If you wish to use older height data in combination with map data produced after the transition to NN2000, a location-dependent map transformation has to be executed.

For a smaller area, e.g. 300 x 300 metres, most people will be able to convert their own data with centimetre accuracy as a simple "height shift". Contact the municipality or the nearest county map office to find out the extent of the difference between the old and new height systems in your area, and convert the data using the formula NN2000=NN1954 +/- X cm.

If you're working with a larger area, e.g. municipality-wide datasets, you can transform the data using a model file (.bin file) and suitable software. The .bin file can be downloaded here.