select just one hub per session. This
strategy is explained fully in the Technical
Guide portion of this document. When
this happens, the
Session Network
Baseline Design selection will change to
USER simply to indicate that the selections
no longer conform to one of the other
strategies. Figure 1.23 is an example in
which the user selected one mark (2139)
and one CORS (covg) as hubs.
As a result,
the
Network Design selection changed to
USER, since these choices are not
standard to any of the other options and
the resulting baselines are indicated on
the map. Processing should follow the
intent of the field survey plan.
CORS: The CORS Network Design strategy
automatically selects all included CORS,
and only the CORS, as hubs (Figure 1.24).
This emphasizes the number of baselines
to the CORS which can be a useful design
if the CORS are to be constrained in the
processing.
CORS are monitored daily,
have well determined NSRS coordinates
and provide strong ties to the global
network. Furthermore, CORS that have been in existence for more than 3 years have computed
(measured) rather than modeled velocities, a characteristic that is desirable. Managers should weigh
CORS quality when planning their surveys.