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There hasn't been much research done on the orientation of the Dutch dolmens.
Four studies stand out: 1929, van Giffen, De hunbedden van Nederland 
1997, Reijs, www.iol.ie/~geniet.
2003, Gonzalez-Garcia en Costa-Ferrer, Possible astronomical orientation of the Duch hunebedden (SEAC 9)
2004, Langbroek, Huilen naar de maan (PIT nr. 2)
All studies aim at the orientation of the long axis of the dolmens and have investigated trends only.
Gonzalez-Garcia and Costa-Ferrer also involve the orientation of the entrances.
Langbroek has taken the mutual orientation of dolmens into account.
Studies of the correlation between the structure and the orientation are unknown.
The study of van Giffen
From 1925 to 1927 van Giffen has attended all Dutch dolmens, made ground plans and measured their orientation.
This is the first systematical study. Van Giffen writes about this himself, that it was hard to have exact measurements
by the lack of a good instrument and by the fact that it's not clear how the axis should be taken 
With the second argument he appears to have the same view as Hoskin (see the page Account
that there is no sense in measuring an orientation with the accuracy of one degree, since the long axis never
can be determined closer then a few degrees. Furthermore van Giffen applies a declination of 13½° west 
which probably is a few degrees to much. His data is good for a general impression, but not for depending a study upon.
The study of Reijs
Reijs bases himselfs on own measurements. Since the orientations of most of the dolmens can be found between +/- 25° around the equinox,
initially Reijs suspects an orientation on the setting of full moon around the spring/autumn point (+/- 26°).
Next he introduces two statistical methods to check the spread of the orienations of the setting moon and the dolmens.
In both cases the hypothesis has to be rejected.
The study of Gonzalez-Garcia and Costa-Ferrer (GG&CF)
The investigators have measured the orientations of the dolmens by themselves.
GG&CF find a double peak around south for the orientation of the entrances.
The peaks compare to the rising and setting of celestial bodies with a declination of -35°.
The orientations of the long axis are located between the solsticies for 83% and between the extreme lunar standstills
The study of Langbroek
Langbroek tries to interprete the data of the survey of van Giffen (via van Ginkel). First he concludes, that a determining influence
of the Hondsrug (a geological feature) is not presumable. On the indication of Rossenberg, Langbroek distinguishes between
dolmens that are and that are not having a mutual dependend orientation. For the second type he concludes that they have
an orientation between the small lunar standstills. 88% of the orientation find themselves between the northern and southern
minor lunar standstills. From the data the same distribution accounts for the solstitia, but this is not mentioned.
Langbroek doesn't give any information about the mutual dependend orientations:
No conclusions and no criterions on how he selected them. The orientations outside the lunar standstills aren't explained neither.
When the study of Langbroek is adjusted by the data of GG&CF, the image doesn't change. Still 86% of the orientations
fall between the lunar standstills. On the other hand, the separation of mutual orientated dolmens loses its
eloquence, because GG&CF find 88% of the orientations between the lunar standstills. None of the irregularities in the study
of Langbroek can be explained by the double peak around south found by GG&CF.