Development of the Iraqi Geospatial Reference System

01 July 2007 | 06:50 Code : 14356 Geoscience events
A critical component of national infrastructure, which is often taken for granted in....

A critical component of national infrastructure, which is often taken for granted in highly developed nations such as the United States, is an accurate spatial reference system. That is, a reliable network of permanent survey marks that have established horizontal coordinates and/or elevations which are referenced to a defined coordinate system. These survey control stations form the basis of a nation's GIS databases, navigation systems, cadastral records, horizontal and vertical construction surveys, and resource management programs. Control networks are still an essential component to the complicated science of positioning objects on, above, and below Earth's surface. But, you may ask, didn't we solve that problem with GPS? Simply, no. GPS has greatly enhanced the capabilities of a land-survey crew, but GPS alone is not accurate or reliable enough to support the multitude of projects which require positioning at levels sub-meter and below. GPS and conventional surveys must be based on an accurate set of reference stations in order to, for example, lay out a bridge that successfully joins in the middle or position a runway on which planes conduct precision approaches. The United States Army 175th Engineer Company (Topographic)(Airborne Corps) arrived in Baghdad in December 2004 with the mission to provide geospatial engineer support including terrain analysis, geodetic survey, and map production to Operation Iraqi Freedom. While preparing for their first mission in Iraq, an aeronautical survey at BaghdadInternationalAirport, the 175th survey section discovered that Iraq did not have an established spatial reference system. Further research uncovered the fact that data from previous British (circa 1917) and Polish (circa 1975) control surveys was available for some parts of Iraq; however, the historic surveys were performed to satisfy separate project requirements, and the segregated survey networks were not joined to form a continuous network that would benefit the nation at large. Our analysis and reconnaissance determined that the historic Iraqi control surveys were plagued with problems common among pre-GPS networks. Namely, survey data was scattered and difficult to locate, and the historic stations with established elevations did not have accurate horizontal positions and vice versa. In addition, many survey markers had been destroyed or were otherwise impossible to locate. These shortcomings were cumulative and made the existing geodetic networks generally useless to a modern surveyor. Rather than allow this deficiency to limit geodetic survey operations in Iraq, we chose to use it as a justification to greatly increase our surveyors' influence on the War on Terrorism and reconstruction of Iraq. We boldly decided to take the leading role in developing a modern, GPS-based geodetic control network throughout the country called the Iraqi Geospatial Reference System (IGRS). With great amounts of technical guidance from the U.S. National Geodetic Survey (NGS), we engineered IGRS in the likeness of the National Spatial Reference System (NSRS), which is developed and maintained by NGS. Most developed nations maintain similar GPS-based reference systems.To purchase the equipment and supplies IGRS will require, the 175th applied for approximately $400,000 of contingency funds from the U.S. military's Joint Acquisition Review Board (JARB). The JARB unanimously approved the entire budget for IGRS in February. Contingency funds or "war-money" is paying for IGRS because the military manages the program's equipment and will at least initially perform all of the surveys. Alternatively, Iraqi reconstruction funds are controlled and approved by a separate authority because those funds pay for projects undertaken by multi-national civilian contractors.The foundation of IGRS is a series of six GPS Continuously Operating Reference Stations (CORS) which are spread geographically throughout the country. The CORS network supports post-processed differential GPS (DGPS) positioning in all regions of Iraq. Given a complete CORS network, a survey crew working to position critical infrastructure features in Tikrit, for example, could conduct their survey using autonomous accuracies of 3 to 10 meters horizontal, 10 to 15 meters vertical. Then, the crew could download appropriate CORS data from stations in Baghdad, Mosul, and Al Asad and use it to differentially correct the field data to obtain centimeter-level accuracies for the final product. This dramatic increase in accuracy is extremely valuable and often essential to both military GPS applications such as mine clearing and remote sensing and civil GPS projects including road construction and precision agriculture. Each CORS site consists of a Trimble NetRS dual-frequency survey-grade GPS receiver that has a permanently mounted antenna and logs position and time data 24 hours every day. Solar panels and large batteries provide the stations with uninterruptible power to ensure the receivers never fail to record an epoch of data. The stations are located at secure installations, and their data transmits over Internet Protocol (IP) to a server in Baghdad. Trimble's GPS Net software controls the server which stores and compresses the incoming data, then sends it over File Transfer Protocol (FTP) to NGS for analysis and publication. The dataset is also transferred to the public archives of the International GPS Service (IGS), a global network of geodetic reference GPS and GLONASS (a Russian satellite positioning constellation) reference stations formed by the voluntary cooperation of many worldwide agencies, including NGS. The pooled IGS dataset and its analysis products are widely utilized for global and regional geoscience and applications, and obviate the need for regional investigators to deploy their own global reference networks. Because their purpose is to be used as reference stations for precise applications, CORS