Surveying Little Egypt
When a state-of-the-art surveying firm is asked to modernize an elevation system in the very heart of our nation, where does it turn? Since the area involved is the great Mississippi River and its drainage basins, perhaps the first place to look to is the rich history of the region.
In 1880, while running for president, General James A. Garfield said, “the Mississippi river, with its great tributaries, is of such vital importance to so many millions of people that the safety of its navigation requires exceptional consideration.” This is just as true today as it was 120 years ago.
The Mississippi River and its tributaries are like the arteries of the human body. The commerce and shipping on the Mississippi River is the great lifeline of Middle America and plays host to such great American cities as New Orleans, St. Louis, Minneapolis and Memphis. Chicago, Kansas City, Louisville, Cincinnati and Pittsburgh, through their river tributaries, are also tied to the Mississippi River.
One of the first attempts at keeping the river channel open to river traffic occurred in the St. Louis Harbor when U.S. Army Engineer Officer Robert E. Lee, later to gain fame as the leader of the Confederate Army, developed a system of dikes and revetments to keep the channel flowing next to the St. Louis harbor in 1837. Early interest in the Mississippi River was two-fold: first was the transportation of products and people along the river. Before 1830, most of this traffic was one way, from north to south, or from Pittsburgh along the Ohio River to New Orleans. There was some limited travel up the river, but it meant four to six months of rowing, poling or dragging the boat from the bank with ropes, by manpower or with horses. Many of the boat hands hired for the trip jumped ship because of the hardship placed on the crew. Others were held prisoner to prevent them from doing so.
In about 1830, this changed completely. With the invention of the steamboat, cargo and people had a way of returning back up the river. The New Orleans port became a beehive of activity and a major international river harbor.
This year-round navigation opened up a whole new set of problems. In the fall of 1830, low water restricted river traffic in certain areas of the Mississippi and major parts of the Ohio River. The Army Engineers began to install lock systems along the rivers that captured water in times of high flow and would store this water in reservoirs or pools. These reservoirs were used to keep the water level at an elevation that would allow year-round navigation.
The second important consideration in the development of the Mississippi River was an effort to control flooding. Alluvial lands lying between Memphis and New Orleans along both sides of the Mississippi River had the potential of producing vast crops of cotton and other products but suffered from annual flooding—sometimes in disastrous proportions. This problem resulted in the construction of privately owned levees along the river. These levees were all set at different elevations with little coordination from one to the next. If the problem of flood prevention could be solved, the value of this area of the country could be greatly increased.
In 1850, pursuant to an Act of Congress, the Secretary of War directed Mr. Charles Ellet Jr. to make a complete survey of the Ohio and Mississippi Rivers, with a view toward a master plan for flood prevention and navigation. In 1876, before the Mississippi River Commission was formed to coordinate all activities on the river, a survey of the Mississippi was begun in the vicinity of Cairo, Illinois, nicknamed Little Egypt. A temporary datum was adopted at 300 feet below a plane known as the Cairo City Datum of 1871. When the same survey was begun in the vicinity of Memphis in 1877, another temporary datum was adopted at 225 feet below the high water of June 23, 1858 at Memphis without any connection to the lower Delta Survey Datum of 1858. The first connection by precise levels between Memphis and Cairo was completed in 1880. The Mississippi River Commission established a tide gage at Biloxi, Mississippi. In 1882, a final value was adopted for Mean Gulf Level by the Mississippi River Commission based on the mean years of 1882, 1884, 1896, 1897 and 1898. In 1890, re-leveling was started at Fort Adams, Mississippi. The re-leveling ran south to Baton Rouge, Louisiana and north to Cairo, Illinois. In 1910 the level line from Memphis to Cairo was completed.
The U.S. Coast & Geodetic Survey (USC&GS) adopted the Mississippi River Commission value of Mean Gulf Level of 1899 and used it in the general adjustment of 1898, 1903 and 1907. The USC&GS later performed the General Adjustment of 1929, in reference to adjustments and datum relationships. The published elevations of the Mississippi River Commission for level lines between Biloxi and New Orleans and along the Mississippi River are mainly observed elevations based on one tide station, without orthometric corrections applied or corrected for closure. The relationship of Mississippi River Commission Vertical Datums with the Mean Sea Level Datum of 1929 will vary as a function of observational error and as the orthometric height varies. In 1944, the varying difference was noted between Mississippi River Commission Vertical Datum and USC&GS 1929 resulted in the tie-point method being established. However, the tie-point method seems to have faded from use. The Mississippi River Commission Vertical Datums have evolved into merely a number of indices that are transformed by algebraic addition. The true relations between the various Mississippi River Commission Vertical Datums and Mean Sea Level 1929 are now obscured by time and no longer used. The index relationships are as follows:
|Datum||Conversion to Mean Sea Level 1929|
|Ellet Datum of 1850||unknown|
|Delta Survey Datum of 1858||0.86|
|Old Memphis Datum of 1858||-8.13|
|Old Cairo Datum of 1871||-21.26|
|New Memphis Datum of 1880||-6.63|
|Mean Gulf Level Datum (preliminary) 1882||0.318|
|Mean Gulf Level Datum of 1899||0.00|
|New Cairo Datum of 1910||-20.434|
|Mean Low Gulf Level Datum of 1911||-0.78|
Today’s Memphis DistrictNow let’s move to the modern day Memphis District of the U.S. Army Corps of Engineers. The Mississippi River is divided into different areas controlled by different districts of the Corps of Engineers. The survey section of the Memphis District had operated under the multiple vertical river datums across portions of six states. The Corps realized there had to be a better solution to the problem of relating projects developed on various historical datums. One of the catalysts for improvements was the need to create a common, consistent, reliable adjustment that would enhance the use of technologies such as GPS, geographic information systems, electronic charting and LIDAR.
