The level network was performed using a two-man crew with a survey chief of party from Patrick Engineering, Inc. (PEI) and an assistant from the City of West Chicago Engineering Department. The equipment utilized consisted of a Leica NA2002 digital level and a bar-coded fiberglass staff. Data was recorded using a Sokkia SDR33 electronic field book and downloaded using Softdesk Survey Complete version 2.0.
Back sight and foresight distances were less than 300 feet at all times and efforts were made to balance the back sight distances with the foresight distances. The collimating error of the instrument was determined to eliminate any source of error due to unbalanced back sight and foresight distances.
It was determined by the City that the vertical datum of the control points was to be the same as the DuPage County Benchmark System. Information on County benchmarks in proximity to the City was released and eleven benchmarks were recovered. The benchmarks included in the leveling survey are:
- Seminary 2 D CO
- 59 North C NW RR D CO
- DuPage Airport D CO
- 59 South C NW RR D CO
- Route 38 – KRESS D CO
- Kress – N. WESTERN D CO
- Route 38 – GENEVA D CO
- NOF 38 – Town D CO
- Route 38 – JOLIET D CO
- Route 59 and Route 38 D CO
- TT 25 GT WILSON D CO
STAR*LEV, a program by Starplus Software, Inc. was the program used to adjust the leveling data. The data from the SDR33 field book format was converted, using a C language program developed by PEI, to the format compatible with STAR*LEV. During the conversion, the date was calibrated using the collimating correction factor.
The first step in the adjustment consisted of performing a minimally constrained adjustment for all of the level observations. This intermediate adjustment indicated if the observations agreed with one another and if they were free of errors. This adjustment was performed by constraining the elevation of SEMINARY 2 D CO. The results of the minimally constrained adjustment indicated there were no blunders in the data.
The second step was to determine which DuPage County Benchmarks would be held fixed for the final adjustment. The results of minimally constrained adjustment indicated that the leveled elevations for the other 10 county benchmarks were within two to three hundredths of a foot of the published elevations except for DuPage Airport D CO which deviated by about three tenths of a foot. The precision of leveled elevations was determined to be about equal to the difference from the published elevations with the County benchmarks were acceptable. Therefore, it as determined that all of the DuPage County Benchmarks were to be held fixed for the final adjustment except for DUPAGE AIRPORT D CO.
The final step of the adjustment was to hold the acceptable elevations as published by DuPage County and perform the adjustment using the STAR*LEV program. An observation weight factor was used in order to properly adjust the leveling data. The leveling data was broken down into elevation differences between points. These elevation differences were then adjusted based on a function of the distance between the two leveled points. For example, an elevation difference between two close points would be corrected less than two points that were far apart.
An observation weight factor of 0.016 feet divided by the square root of the distance in miles was used for the adjustment. This was determined by modifying the weighting factor until the error factor for the entire adjustment was 1.0, this means that there was an acceptable adjustment. This converts to 3.8mm per square root of the distance in kilometers. In comparison, 12mm per square root of the distance in kilometers is the Third Order specification for loop misclosure for leveling. Therefore, we are confident that the level network is of Third Order Accuracy.