12.1. Recommended Network Adjustment Sequence for Alignment with the NSRS

A sequence of five (5) network adjustments ensures that your project is aligned with the NSRS both horizontally and vertically. These steps are automatically generated when a tracking ID is associated with the project and will enable the final coordinates to be consistent with the latest horizontal and vertical datums of the NSRS.

1. Preliminary Adjustment

2. Horizontal Free Adjustment

3. Horizontal Constrained Adjustment

4. Vertical Free Adjustment

5. Vertical Constrained Adjustment

Caution

A user may perform a similar sequence of network adjustments without associating a Project Tracking ID, but the corresponding constraints will have to be selected manually, and all solutions will be provided by GPSCOM instead of ADJUST.

12.1.1. Preliminary Adjustment

To begin the network adjustment sequence, the software package GPSCOM is used to combine and adjust all selected sessions into a single network solution in the ITRF (and then transform it into the selected output reference frame; see Preferences Section 4.4). The output of this adjustment provides the necessary inputs for the ADJUST suite of programs in the following steps.

12.1.2. Horizontal Free Adjustment

This and the following three network adjustments use the program ADJUST, aligning the project with the NSRS. Although OP uses the term “horizontal” in reference to an adjustment, it is more correctly interpreted as “geometric,” as all three geometric (lat/long/ellipsoid height) coordinates are involved. By constraining only one mark in 3-D, the adjustment allows the least squares solution the maximum freedom to help detect outliers and other problems.

12.1.3. Horizontal Constrained Adjustment

All marks with known geometric coordinates in the NSRS reference frame are constrained to those coordinates in 3-D. This forces alignment of the user marks in the project to the constrained marks. This is sometimes described as propagating the reference frame from the constrained to the unconstrained marks.

12.1.4. Vertical Free Adjustment

As with the horizontal free adjustment, the vertical free is a minimally constrained adjustment used for quality control. The adjustment aligns the marks to the vertical datum. Note that areas with little or no existing control in the appropriate vertical datum may be problematic, but can still be adjusted vertically. For such special situations, take the time to describe it in detail in your Survey Project Proposal.

12.1.5. Vertical Constrained Adjustment

This adjustment forces alignment of the constrained marks to the vertical datum and propagates the vertical datum to the vertical coordinates of the unconstrained marks.

---