Unification of the Georeferencing Systems of GIS Spatial Data Infrastructure
6. Vertical Control
Vertical control networks are a series of points on which precise heights, or elevations, have been established. Vertical control stations are typically called Bench Marks (BM) as part of a vertical information network, the benchmark's elevation is known relative to a vertical datum, usually approximating the mean sea level. The vertical accuracy standard is usually referenced to the standard specifies a linear value within which the true or theoretical location of the point falls 95 percent of the time. Once the levelled height difference is obtained from field observations, one has to add to it a small correction based on gravity values along the way, to convert it to either the orthometric, dynamic or normal height difference. Geodetic levelling is probably the most accurate geodetic relative positioning technique. Modern techniques are inherently three-dimensional: simultaneous observations at two points yield three-dimensional coordinate differences that can be added directly to the coordinates of the known point to get the required coordinates of the unknown point.
The zero surface to which elevations or heights are referred is called a vertical datum, which is associated with the Mean Sea Level (MSL). The mean sea level (MSL) is determined by continuously measuring the rise and fall of the ocean at tide gauge stations. This averages out the highs and lows of the tides caused by the changing effects of the gravitational forces from the sun and moon which produce the tides. The MSL then is defined as the zero elevation for a local or regional area. MSL is a close approximation to the geoid, which is the true zero surface for measuring elevations. Heights obtained from geodetic leveling are known as orthometric heights (Figure 1). The orthometric heights are usually reflect local variations in gravity as well as changes in topography.
As stated, GPS, on the other hand, produces a much different kind of height. Whether one uses point, differential, or differenced carrier phase positioning, one obtains a set of XYZ coordinates that depend upon locations of base stations and satellite positions. These three-dimensional, geometric relationships, in contrast to geodetic leveling, do not depend on local gravity variations. Since XYZ coordinates do not directly express ellipsoidal or orthometric heights, it is necessary to transform them into a different coordinate system. Typically, XYZ are transformed into geodetic latitude, longitude and ellipsoidal height (see equation 3 ).
The justification of a global vertical datum adopting unification approach, can be summarized (Heck and Rummel, 1990), as follow:
- The implementation of a global vertical datum will lead to a better accuracy in the connection of national and continental datums.
- Removal of the systematic regional biases in gravity anomaly data banks, due to referencing heights to different level surfaces in different vertical datum zones, requires the definition of a global vertical datum, in such case the gravity anomalies can refer to one unique geopotential surface.
- Discrepancies in results obtained by geodetic levelling and oceanographic procedures, used for the determination of sea surface slopes, need a consistent vertical datum.
In describing a vertical reference datum, the heights are referring to the geoid. orthometric heights can also be determined through the use of a geoid model. Differences between the geoid model and the vertical datum can indicate any biases that may be present in the vertical datum. Effectiveness of the geoid model may be determined by the relatively small geoidal heights exhibited in the region with the use of a particular datum, and obviously the amount of data sourced will have an effect on the accuracy of the geoid. The most recent global geopotential model that being adopted as the geopotential model for the unification of global vertical datums is, the Earth Geopotential Model 1996 (EGM96). In connecting the vertical datums, every datum zone contains a fundamental station allowing the geometric height relative to a selected reference ellipsoid and orthometric height relative to the fundamental station to be measured. By using this procedure, the geoid height or the height of the equipotential surface through the fundamental station above the referenced ellipsoid can be determined.
7. Georeferencing and GIS Spatial Data
Geographic Information Systems require georeferencing of spatial data for a variety of reasons, such as: collection and processing of spatial data, establishing of mapping relationship to other existing data, updating, discovering and potentially reconciling discrepancies between conflicting data sets; providing a reference coordinate system for GIS applications, to reduce mapping errors as new data are created , to constrain mapping errors within a geographic area, to improve the spatial accuracy of existing data sets; conversion of maps into digital format for GIS. Georeferencing is fundamental in data sharing and in the integration of spatial data from various sources into a single consistent data set. This is often achieved using a single common geodetic reference frame, or datum. A number of factors have led to an increasing need to georeferencing and to base spatial data products on a common reference frame that extends across the whole globe. These factors include growing reliance on satellite positioning systems and development of satellite based mapping systems affording increasingly higher resolution. Another major influence is the trend to spatial data infrastructures with national, regional and even global coverage.
