+1.5 m, and their mean is 0.27 m. No systematic translation 
appears at the regional scale. On the contrary anomalous values 
appear in several subgrids (a clear example is provided by D6): 
these anomalies require deeper investigation that will be 
performed in the next future. 
| 
    
  
  
  
tS 
Figure 7. Estimates of the translations between the LR 
Lombardy and the HR LiDAR DTMs. Individual translations 
have been estimated for the subgrids of the regional DTM. Grid 
C2 has no overlap with the Lidar DTM. 
4. CONCLUSIONS 
The presented research is in the framework of the Interreg Italy- 
Switzerland HELI-DEM  (Helvetia-Italy Digital Elevation 
Model) project. HELI-DEM is aimed at creating a transalpine 
DTM by merging all the available height information for the 
alpine area. Particularly the LR DTMs of Piedmont (50 meters 
of planimetric resolution) and Lombardy (25 meters) Italian 
regions and the national Switzerland DTM (25 meters) are 
available. Moreover, local HR LiDAR DTM (1 meter) is 
available for the main hydrographic basins in Italy. 
The paper has discussed the first operations needed for the final 
merging. At first, all the DTMs have been transformed to the 
same 3D reference frame, that is the european ETRF89. Then, 
several cross-checks have been performed: in particular, the LR 
DTMs have been compared in their overlapping boundaries and 
have been compared with the HR DTMs where they are 
available. 
To these purposes, several algorithms and procedures have been 
developed in MATLAB and GRASS. 
The cross-check between Lombardy and Switzerland has not 
shown any general bias and the standard deviation of the 
differences, even if bigger, is comparable with the nominal 
accuracy. However, the spatial distribution of the differences 
presents an anomalous behavior. The magnitude of the 
differences is not correlated with the heights or the slopes and 
the biggest differences are localized in particular subgrids of the 
Lombardy DTM: this clearly implicates some blunder in the 
production of those particular datasets. 
A bias of about 1.5 meter exists between Piedmont and 
Switzerland DTMs: obviously, the bias is below the nominal 
accuracy of both the datasets but it is everywhere present. 
Moreover, it is confirmed by the comparison with the Lombardy 
DTM. 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B4, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
In some valley, the comparison between Lombardy LR DTM 
and the HR DTM shows an apparent planimetric translation. A 
software to estimate the translation between two DTMs by least 
squares has been implemented. Globally, a common translation 
does not exist: this is also true at the level of the several sub- 
grids of the Lombardy DTM. Therefore, no bias in the reference 
frames of the two DTMs should be suspected. However, more 
investigations should be done on the subgrids and valleys where 
the bias is more evident. 
In particular, external validation by GPS surveys will be done in 
the next months. 
5. ACKNOWLEDGMENTS 
This research is a part of the HELI-DEM (Helvetia-Italy Digital 
Elevation Model) project, funded by the European Regional 
Development Fund (ERDF) within the Italy-Switzerland 
cooperation program. More details about the project can be 
found on the website: www.helidem.eu. 
References: 
Altamimi, Z. and C. Boucher, The ITRS and ETRS89 
Relationship: New Results from ITRF2000, Report on the 
Symposium of the IAG Subcommission for Europe (EUREF), 
Dubrovnik, 2001. 
Barzaghi R., Borghi A., Carrion D., Sona G. (2007). Refining 
the estimate of the Italian quasi-geoid. Bollettino di Geodesia e 
Scienze Affini, ISSN 0006-6710, fascicolo 3/2007, pp. 145-159 
Benciolini B., Biagi L., Crespi M., Manzino A., Roggero M., 
Reference frames for GNSS positioning services: some 
problems and proposals. Journal of Applied Geodesy, N. 2-2008 
Biagi L., Brovelli M.A., Campi A., Cannata M., Carcano L., 
Credali M., De Agostino M., Manzino A., Sansó F., Siletto G. 
(2011). Il progetto HELI-DEM (Helvetia-Italy Digital Elevation 
Model): scopi e stato di attuazione, in Bollettino della Società 
Italiana di Fotogrammetria e Topografia, n^1/2011, pp. 35-51, 
ISSN: 1721-971X. 
Brovelli M. A., Cannata M., and Longoni U., LIDAR data 
filtering and DTM interpolation within GRASS, Transactions in 
GIS, vol. 8, no. 2, pp. 155-174, 2004. 
Donatelli D., Maseroli R., Pierozzi M., La trasformazione tra 
sistemi di riferimento utilizzati in Italia, Bollettino di Geodesia 
e Scienze Affini, Anno LXI, N? 2, 2002 
El-Sheimy N. , Valeo C., Habib A. , 2005. Digital Terrain 
Modeling — Acquisition, manipulation and applications. Artech 
House. 
Koch K. R., Parameter estimation and hypothesis testing in 
Linear models, Springer Verlag, 1987 
Li Z., Zhu Q., and Gold C., Digital terrain modeling: principles 
and methodology. CRC, 2005. 
Marti, U. Comparison of High Precision Geoid Models in 
Switzerland. Dynamic Planet, International Association of 
Geodesy Symposia, 2007, Volume 130, Part III, pp. 377-382. 
Rees W.G., The accuracy of digital elevation models 
interpolated to higher resolutions. International Journal of 
Remote Sensing, vol. 21, no.1, pp. 7-20, 2000. 
Wehr A., Lohr U., Airborne laser scanning-an introduction and 
overview, ISPRS Journal of Photogrammetry and Remote 
Sensing, vol. 54, no. 2-3, pp. 68-82, 1999. 
      
   
  
   
   
  
  
   
  
  
  
  
  
   
  
  
  
   
  
   
  
   
  
  
   
  
   
   
   
  
  
  
   
  
   
  
   
  
  
   
  
   
   
  
   
  
   
   
   
   
  
   
    
   
   
   
   
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