He made a further visit to Wallachia in 1856. From inspector-general (1867), Lalanne in 1876 became director of the École des ponts et chaussées. Further experiences along the Danube involved Lalanne in Budapest, and in Silistra with a bridge project, and monitoring involved with demarcation after the Treaty of Berlin (1878). In 1879 Lalanne became a member of the Académie des sciences. In 1881 he retired, and was made a Grand Officer of the Legion of Honour. He became involved in politics again in the moderate left-wing. In 1883 he became a senator for life ("sénateur inamovible"). In 1882, he was Chairman of the Compagnie des omnibus de Paris. He died in Paris. The context of much of Lalanne's work was the construction of the Belgian and French rail systems. With Charles Joseph Minard, another civil engineer of the Ponts et Chaussées, Lalanne is considered to have made important contributions to rigorous cartography. He first clarified the distinction between an isometric line and an isopleth. Interested in population density and centres, Lalanne plotted innovative maps (1845) with contours of the same population density, and announced an "equilateral law" of equal spacing of the centres (1875), which he found a good fit in France. A paper of 1863 raised general considerations on transport routes and population distributions, direction later covered in work of Walter Christaller. With Philbert Maurice d'Ocagne Lalanne is considered the inventor of the nomogram. The intention was to replace the slide rule. In a paper of 1846 Lalanne contributed to the subject the ideas of anamorphosis and the use of projective transformations. There were applications to the solution of the cubic equation. He called his nomographic device the "abac". This "universal calculator" had 60 functions implemented graphically. Half a century later Maurice d'Ocagne provided a general theory, founded in projective geometry, for the nonlinear scales related by nomography. In 1840 Lalanne announced balanced ternary as an arithmetical system, in "Comptes Rendus"; this followed earlier work on signed arithmetic by John Leslie and Augustin Cauchy. Also in 1840 he devised a mechanical computer capable of solving numerically polynomial equations up to degree seven. Charles Darwin cited a paper by Lalanne from the same year 1840 on the geometry of honeycombs, in his "Natural Selection" manuscript.