Porro, Ignazio

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(b. Pinerolo, Italy, 25 November 1801; d. Milan, Italy, 8 October 1875) topography, optics, geodesy.

After receiving his education in Turin, Porro entered the Piedmontese Corps of Engineers, in which he remained until 1842, attaining the rank of major. In that capacity he conceived and constructed several optical surveying instruments that revolutionized topographical and geodetic practice, and brought it essentially to its modern status. The science of rapid surveying (tachymetry; in Italian, celerimensura) was then new, and Porro states in his text on the subject that it originated in Italy in 1823. His first invention was the stereogonic telescope, which permitted the optical measurement of the distance from the objective to a graduated stadia rod; attaching this telescope to a theodolite, an instrument then becoming popular, Porro constructed the telemeter in 1835. It thus became possible to measure distance, elevation, and orientation with one instrument. The remarkable saving in time and the improvement in accuracy that it afforded led prominent French engineers to write impressive testimonials to Porro’s innovations in the introduction to the French edition of his Tachéométrie.

The practical test of his instruments had been made in the construction of roads, railroads, canals, and military fortifications by the time Porro left the army. From 1842 to 1861, in Turin and then in Paris, he improved existing instruments and developed new ones; he also wrote articles and textbooks popularizing them. The results of this activity, despite its acknowledged scientific merit, were not satisfactory, partly because of Porro’s apparent lack of organization and administrative skill and partly because of a tendency to modify his work before it had been fully tested.

After returning to Italy in 1861, Porro taught tachymetry in Florence and then in Milan, where he founded the journal Tecnomasio italiano and the Società Filotecnica (1865). He continued his scientific activities there until his death.

In 1848 Porro invented a telescopic objective (obiettivo anallattico) that furnished several views of the same subject at different scales. The forerunner of range finders and of modern objectives for telemetry and telephotography, it was adapted by him to topography. In the last years of his life Porro, who in 1855 had divided a circle with a 35-millimeter diameter into 4,000 parts, constructed the cleps, an improved tachymeter that he hoped would become the universal instrument, easy to transport without impairing accuracy and facility of use. Porro’s other contributions include the introduction of inverting prisms in binoculars in 1851 and the subsequent construction of the modern prismatic instrument; the application of photography to topography, showing how the measurement of both distances and angles could be performed on the basis of photographs; and some of the earliest astronomical photographs of the eclipse of 1857.


I. OriginalWorks. Porro’s principal work is the text La tacheometria (Turin, 1854), best known in its rev. French ed., La tachéométrie, ou l’art de lever des plans et de faire les nivellements avec beaucoup de précision et une économic de temps considérable (Paris, 1858). The French ed. was an expanded rev. of a memoir in the Annales des ponts et chaussées, sec. 5. 4 (1852), 273–390, and appeared with the first-name initial J. He also wrote Applicazioni della celerimensura alla misura generale porcellaria ed altimetria in Italia (Florence, 1862).

II. Secondary Literature. A review of the development of optical instruments, which places some of Porro’s inventions in historical perspective, may be found in Encyclopaedia Britannica, 11th ed. (1910), articles “Binocular.” “Photography,” “Range-Finder.” and “Stereoscope.” For a comprehensive historical treatment of tachymetry, sec N. Jadanza, “Per la storia della celerimensura,” in Rivista di topografia e catasto, supp. to Giornale dei lavori pubblici e delle strade ferrate (1894).

Bruno A. Boley