Horizontal Coordinates Based On NAD83 (1997)
The horizontal positions of all West Chicago points have been readjusted relative to nearby High Accuracy Reference Network (HARN) stations provided by NGS. The HARN stations have published positions which represent a new datum called NAD83(1997). Previous data from NGS were on a datum called NAD83(1986). Although the origin of NAD83 State Plane Coordinates has not been changed, the coordinates of NAD83(1986) points will differ from NAD83(1997) points.

It was found that the West Chicago points’ State Plane Coordinates were shifted by 0.02 feet South and 1.09 feet East. This shift is the difference in the calculated NAD83(1997) and published NAD83(1986) coordinate for the First Order point SE 7-39-9.

The HARN stations that were used were the Order A point ZAU B, and the Order B points Salt 2 and EWT IL83 A. The published horizontal positions of these points were held fixed in the adjustment.

A minimally constrained horizontal adjustment constraining ZAU B was performed to check the new published HARN coordinates versus our GPS network. The difference between the published and observed coordinates were calculated and the results are summarized below.

Changes in position from NGS values
By holding ZAU B fixed
Change = Calculated - Published

Upon inspection, it can be seen that the differences are very small. Compared to the long distances between the three HARN points, these differences correspond to distance a precision of 1 in 500,000 or better, which exceeds First Order accuracy (1 in 100,000).

As a result, the coordinates of all three HARN points were held fixed in the final adjustment. The residuals in the GPS observations that were caused by the mismatch of coordinates shown by the above table would be placed mostly in the long vectors extending from The City to the HARN points. The short vectors that were observed in The City would essentially be unaffected and the relative geometry of the GPS network within The City would remain the same.

NAD 83(1997) Horizontal Control

NAD 83(1997) Illinois East Zone (1201)
State Plane Coordinates