Illinois Height Modernization: Benchmarks, Datums, and Leveling Efforts

Benchmarks in Illinois

A benchmark is a relatively permanent object whose elevation above an adopted datum is known. All types of property and engineering surveying rely on benchmarks. Illinois has two major networks of benchmarks, managed by the National Geodetic Survey (NGS) and the U.S. Geological Survey (USGS). These two geodetic control networks are supplemented by several other networks. None of these networks are integrated into a single database.

Illinois has lost nearly half of its benchmarks, and continues to lose them, because little funding is available for replacements. Existing benchmarks can be destroyed by ongoing construction, farming, or other activities; threats to surviving benchmarks are increasing.

The figure at the right shows all of the benchmarks currently in the NGS database for Illinois. Benchmarks in red reported in good condition within the past 10 years. Benchmarks in blue had no reported recoveries in 10 years.

Datums in Illinois

Each benchmark is tied to a specific known elevation marker, called a datum. Agencies working in Illinois rely on several vertical datums which are not easily related to each other. Some of these include:

• NAVD88, the North American Vertical Datum of 1988.
• NGVD29, the National Geodetic Vertical Datum of 1929.
• The City of Chicago datum, one of numerous legacy municipal datums.
• NGVD12, used by the USACE Rock Island district for management of the Mississippi River.
• The Ohio River datum, used by the USACE Louisville district for management of the Ohio River.

Approximate conversions between these legacy data exist, but their use degrades the precision of the measurements. During emergencies, these inconsistencies can cause confusion and waste valuable time. Multiple datums also make it virtually impossible to create useful, accurate, seamless maps and supporting flood data for the state. An example of the problems that multiple vertical datums can cause can be found in the Mississippi Valley in Northern Illinois, where levee top surveys are in NAVD88 and water level measurements are in NGVD12.

Because of the large number of different datums, numerical conversions between networks are approximate. This fact which lowers the accuracy of existing elevation data sets.

Leveling Efforts: Proposed Improvements to Benchmarks and Datums

The ILHMP project aims to replace lost benchmarks and resolve issues related to conversion between datums. The "Standards and Specifications for Geodetic Control Networks" specifies that benchmark spacing should not exceed 3 kilometers along a level line for all leveling, regardless of order. For all first order and second class 1 surveys, the average benchmark spacing should be 1.6 kilometers, while for second class 2 and third order surveys the average benchmark spacing is 3 kilometers. The actual benchmark spacing will also depend on the spacing of the level lines. Spacing of level lines depends on the infrastructure that we are trying to support. A denser network is needed in rapidly developing areas, such as the Chicago metro area and the St. Louis metro east area, than in predominantly rural areas.

Year 1 Leveling improvement efforts have included the release of an open contract for geodetic leveling for about 40 miles in northern Illinois. Geodetic leveling and installation of 22 monuments in Winnebago and DuPage Counties will be completed by a group led by Patrick Engineering and including Berns, Clancy & Associates. Year 2 funding will support additional geodetic leveling work.

A resulting statewide network would consist of points that are all GPS accessible, are accurate both horizontally and vertically, and are on one consistent datum. Height modernization will provide the opportunity to standardize the horizontal and vertical control used in the measurement and modeling of all the various watersheds, rivers, roadways, floodplains, farm fields, landscape changes, landslides, and well locations throughout the state. These improvements will result in significant savings in costs for services to the public.