Unification of the Georeferencing Systems of GIS Spatial Data Infrastructure
Dr Kamal Abdellatif Abdalla
Surveying and Mapping Directorate,
Al Ain Town Planning Department
P. O. Box 1339, Al Ain, UAE
Email: drka@alainplanning.gov.ae
1. Introduction
Technological advancements in GPS positioning and GIS implementations and processing have provided the ability to perform precise georeferencing and produce high quality spatial data infrastructure. This ability, together with the applications of continuously operating reference stations (CORS) and the expanding use of geodetic control in support of spatial information systems, demands the adoption of reliable procedures to support georeferencing of spatial data and GIS activities. The continuing evolution of GPS and GIS hardware and software as well necessitate the adoption of standards and specifications in planning, methodologies, and implementation of both systems.
The impact of GPS on geodetic control surveying, assisted the geomatics community with a powerful tool for the establishment of precise geodetic control. This evolution has brought with it reduction in cost and an increased demand for geodetic control services. GIS users and spatial data developers rely on georeferencing and on the standards and specifications adopted to ensure the given level of product consistency. In order to produce or update spatial data a geodetic network is necessary. Such a network consists of a number of points spread across the area under consideration, these points are usually in the form of monuments established and placed in the ground, along with a high-accuracy positional value for each point. Traditionally, there have been completely separate networks for horizontal and vertical control, but some and recent networks combine the two on common monuments. By referencing spatial data to such a network, the resulting data and information from multiple local survey activities can be accurately connected.
The diversity of datums in use today and the technological advancements that have made possible global positioning measurements requires careful datum selection and careful conversion between coordinates in different datums. It is well known that the current, and growing, trend towards the use of satellite positioning systems and global satellite mapping systems to produce position-based products in a global reference frame can introduce serious practical difficulties if the results need to be related to older maps and/or digital data. Special problems arise, for instance, in the fields of updating, map revision, cadastral surveying and geomatics operations to support spatial data production. The difficulty fundamentally arises because of the need to transform the data into the coordinate systems used to describe the older data. In principle, coordinate transformations are straightforward mathematical procedures but in practice they can cause serious problems.
The required accuracy for a control survey depends primarily on its purpose. Factors that affect accuracy are type and condition of equipment used, field procedures adopted, and the experience and capabilities of personnel employed. Through years different sets of detailed standards of accuracy and specifications for geodetic control are published.