themselves must have positions that are virtually free of error. NGS calculates the positions and velocities of the Iraqi CORS by processing data from each new station with data from a set of "hub" stations that are part of the existing global CORS network. Sites having a long history of reliable operation as well as accurately measured plate tectonic velocities are candidates for hub sites. Chosen stations for the positioning of Iraqi CORS include BAHR (Manama, Bahrain), GLSV (Kiev, Ukraine), KIT3 (Kitab, Uzbekistan), LHAS (Lhasa, China), MALI (Malindi, Kenya), MATE (Matera, Italy), NICO (Nicosia, Cyprus), POL2 (Bishkek, Kyrghysstan), TRAB (Trabzon, Turkey), and URUM (Urumqi, China). NGS processes the positions of new CORS with their custom-developed software PAGES. This program uses the method of double differences to remove clock errors in GPS satellites and the hub stations. Once synchronized time is established, the precise locations of the satellites along their arcs of orbit may be calculated epoch by epoch. The positions of the Iraqi stations are then determined through triangulation to the solved locations of the satellites. For this triangulation, the software forms the L3 or ionospheric-free observable from the L1 and L2 carrier phases which are broadcast by the satellites. Measurements from a minimum of four satellites are necessary at each epoch of triangulation to solve the four unknowns of X, Y, Z position and time. GPS satellites circle the earth's center of mass; therefore, their locations in space, and the resulting triangulation of points on the earth, are determined in an X, Y, Z Cartesian coordinate system whose origin is located at the earth's center of mass. Because site coordinates are determined in the reference frame of the GPS orbits, the mathematical Earthmodel for satellite orbit calculation is all important, including the slow rate of drift of the earth's tectonic plates and of all the features embedded on those plates, such as CORS antennas. Through the continuous accumulation of daily coordinates for a worldwide collection of GPS trackers similar to CORS, plus other technological advances, Earth scientists are able to refine and refine again the International Terrestrial Reference System (ITRS), leading to the creation of International Terrestrial Reference Frames ITRF92, ITRF94, ITRF96, ITRF97, and ITRF00. Similarly, the World Geodetic System 1984 (WGS 84), or the satellite reference frame manifested as orbits that are broadcast in real-time by the GPS satellites themselves, has undergone four revisions. ITRF coordinates are published for some common date of definition, usually EPOCH 1997.0, representing January 1, 1997, and the positions are then projected to any time of interest by means of a site's plate tectonic velocity that, barring sudden displacements from earthquakes, is well represented as a constant, linear velocity. The Horizontal Time Dependent Positioning (HTDP) model, which mathematically predicts the drift of tectonic plates, is a more detailed and feature-rich product for sites in the continental U.S. than the generalized global plate tectonic model, which is known as NNRN UVE L-1A. Developed by DeMets et al in 1994, the NNRN UVE L-1A model is perhaps the most well-known model that may be used to predict horizontal plate tectonic velocities for locations outside the United States, usually with an accuracy of a few millimeters per year for locations within stable plate interiors. However, this model does not make allowance for deformable plate boundaries, such as the boundary between the Eurasian Plate and the Arabian Plate upon which Iraq lies. Predicted velocities for CORS in Iraq developed from any plate-tectonic model are expected therefore to be relatively inaccurate, requiring revisions of coordinates every few years until an accurate velocity can be calculated from the accumulated record of data. Whether a surveyor chooses to ignore or include the contribution of plate tectonics to site coordinates depends on the duration and accuracy requirements of the application. Nonetheless, the development of accurate site velocities remains the Holy Grail of coordinate computations and reference frame definition; therefore, data generated about the Arabian tectonic plate may prove to be a valuable, long-term benefit of the Iraqi CORS network. The shortcoming of even the most reliable and accurate CORS network is its inability to provide starting control for conventional surveys such as level-lines and traverses. Also, CORS stations only provide service so long as they are operational. To aid Iraq in the development of a complete spatial reference system that would support all types of surveys and would not be completely reliant on Iraq's only modestly reliable power systems and internet communications, we designed a nationwide High Accuracy Reference Network (HARN) of permanently monumented survey control stations. The proposed Iraqi HARN is similar to the 1st Order survey control network component of the NSRS that covers the United States. The American network has provided invaluable service since its origins which can be traced to the Survey of the Coast in 1816. Our goal is to achieve a station spacing of not more than 50 km, which will equal a total network of 200-300 stations. Each HARN station consists of a Berntsen 9/16" stainless-steel rod monument driven to refusal with a jack-hammer and delineated with a custom access cover and witness post. We chose 3D rod monuments because they maintain superior horizontal and vertical stability in Iraq's soil, and their construction requires less concrete, labor, and most importantly time than pillar-style monuments. The points will be surveyed with static DGPS for durations determined by each station's distance from the nearest active control station (either an established HARN running a simultaneous GPS session or an Iraqi CORS). By employing flexible session schedules, the HARN teams will maximize efficiency and reduce security requirements while maintaining a homogeneous accuracy rating While the previously mentioned historic surveys were referenced