Lahti leads the way in the transformation of local coordinate systems
By Petri Honkanen
Pdf-file of the article, from Positio ICC Special Issue 2011
A new reference frame EUREF-FIN and a new national height system N2000 in Finland keep surveyors busy. Transformation to the new systems is a current project in several Finnish municipalities. The city of Lahti is giving an example.
Increasing regional co-operation and consolidations of municipalities have created the need for the harmonization of spatial data and the introduction of common coordinate systems. The municipalities of the Lahti region have carried out long term GIS co-operation. One of the main projects has been the creation of a common GIS database for all these municipalities.
The main requirement of a common GIS database was the quantification of common coordinate and height systems in the area of co-operation. Originally, the municipalities of the area used the old national grid KKJ and the local system of Lahti as their plane coordinate systems and N60, N43 and the so called Lahti system as their height systems. Land in Finland rises approximately 3-9 mm annually. The uplift alters heights, and this is the main reason for updating the height systems.
First the plane coordinates
The city of Lahti was the first Finnish municipality to begin the EUREF-FIN transformation process in 2004. In the first phase, the land use department of the city of Lahti measured EUREF-FIN coordinates to the 10 first order control points in its area. The network was connected to FinnRef ® that is a network of permanent GPS reference stations, the backbone of the Finnish realization of EUREF. Transformation between the new and the old systems was carried out by a 7-parameter affine transformation. Helmert transformation parameters were determined for later use as well. The RMS values of the affine transformation were calculated as 19 mm.
Because of the lack of practical references, the basics of the transformation and its testing were carried out on an extensive scale. One of the main interests was the changes in the areas of real estates caused by the transformation. The areas were investigated and compared between the old and new systems. Relative changes of the areas turned out to be 0.182 ppm. In practice that means that the area of an ordinary 1000 m2 real estate decreased by 0.09 m2.
Testing included the measuring of the plane coordinate network of Lahti. The results showed that the network is homogenous and there was no need for local transformations. So Lahti was the first Finnish municipality to introduce the EUREF-FIN coordinate system with ETRS-GK26 projection in November 2005. In 2006, the land use department of the city of Lahti executed similar transformations in other municipalities of the area of co-operation.
Then the height system
During 2008, the National Land Survey measured enough N2000 height control points in the area of the city of Lahti and the transformation of the height system could be carried out. The height difference between the Lahti system and N2000 was solved by leveling N2000 heights to the 94 control points of the Lahti system. The height difference was determined to be 382 mm. Leveling showed that the height network was homogenous and the transformation could be carried out by using one rigid vertical correction for the entire city area. The GIS database of the city of Lahti was transformed to the N2000 system in January 2010.
A laser scanning of the area was carried out in parallel to the transformation process, and now the city has an accurate elevation model and contours of 1 meter interval in the new height system.
The transformations of the height systems in the other municipalities of the co-operation area are being realized this year. They are carried out again by the land use department of the city of Lahti. The experience gained during the transformation process in Lahti can be applied.
Information campaign proved useful
EUREF-FIN has been the operational coordinate system in the city of Lahti now for more than five years. Before the transformation procedure, the city ran an extensive information campaign aimed at planning and construction industries as well as the general public. Because no major problems appeared after the transformation, it seems that the campaign was worth the effort.
The common plane and height systems have benefited subsidiary and private companies working with GIS-solutions in the Lahti area. Dealing with many transformations is now unnecessary when only one plane and one height system is in use in the operational area. Also data transfer between the municipalities and governmental institutions is now more straightforward than it was when dealing with many local coordinate and height systems.
Petri Honkanen works as the head of the land survey department of the city of Lahti, contact: petri.honkanen[at]lahti.fi
City of Lahti
Lahti is located in southern Finland, only 100 kilometers from Helsinki. With a population of approximately 100 000, Lahti is the eighth largest city in Finland. Worldwide, Lahti is probably best known for the Lahti ski games and many other world class sporting events. Lahti is also well-known for its nature and cultural events.
6.8.2011 11:04