To handle most of the problems of spatial data, there is a need for a good understanding of the definition and realization of georeferencing. A practical approach to georeferencing implementation is the need to be in accordance with agreed standards and guidelines addressed by several international organizations. As well the information describing coordinate reference frames, often referred to as coordinate system metadata that used by the industry and many national and international standards organizations as well as industry are categorized as standards for geometics and non-geomatics activities. Beginning in the 1980s, national standards for spatial data infrastructure and geographic information have been drafted in several countries and international organizations.
Practically spatial coordinates based on local or global reference systems are usually converted to a map projection system for both spatial representation and map production. The projections are not defining ellipsoid or geodetic datum. The most common projection used for spatial data to high accuracy is the Universal Transverse Mercator projection (UTM), which is usually adopting the required reference system and practically suited to maps of north south extent. In the future, it is more likely that spatial data is to be drawn on the surface of global ellipsoid based on latitude and longitudes rather than UTM grids. Airborne and space borne georeferencing techniques are likely to include 4- dimensional approach, and be integrated with georeferencing systems in conjunction with aerial or space frames for digital photogrammetric and remote sensing mapping.
Unification and georeferencing will help in increasing the quantity and quality of data sharing and integration capabilities of GIS networks, and can be used for studying the physical phenomena (Cmbrinck et. Al, 2003), that related to spatial data as a function of time, such as crustal dynamics and movements, tide effects, gravity fields, magnetic fields and ionospheric mapping. This will also help in initiating regional geoids and gravity anomaly maps production.
Different sets of detailed standards of accuracy and specifications for geodetic surveys are to be considered to get reliable georeferencing of spatial data. Many GIS systems handle high quality vector and raster data and digital terrain models (DTM) from a wide variety of sources including, satellite imagery, aerial photography and Ladar scanning, the quality of their end product depends very much on the accuracy of georeferencing..
The revolution in GPS has not been confined to the surveying community, but has extended into mapping, spatial data infrastructure, and Geographic Information System (GIS). Now widespread integration between GIS and GPS are available, in various activities of GIS applications including mobile GIS, monitoring and tracking activities. Many of these applications require accurate positioning, and, a common requirement is the transformation of GPS heights into heights above mean sea level or the geoid. The adoption of georeferencing system will enable the production of a homogeneous series of maps and spatial data, which will meet user requirements. However, this change will also have far-reaching implications for the users and producers of maps and spatial information systems all around the world. However, georeferencing deals with positioning which is the fundamental element of GIS base maps and spatial data in terms of usage, analysis, and features location accuracy and the reliability of any information derived from the GIS system. Georeferencing will also, help in ensuring that all maps or layers in GIS database overlay accurately and the data set is georeferenced to a common coordinate system, and it playing a major role in constructing, displaying and analyzing different GIS layers and giving more abilities to the GIS system in using spatial data for analysis and measurements of length, size, area and shape.
9. Conclusions
Georeferencing can be considered as a common geospatial data item to many GIS applications and provides key elements for integration and sharing of spatial data and thematic information. The unification and implementation of global reference systems will lead to: better accuracy in connection with national and regional datums; removal of systematic regional biases in gravity anomaly data; enhancing, and converting national and regional datums to global reference systems.
The paper recommended that future spatial data representation should focus on the surface of reference ellipsoids; and the planning for unification of georeferencing systems should incorporate IGS stations at national and regional levels. Global or regional spatial reference systems should be initiated to provide national and global framework for spatial data. It should be characterized by accuracies compatible with modern positioning technologies and facilitate sharing of geospatially referenced data between various governments and spatial data users. Georeferencing is important as it relate the global reference systems to the time dimension. This time dimension will give another angle of view to the spatial data that represent a region geometry and spatial topology at certain time.
References
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