Sect_12.1.4

12.1.4 Constraining GPS Networks

GPS AND THE FUTURE OF GEODETIC NETWORKS

The challenge for geodetic authorities is to promote the use of GPS in the most effective manner, but to still maintain the basic fabric of control network methodologies, such as:

The notion of a "hierarchy" of control network points with high precision, or so-called "first order", points established at a lower density than the "second" or "third" control points. This is consistent with the basic survey practice of "working from the whole-to-the-part". The issues therefore are:
- Given that GPS surveying is able to deliver relative accuracies of the order of several parts per million (better than the old "first" order standards), should the same multi-level primary, secondary, tertiary network system be maintained?
- Efforts should be made to redefine or renovate the geodetic network to a level of accuracy at least an order of magnitude better than that possible using the "standard" GPS surveying methods.
That control points should be located where they are needed, hence:
- The practice of even geographic distribution of points may need to be changed. For example, where the population is sparse and no resource development needs must be met by the geodetic network, a close and even distribution of geodetic marks may not be necessary.
- The control points should be located where they are most accessible, such as in the valleys rather than on hilltops. However, a degree of station intervisibility must still be assured.
A geodetic control network is an asset that must be maintained and upgraded by the geodetic authority, hence:
- Policies must be established that ensure continuity with existing practice of station marking, maintenance, documentation.
- The geodetic authority must develop and enforce survey methodologies and practices that will ensure the continued maintenance, as well as the extension or densification, of the geodetic network to some specified level of accuracy.
- Given that the cost of surveying a station using GPS may be significantly less than the cost of establishing a permanent mark, there may be justification for a much sparser network of high quality GPS points.
- What are the respective roles of the federal geodetic authorities vis a vis state lands departments?
The definition of a geodetic network in terms of classical notions of a non-geocentric datum based on a single local origin station, with the reference ellipsoid oriented in such a manner as to ensure a "best fit" of the geoid over the area of interest, is outmoded because:
- Users of GPS technology in point-positioning (or navigation) mode want to easily relate their position information to the datum (and the maps based on that datum). Although point-positioning accuracies are 100m (95% of the time), datums such as the AGD are several hundred metres offset from the geocentre!
- GPS survey users can avoid the need for coordinate transformations if the local geodetic datum is "close" to WGS84.
To make use of ALL geodetic data in the establishment of the geodetic network, hence some of the issues are:
- How will GPS baseline results be used to strengthen the network?
- What network adjustment strategies should be used when combining GPS and historical geodetic data (terrestrial measurements of distance, angles, levelling, etc., and space-based observations such as TRANSIT and SLR coordinates, and VLBI baselines)?
- How is the network recomputation (or renovation) to be carried out in the context of datum redefinition?

With the progressive improvement in GPS "geodesy" methodologies, relative accuracies of the order of 0.01ppm can now be achieved. These accuracies are necessary, for example, for scientific studies of crustal motion. Some of the most important legacies of such studies are:

The establishment of an international network of permanent GPS tracking stations supporting the production of high precision orbits by the International GPS Service for Geodynamics (IGS). This fundamental network makes it possible for geodetic authorities to carry out GPS surveys to a higher accuracy than possible previously by providing a direct connection to a superior network as well as satellite ephemeris information with accuracies at least an order of magnitude higher than that available from the Navigation Message.
In addition to precision orbits, the IGS stations contribute to the definition and maintenance of a global high precision geocentric datum known as the ITRS (International Terrestrial Reference System -- section 2.1.4). This is "close" to WGS84 (on average within a metre), but of higher internal consistency.
The notion of four-dimensional geodesy, whereby station coordinates expressed in the ITRF must be accompanied with an epoch identifier. Making connections to this network therefore gives rise to issues of precise datum definition that were never previously considered when geodetic datums were defined in isolation on a local or regional basis.

The IGS and ITRF therefore provide for the first time a convenient means of densifying high precision, so-called "zero order" networks to the local, regional or continental level. These "zero order" networks can be expected to have internal accuracies at the 0.1ppm level or better, and hence are ideal for providing the "backbone" for the new high precision datums for the 21st century.

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