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Activities

The project is divided in 9 work-packages managed by the project partners. A summary of several activities of HELI-DEM is presented here.

CONTEXT: LOCAL ADMINISTRATION INTEREST FOR DIGITAL ELEVATION MODEL (DTM)
The need to represent the morphological and topological features of the terrain comes from ancient times, and it becomes more and more critical as complex construction are to be planned. Nowadays land planning and management activities use digital terrain models, which are available with different degrees of accuracy and precision. In this context, the different DTMs of the partners of the project are described, together with their main uses.

DATA AVAIBILITY
In the last years, several Digital Terrain Models have been made available for Italy and Switzerland: they have been produced with different technologies, resolutions and accuracies and are georeferenced in different reference frames. The first Chapter provides a detailed discussion of the data that have been acquired for HELI-DEM.

IMPLEMENTATION OF GNSS PERMANENT NETWORK
A GNSS permanent network has been established covering the alpine areas of the Piedmont, Lombardy and a part of Switzerland. The network is a prototype of a transnational positioning service obtained by the unification of already existing regional networks: moreover, the network aims at materializing the reference frame for the real time and post-processed survey, finalized to the merging of the DTMs and to the estimation of the model of heights. This chapter describes the prototype network and its adjustment in ETRF2000. It also describes the first test of kinematic positioning, both real-time and post-processed.

CROSS VALIDATION
The acquired DTMs should be cross and externally validated. This chapter presents the results obtained from the Research Units of Politecnico di Milano and Politecnico di Torino.
The basic analyses are common to both groups because all the methodological choices had been previously discussed and shared. However, according to the first results, the insights are different. Therefore it was decided to maintain separate contributions.
The work of the Politecnico di Milano focuses on the cross-validation of the Lombardy and Switzerland DTM on the border. After that, Lombardy low resolution and LiDAR high resolution DTMs are compared: because the differences seem to show some anomalies, the 3D translations between the two DTMs are estimated. Finally, for some sample areas, where the results are decidedly abnormal, an external validation is performed using GNSS RTKtechnique. The work of the Politecnico di Torino focuses on the cross-validation of Piedmont DTM with Lombardy and Switzerland DTM on the respective border. Also in this case the comparison between the regional low resolution DTM and some higher resolution DTMs, as for example the LiDAR DTM, is computed. Measures on the terrain in the Valsesia area are done, where two DTMs with different precisions and discordant are available, to find out if some altimetric or planimetric sistematisms between the two DTMs exist.

LEVELING LINES CAMPAIGNS
In the HELI-DEM project some leveling lines have been commissioned and performed both in the Piedmont and Lombardy Region. The goal of these campaigns was threefold: to connect the Italian height system to the Swiss one, to close some Italian leveling rings that IGM had not yet completed and finally to evaluate the geoid undulation through joint GNSS measures on some well-known leveled points, in order to compare the differences between these results of geoid undulation with those available by the three gravimetric geoid models available for the project. The leveling lines have been measured and tested in both Italian regions.

DATA STRUCTURE PLANNING AND HELI-DEM GEOSERVICE
The data structure that was planned as suitable according to the different levels of height data for the project. Moreover, the implemented HELI-DEM geoservice is presented. The requirements are studied and the available input data are discussed: according to the above characteristics, the output levels are defined.

INTERPOLATION OF DTM DATA
The DTM named HD-1 in the previous action must cover the whole project area and should have a similar resolution to the input data. It is realized through individual interpolation of the three DTMs of Lombardy, Piedmont and Switzerland on the nodes of the grid created for this purpose. To perform the interpolation, where possible, a local polinomial bicubic model is used; for the nodes in which this approach produces a bad result, the polinomial surface is reduced to a bilinear. In the overlapping areas where two or more input DTMs exist, the average of the interpolated elevations is then computed.

DTM CORRECTION
This workpackage discusses a procedure to correct a regional low resolution DTM with local high resolution data, where they are available. In particular for HELI-DEM, the HD-1 DTM, that was obtained by merging the regional, medium resolution DTMs, does not exploit all the elevation data available: indeed, Lombardy and Piedmont river basins are covered by a LiDAR DTM, that is characterized by higher resolution and accuracy and could provide data to locally correct the HD-1 DTM. Anyway, the simple substitution of the HD-1 data with LiDAR data might be not sufficient, because of local biases between the two models; these can produce discontinuities at the borders of LiDAR area. A possible solution could be given by the following process: the differences between the two models are computed and are filtered by a low pass filter; then, the filtered differences are applied to correct the low resolution DTM. This chapter presents the adopted methodology and the results obtained by its application to the real HELI-DEM case.

THE GEOID
The geoid, i.e. the equipotential surface of the Earth's gravity field that best approximates the mean sea level, has a great importance in cartography and in environmental and civil engineering in general. In fact it allows to convert ellipsoidal heights to orthometric ones (and vice versa) and it implicitly defines the direction of the vertical at given point. 
Local geoid models of neighboring countries usually have different resolutions and accuracies and moreover often present inconsistencies and/or discontinuities at the borders. These anomalies are mainly due to the different height datum used in the estimation of the local geoids. However, for some engineering applications cross-border merging of multiple local geoids is necessary and cannot be based on a simple average of the two datasets in the overlapping area without taking into account the various inconsistencies and biases.
To solve this problem one can take advantage of the global Earth's gravity field models estimated from satellite data: these models are not affected by local biases since they are not based on ground gravity data and therefore they may provide a correct estimate of the low frequency of the geoid undulations (wavelengths greater than 100 km).
In the present work a two-step algorithm to merge local geoids using GOCE global model is presented. Firstly we estimate the biases of the two local geoids by a least squares adjustment and after that the unbiased geoids are combine by means of a collocation procedure.
Particular attention is given to the estimate of the covariance function of the high frequency signal and the covariance matrix of the error of the GOCE model. After estimating a bias of about 1 m for the Italian geoid and about 0.8 m for the Swiss one, a unique Italian-Swiss "unbiased" geoid with an accuracy of few centimeters has been computed.

THE GEOPORTAL
The geoportal HELI-DEM provides the cross-border Digital Terrain Model developed by this project and exposes some capabilities of analysis based on elevation models. The system is composed by a server and a client component. The server adopts a service-oriented architecture, where different servers implementing different web services are properly orchestrated. The interface is designed to provide an intuitive access to the features offered by the services. The system, based entirely on open source technologies and open standards, shows how interoperable services can interact and demonstrates the maturity of the Open Source solutions adopted.

PROJECT RESULT DISSEMINATION
One of the most important request of the European Commission to the beneficiaries of EC funds is to give visibility to the work carried out during the project and to disseminate the results to the widest possible audience.
Since the beginning of the project, HELI-DEM partners tried to respond to this request in a positive way. Project themes were explored in collaboration with external stakeholders and results were discussed with sector experts and also non-specialists. A comprehensive strategy has been implemented: creation of a dedicated website, participation to conferences and meetings, publication of articles in scientific and nonscientific magazines, realization of a regular newsletter, distribution of brochures and production of a publication.


 

The project

The HELI-DEM (Helvetia Italia Digital Elevation Model) project aimed at creating a unified digital model of the heights for the alpine and subalpine zones along the border between Italy and Switzerland. This model should be properly geo-referenced and produced combining in a single model all the available information...read more

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