Geocentro Magazine - numero 6 - novembre/dicembre 2009

66
ANNO I | n. 6 | NOVEMBRE - DICEMBRE 2009
territorio dei tanti agrimensori e periti del suo tempo, anche
se il suo avvicinamento alla rappresentazione cartografica
non deriva certo da intenti di tipo professionale.
Leonardo non è un cartografo in senso stretto, ma si occupa
di cartografia per specifiche necessità di studio e per esigenze
di analisi, finalizzate alla progettazione territoriale o alla
pianificazione di attività belliche.
Per entrare più direttamente nel merito delle sue opere
cartografiche bisogna preliminarmente spostare l’attenzione
dal mondo delle mappe a quello della rappresentazione in
generale, cioè del disegno e della pittura, che per Leonardo non
sono semplicemente un linguaggio per ripetere visivamente
cose già note, ma, come ha osservato Giulio Carlo Argan,
“sono la chiave con cui si penetra nel mondo dei fenomeni”.
È infatti dai precetti derivanti dal Libro di pittura che si
recuperano i fondamenti innovativi della sua produzione
cartografica.
La formula adotta da Leonardo nel rilevamento e nella
rappresentazione cartografica è sintetizzata nel precetto che
egli stesso ferma nei suoi appunti del Manoscritto L dell’Istituto
di Francia, dove egli annota: “scorta sulle sommità e in su’ lati
dei colli le figure di terreni e le sue divisioni e nelle cose volte
a te, fale in propria forma” (Ms. L dell’Istituto di Francia, f. 21
are solved through the use of the fi rst principle of similitude
between right-angled triangles and through the so called “rule
of three”. However, the most relevant contribution of this work
consist in a new method of topographical surveying that is
meant to solve the question of the mutual position on the urban
scale through a triangulation work. Such a work had already
been anticipated by Giovanni Fontana in his Tractatus de
trigono balistario abbreviatus […] in 1440. Th is essay was
recovered and completed by diff erent Renaissance scholars, and
was furthermore ameliorated for other fi ve centuries, until
nowadays.
For this triangulation, they used a goniometer divided in 48
parts, still without an alidade and a compass pointing to the
magnetic North, and still with the help of a plumb line.
Alberti fully describes his method, explaining how to
manufacture the adequate instrument and, step by step,
how operate in order to obtain a correct survey. Moreover, he
explains all the rules to follow in the observation of the angular
directions among the diff erent sites and in their registrations.
Nevertheless, there are no instructions about the representation
of the dimensions and the orientation of the various polygons
surveyed.
Th e second essay, Descriptio urbis Romae, is a much more
original step forward. It shows how an urban plan could be
sketched on a map, according to polar coordinates collected
and organised within standard tables.
Th e instrument that enables the technicians to draw such
tables is a protractor, called Orizon. It is provided with a
graduated radius that is hinged in the middle of the instrument
itself, exactly as it happens in the modern, graduated circular
protractor having a pivoted arm, used for measuring or
marking off angles.
When it has represented on a sheet of paper the plan of a town
measured through the triangulation method explained in the
Ludi, a surveyor establish in this representation the basis of a
cartographic system of reference: starting from a central point,
he can measure with the Orizon the polar coordinates of every
single place on the graphic. We are talking about distances and
angles from a starting point that can give way to the setting
down of a table with the name and the position of every
represented topographical detail, a sort of encrypted map. Such
a table, obtained through a graphical representation and not
by a practical survey on the fi eld, represents a revolutionary
step forward: the cartographer willing to obtain another map,
identical to the original one, could certainly be able to make a
copy perfectly matching the fi rst sample.
Considered from this point of view, the whole method suggested
by the Descriptio could appear like a mere academic exercise:
the advantages of taking polar coordinates as a reference are
obvious at fi rst sight, as such a process allows the creation of
a faithful reproduction from an original map. However, this
method is by far more productive than a simple technique
for a geometrical copy of a topographical map (a copy that
http://it.wikipedia.org/wiki/File:Leonardo_self.jpg