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.
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:
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.
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.
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.
By September of 2011, 187 new benchmarks and 114 visited horizontal monuments will add a total of 301 new benchmark records to the National Spatial Reference System (NSRS).
Year 1 included geodetic leveling along 1 level line for 48 miles in northern Illinois. Geodetic leveling in Winnebago and DuPage Counties included installation of 24 monuments and incorporated 16 existing benchmarks. The new level line has a total of 40 monuments and was completed by a group led by Patrick Engineering and included Berns, Clancy & Associates.
Year 2 efforts included geodetic leveling along 3 level lines for a total of 141 miles in northern Illinois. These level lines installed 68 new monuments and incorporated 34 existing monuments. The new level lines have a total of 102 monuments and were completed by three groups which included (1) a group led by Patrick Engineering and included Berns, Clancy & Associates, (2) Trotter and Associates, Inc., and (3) American Surveying & Engineering.
Year 3 efforts included geodetic leveling along 4 level lines for 334 miles in northern and eastern Illinois. One of these lines has been designed to extend westward from Year 2 activities along the northern border of Illinois in Winnebago County. The other three lines extend southward through Cook, Kankakee and Iroquois Counties. These level lines installed 95 new monuments and incorporate 64 existing monuments. The new level lines have a total of 159 monuments and were completed by four groups which included (1) a group led by Patrick Engineering and included Berns, Clancy & Associates, (2) Trotter and Associates Inc.,(3) American Surveying & Engineering, and (4) Willett Hoffman & Associates Inc.
Year 4 efforts included geodetic leveling along 5 level lines for over 700 miles in northwestern, eastern, and southern Illinois. One of these lines was designed to extend westward from Year 3 activities along the northern border of Illinois in Winnebago, Stephenson and JoDaviess Counties. The other four lines extend southward through Champaign, Vermilion, Edgar, Clark, Crawford, Lawrence, Wabash, Edwards, White, Gallatin, Hardin, and Pope Counties and then westward through Massac and Pulaski Counties before turning north into Union County. These level lines will install about 708 new monuments and incorporate 108 existing monuments. These level lines have a total of 811 monuments and were completed by three groups which included (1) a group led by Patrick Engineering and included Berns, Clancy & Associates, (2) Trotter and Associates Inc., and(3) American Surveying & Engineering.
Year 5 efforts propose geodetic leveling along 5 level lines for xxx miles in along the western edge of Illinois. The five lines extend will complete a continuous perimeter of the state, starting at the terminous of Year 4 activities in Carroll County and extending southward through Whiteside, Rock Island, mercer, Henderson, Warren, Knox, hancock, Adams, Pike, Calhoun, Greene, Jersey, Madison, St. Clair, Monroe, Randolph, Jackson, and Union Counties. These level lines will install about 623 new monuments and incorporate 176 existing monuments. The new level lines have a total of 799 monuments.