Man is both a “visual” and “visualising” animal. Our species uses sight as its primary sense, a characteristic which it shares with the rest of the primates, and it is above all on the information provided by our eyes that we base our cognitive universe. Our species is able to visualise thoughts and ideas, to create diagrams, graphs and paintings. Dreaming is probably the most powerful instrument nature has given us for organising action and facing reality. To visualise things is to represent them to ourselves before others, to represent memories, to represent things as they are but also as they could be. Terms such as “imagination” and “idea” do not come about by chance: “idea” comes from the Greek eidos which means sight, intuition, image. Imagination is perhaps the main gift required by scientific researchers, as with craftsmen; people who act, imagine and organise their ideas, visualising them and crystallising them graphically. To visualise situations means that they can be analysed rapidly, and makes it possible to compare scenarios (another term linked to visual representation) and to plan. And planning means deliberately changing the world – or keeping it the way it is. Graphic representations have an enormous capacity to summarise, especially when they visualise measurements, quantitative relations, numbers and data. Graphic representation facilitates the acquisition and comprehension of data, of the numbers from which it was created. To estimate a distance or recognise a landscape are fundamental survival skills for all animals living above ground, be they predator or prey, and for this reason a representation which translates quantitative relations into spatial relationships is more easily understood and assimilated. Maps have long been used for representing the territory, but only recently have thematic maps been created for representing what goes on in that territory. We may be interested to know how air temperatures vary in space rather than in time, or to know what the temperature is in a given moment in Piazza San Marco, on the Lido, in Marghera, or at all the measuring stations. It is sufficient to replace on one axis (generally the abscissa) the spatial variable (the measuring stations) with the temporal one (time) to generate a new graph showing temperatures at a given moment, or their average. The task is more complicated if we seek to understand the distribution of temperature throughout the Lagoon of Venice. The stations occupy a place in physical space, they are not collocated along the lines of a regular grid (and it is not always a good idea for them to be so), and it is thus necessary to take account of their actual distribution in the territory. The values recorded may be represented with appropriate symbols of suitable dimensions (for example, coloured dots on the map near each measuring station), or interpolation techniques can be used to calculate temperatures in areas between stations, where they were not measured. Interpolation techniques therefore enable us to deduce the value of a variable (in this case, temperature) even in places where it was not actually measured, and so to create a continuous representation. Examples of this are shown in the first two plates of the Atlas. Mathematical reconstruction (interpolation) of the values of variables in places where measurements were not actually taken is a point on which to reflect carefully. If the area were covered with stations set very close to each other, the problem would not exist; the mass of coloured dots would be sufficient to describe thermal differences with precision. But the measuring stations are necessarily limited, and statistical models need to be employed to estimate what happens in the space between them. But how accurate is such an estimate? Great care must be taken when reading a map; the proper degree of importance can only be attributed to a cartographical representation if we have a good understanding of what it represents, what its spatial variability is, how the measuring campaign was conducted, and how the interpolation was calculated. Many of these aspects are described in detail in a technical note at the end of this introductory chapter. A representation is not the same as the object itself, although cinema and television have shown how this distinction is not always clear to everybody. Images have a great capacity for persuasion, they touch emotional chords unaffected by logic: when we do not believe somebody, we say: “Show me” or perhaps “Demonstrate it”. It is precisely for this reason that to propose graphic and especially cartographic representations carries a great responsibility. An atlas is a tool to facilitate thought. Those who work on its creation always bear in mind these simple but fundamental considerations and so, it is hoped, will the readers of this Atlas.