RICERCA ITALIANA     

Research program

The geomatics in support of the actions of Government of the territory

University Co-ordinator

Politecnico di TORINO - INGEGNERIA DEL TERRITORIO, DELL'AMBIENTE E DELLE GEOTECNOLOGIE - ()

Research Unit Leader

Alberto Cina

Description

Our research program, planned in two years, will start with a in-depth bibliographical survey, in the field of the coordinate transformation, from a cadastral to a global system.
In the following, a number of stages are envisaged, namely:
a) theoretical analysis of reference systems and their transformations;
b) possible use of modern GNSS receivers for cadastral surveying;
c) validation of used techniques for a field survey.
In more detail, operations will proceed as follows.

Stage 1.Geodetic and cartographic procedures aimed to transfer cadastral mapping into a global system

A conform-type transformation (a rototranslation with scale factor) is normally used, in order to convert Cassini-Soldner coordinates into Gauss’ ones. The four parameters are estimated considering the coordinates of at least 2 points in both reference systems.
This transformation is possible over a limited area, for which parameters may be judged constant. However, the method has some defects: it requires a high number of “double points”, with the related charge of surveying work; also the evaluation of parameters is related to accuracy of point determination. Hence a problem of incoherence ensues for adjacent sheets. Moreover, the difference in cartographic deformations should be kept into account: Gauss representation is conform, instead the Cassini-Soldner one is aphylaptic, which gives large errors (about 10 m) especially on axis system with wide extension.
A procedure will be developed which lets a rigorous transfer to be obtained from cartographic coordinates and the related reference systems to WGS 84 systems to avoid the above problem. A proper software should be developed, to make easy this transformation, both for raster and vector coordinates and maps.
The problem due to different orientations of Bessel’s ellipsoids are to be considered, together with problems linked to numerosity of “small origins” and to amplitude of “large origins”. An estimation of some origins will also be possible, with new measurements in the GPS system WGS84.
Data processing is, of course, the basic part of all procedure: it is mandatory a robust snooping of data, in order to eliminate possible outliers.

Stage 2. New tools and new outlines for cadastral survey

Specific fields about GNSS measurements into recent PREGEO procedures are considered but nowadays, real time GNSS positioning based on permanent station networks are increasing. This could have an important impact on cadastral surveying.
In this field, Turin Unit has already a significant experience, having put into being the first Italian RTK permanent station network, in Piedmont. An important contribution to obtain these results has been brought by previous financing on the part of the Ministry.
Other Regions are moving to the same topic, in order to obtain real-time positioning in the centimetric range.
As once measurements were related to network points or control point now they may be seen as real time coordinates. Obviously, RTK coordinates are owed to network software and to the receiver firmware starting from phases and code observations, but the end user perceives them as direct measure of the coordinates.
For optimum usage of such results, mapping should take place in a reference system compatible with the GNSS one.
As the law II/88 have been a little revolution in the professional world, implying the modernization of the surveying equipments (PC, total station), the GNSS modernization could have a bigger impact. Procedures and surveying techniques are not lesser important than equipments. Double frequency receivers are already available, but new class of instruments are envisaged in the near future.
Single frequency receivers are on the market at reasonable costs (about 3000 euro): they allow the use of differential GNSS techniques, which may be improved in the frame of GNSS networks.
Single frequency with RTK option and WAAS Omnistar Racal Systems, with phase measurements, are available with an adequate accuracy to the cadastral surveying, even without ambiguity fixing. GNSS modernization should supply a significant contribution for cadastral surveying: the new GPS code Lc2 is expected to give a decimetrical accuracy.

Now, a purpose of this project is to examine existing types of equipments devote to cadastral relief, together with their cost-benefit ratio.
We shall also check attitude of GNSS receivers to cooperate with permanent-station networks. We shall try to actually assess the accuracy of available receivers, for different acquisition modes and related to the various RTK inputs.
A preliminary test of market conditions will be of help, for optimum choice of receivers suited to the actual survey. The final aim is updating the procedures nowadays in use for cadastral survey.

Stage 3. Practical surveying.

Geodetic and cartographic transformations procedures from cadastral coordinates to Gauss’ one will be pursued and tested in the field by means of GNSS measurements, using low-cost and dual frequency receivers. In addition, accuracy of cadastral mapping will be evaluated, once transformations have been completed: this will be actuated on landscape details, that should be easily recognisable. The equipment shall be handheld and double frequency GNSS receivers taking advantages from the permanent station networks as that one developed and managed by the Politecnico di Torino.
The accuracy of cadastral reference system origins shall be evaluated, estimating some control points. This validation may also be useful to evaluate possible advantages and costs of a re-determination of original points in WGS84 reference system.
The study of the different equipments operability will give many important operative informations about the use of GNSS technologies in cadastral applications.