Tablet representing the profession of surveyor
The picture opposite shows a tablet that the surveyor Sytske Gravius had
put in the façade of his house at Raadhuisstraat 10, Grouw
in 1655. The tablet was made by a fellow-surveyor, Syoerdt Aetesz
Haeckema, who was also a sculptor. At the time they were both working in
the province of Friesland on an ordnance survey project for an atlas, which
eventually was published in 1664.
The tablet consists of three parts:
In 1926 the stone tablet was moved to the Rijks H.B.S. (a secondary
school) in Drachten (Friesland) on the initiative of H.K. Schippers,
who was at that time teaching Maths at
Top: A problem from plane geometry
Centre: The art of surveying
Bottom: A date riddle
In 2007 it moved back to Grouw,
where it can be seen again at Raadhuisstraat 10.
The upper part shows a green, rectangular triangle having the hypotenuse
BC as its base. Underneath is written in seventeenth-century Dutch:
In A recht. AB, BC en CA doen 30. AB is 7 langer als AC.
Vrage na elck side.
"Angle at A right. AB, BC and CA total 30.
AB is 7 longer than AC. How long is each side?"
Two surveyors at work with their various tools. The man on the left is
looking through a so-called
Hollandse Cirkel or Circle of Dou,
an instrument with compass and
graduation to measure angles. At his feet are a sun-dial for measuring
the sun's altitude and an unused surveyor's chain (each part of which
probably had the length of one Rhineland foot). In the middle one sees
a triangle put in the right position to determine the relative height of
the church tower. On the far right stands the other surveyor, ready to
indicate the measuring direction.
Here is written in seventeenth-century Dutch:
Biden geadmitteerden Lantmeter. Ao z van 777.
That means: "By the Official Surveyor - A. D. z of 777."
The 'z' stands for square.
The square of 777 totals 603729, which apparently gives us the
date the stone tablet was put in the wall, since 603729 divided by 365
gives approximately 1655.
The origin and meaning of the 'z', together with an explanation of this date
riddle can be
found in a typed manuscript (dated 1941) by the above mentioned teacher.
The arabic word mal means wealth,
but can also signify the square of a number.
In medieval Latin, accordingly, the word
census (literally measuring of property, tax)
was used for square. In Italian this was written
censo or zenso,
The manuscript mentions as sources of this information the following books:
David Eugene Smith, History of Mathematics, Ginn and Company, 1925,
Vol. 2, p.394
Heinrich Wieleitner, Rechnen und Algebra, Berlin, Otto Salle, 1927.
The picture of the tablet put back at the wall in Grouw which is used here,
was made by Mieke den Boer-Snoei.
I wish to thank the late Fred Ferro for much information.
See also his article on this subject in the Dutch journal
Monumenten 18 (1997), nr. 9, p.15, reprinted in
De Hollandse Cirkel
4 (2002), nr.3. p.7.
The nice collection of photos of gable stones made by the late Fred Ferro
can be seen on the
image bank of Dutch cultural heritage.
to Tom Koornwinder's home page