The work on these projects occurred between Mississippi River mile 595 and 963. The purpose of this GPS survey project in 2001 was to upgrade the current Mississippi River control network from NGVD 29 to NAVD 88. All non-NGS/C&GS marks that were occupied and adjusted during the process of the survey are being Blue Booked and submitted to the National Geodetic Survey (NGS) for inclusion in the NGS database of benchmarks. The manual under which this work was performed is the NOAA Technical Memorandum NOS NGS-58: Guidelines for Establishing GPS-derived Ellipsoid Heights (standards: 2 cm and 5 cm).
The NGS Guidelines envision filling in an area with, ultimately, a “local network” of new stations, spaced less than 10 km apart, for which ellipsoid heights are accurately determined using GPS technology. (Download your own copy of NGS-58 from the NGS website, http://www.ngs.noaa.gov/-
PUBS_LIB/pub_index.html, to follow the terminology as it is used here.) For this project, NGS staff provided the guidance and direction needed to adapt this area concept to fit a basically linear requirement. CORS stations in Illinois (STL-4), Missouri (BLMM) and Arkansas (MEM2) were chosen as “control stations” along with a monumented station in Mississippi (SAT TRI STA NO 133) having a well-determined ellipsoid height. The CORS are, of course, the fundamental references for ellipsoid heights in the conterminus United States. A chain of established stations, mostly published bench marks, was selected to serve as the “primary base” network, the backbone of this project. These base stations are spaced at about 40 km intervals along the Mississippi River corridor.
The standard chosen for this work was the ellipsoid height local accuracies of 2 cm at 95 percent confidence level. The company 3001 Inc. of Sulphur, La., having an existing contract with the Memphis District, and many years of experience performing GPS surveys over large geographical areas for many different governmental agencies, was chosen for the work. The final results of this horizontal and vertical upgrade would become part of the NGS network. The Memphis District chose to register the project with the NGS for a number of reasons. First, the standards are very high and the data checks are very stringent. The likelihood of significant processing and adjustment errors being caught is very high. Second, the Corps of Engineers as a government agency has a responsibility to see that the control that is published and used is accurate and reliable. Third, there are long-term benefits of having the control in the NGS database. When others use this control from the NGS database, the accuracies and reliability tend to propagate throughout the region.
The project manager of the work was Ed Dean of the Memphis District, Corps of Engineers, Russell Carr and Don Eames of 3001 Inc. were responsible for the mission planning, field observation and data adjustment. Before work could begin, a detailed plan of the GPS survey was submitted to NGS and Kurt Shinkle, Geodetic Advisor in the state of Mississippi. Sixteen primary network points were selected, many of which were existing CORS stations. This work resulted in tying together a number of different HARN networks in different states bordering on the Mississippi River. These primary stations received three different five-hour observation periods with Trimble 4700 receivers (Trimble, Sunnyvale, Calif.), in some cases the observation periods had to be extended past the five-hour sessions because of high PDOP spikes caused by undesirable satellite geometry. The five-hour sessions only allowed one complete session to be done each day per unit, and a total of five units were used for this work.
The secondary network consisted of 43 stations, many being existing points along the Mississippi River from older datums including the 1929 vertical adjustment. These points received three different observation periods of one hour each. All the sessions were run during a 32-day period during the fall of 2001. This survey included 81 total sessions and 316 independent vectors. As of this writing the final adjustment of the data continues with the final results looking very encouraging.
The upfront effort between two different governmental agencies and a private company proves the worth of a corporative partnership resulting in a major improvement of the survey data available along this stretch of the Mississippi River. This work will benefit all surveyors working in this area. At this time an expansion of this work in 2002 is planned that will result in an integrated GPS network about 450 miles long and 200 miles wide with a permanent point at the corner of each 15-minute USGS quad sheet.