I have recently accepted the challenge of giving a very short introduction to GIS to an eclectic audience composed of highly educated professionals, albeit not versed in geospatial technologies or geography. This has prompted me to rethink the question “*what is GIS?*“, modifying accordingly my approach to teaching it from scratch. The idea was to achieve that beautiful blend of minimal elementary theory, for meaningfulness, and minimal sufficient practical knowledge, for usefulness, that enable any person to start producing simple results and delving further in the matter, if desired.

After some pondering, and many sketches, I finally came to this:

This is of course nothing else that the classical “*a system to store, manipulate and visualize geographical data*” stripped to its bare minimum.

In this scheme of things, we may start describing the two basic representations of geographical reality: vectors, for discrete features, and rasters, for continuous fields, for which different formats are used (in the most simple cases just *ShapeFiles* and *GeoTIFFs*). Here, I find convenient to introduce the concept of data as layers, which is essential for both processing and visualization in GIS.

We may then continue explaining that, for each layer, we usually have associated data, that are allocated in database tables called *attributes*. These can be used to store information, to choose a particular subset (spatial queries) or make operations on the layers. Vectors and raster data models have associated sets of spatial operations, and also operations for converting one type into the other. **It is at this level that we most often use GIS to explore relationships in spatial data.
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As all this makes sense only in the context of a particular coordinate system, a *Spatial reference System (SRS)*, we may continue by introducing some essential cartography. I found that it is sufficient, and more convenient, to limit myself to the practical basics of a projection of interest for the audience (such as UTM), introduced merely as a coordinate system. I prefer this approach to the more classical lecture on geodesy, with the gory details of the geoid, ellipsoid and the myriad of projections and reference systems, which more often than not are just confusing for beginners.

Finally, I found relevant to say some closing words about where our data might come from: models, field measurements, regional surveys, GPS, satellite images, on-screen digitizing; and where our data might go to: maps, statistics, datasets as input for other studies, etc. This gives context and adds that tiny dose of practical meaningfulness that most people (that is, not GIS nerds) appreciate.

This was my brand of “*GIS in a nutshell*“, as of today. Which is yours?