to a local datum (Iraqi Datum) or a regional datum (European Datum of 1950), all HARN positions are referenced to the ITRF00 using the Geodetic Reference System 1980 (GRS80) ellipsoid. ITRF00 is considered virtually identical to the WGS 84 (G1150) coordinate system as defined in 2001 by the National Imagery and Mapping Agency (NIMA), now the National Geospatial-Intelligence Agency (NGA). This seemingly arbitrary decision to use a global datum is of critical importance to the success of follow-on applications such as border surveys and aeronautical surveys which rely on the agreement of survey data over large geographic distances. Following that argument, the third dimension of a GPS position, which is called an ellipsoid height, is simply the vertical distance between a given point and the WGS 84 ellipsoid. This measurement is not to be confused with an elevation, which is the vertical distance between a point and the Geoid--the equipotential surface that represents global mean sea level (MSL). Because MSL elevations are the traditionally accepted vertical measurement, GPS-derived ellipsoid heights on all HARN stations must be converted to elevations. Here we had a bit of luck; oil exploration in Iraq has produced large amounts of accurate gravity measurements. NGA uses gravity data, including the dataset available for Iraq, to develop ellipsoid height-to-elevation conversions in the form of gravity models. All Iraqi HARN elevations are determined using NGA's Earth Gravity Model 1996 (EGM 96), which is currently the most accurate model for the region. Aside from the project's various technical hurdles, developing the Iraqi HARN presents a formidable logistical operation. The field teams require constant re-supply and protection in order to survive and complete their missions while spread throughout a desert that is extremely hostile. Because the 175th is a technically oriented engineering unit and possesses limited organic resources such as manpower, force-protection, and logistics support, our initial analysis determined that the feasibility of the mission was bleak. Again, a shortcoming became an opportunity. To meet the mission requirements, we augmented the unit's manpower and expertise by partnering with the 42nd Engineer Regiment (Geographic), British Royal Engineers. We found this outstanding group of "mates" in Basrah, and their infectious enthusiasm has provided IGRS ample fuel since the day we sat together and developed the project's initial plans. Presently, after a month of intense technical and tactical training, the combined group of American and British surveyors is breaking into six autonomous teams to survey the first phase of the HARN which covers Iraq's four southern provinces: Muthana, Dhi Qar, Maysan, and Basrah. Each field team is lead by a British or American non-commissioned officer who is responsible for all aspects of his team's safety and prosperity as well as the completion of technical tasks involved in the survey. The IGRS Operations Cell, which is located on Basrah Air-Station, will synchronize the survey plans and coordinate the taskforce's movements and logistics support. To monitor data integrity, the teams will send survey data from each job site to Baghdad using satellite radios. A quick computation at this central location will determine the validity of the data and provide nearly instantaneous feedback to the field crew. This added process will greatly diminish the number of sites that must be revisited to rerun failed GPS observations.The HARN task force will publish the entire data set including station coordinates, elevations, photographs, and descriptions on an interactive web page. The web-interface will provide all reconstruction engineers and surveyors in Iraq easy access to accurate and updated HARN data.The final and possibly most valuable stage in the development of IGRS is the training and integration of Iraqi surveyors into the HARN field teams and the CORS management cell. Considering that goal comes to fruition, in addition to providing support for civil-military construction projects, IGRS could be a powerful catalyst in the reestablishment of a federal Iraqi mapping and survey agency. The engineer staff section of the Multi National Corps Iraq, referred to as MNC-I C7, has made contact with several ministries within the new Iraqi government that are considering the creation of a sub-agency which could take over national geospatial engineering including the management of IGRS. A transition from C7 to Iraqi management will ensure the continued development and use of the survey networks throughout reconstruction and into Iraq's future. Beyond Iraq's borders, many other developing nations still require assistance in establishing modern spatial reference systems. For instance, through progressive outreach programs NGS has provided equipment and expertise in Central and South America, the Caribbean, Africa, and Eastern Europe to nations that are establishing CORS and HARNs. These efforts have proven particularly effective in nations recovering from natural disasters such as hurricanes and earthquakes. War-torn countries require the same geospatial reconstruction; however, military engineering units are more able than civilian agencies to provide support in these regions that generally suffer from decreased security manifested in widespread violence and criminal activity.By using IGRS as a model to create similar programs in nations like Afghanistan and Somalia, the army could effectively package what NGS developed as a successful multi-nation technical assistance program into a military reconstruction effort. Thus, army engineers will further enable nations emerging from conflict to face the daunting task of rebuilding large portions of their infrastructure. Author Note: We would like to express our most sincere gratitude for the assistance provided by Dave Doyle of NGS and Angelyn Moore of IGS. Their technical advice and experience in the field of geodesy were extremely beneficial to this article and essential to the success of the IGRS proje

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