One of the themes running through our book The GIS Guide to Public Domain Data is that maps are representations of reality. While almost everyone reading this statement is likely to agree with it, in the fast-paced world that GIS analysis and creating maps has become, it is easy to lose sight of this fact when staring at tables, maps, and imagery. In a recent video, I discuss just one place where care needs to be made in making decisions based on spatial data. In the video, observe my surroundings as I stand near the traditional “line” that divides the deciduous forest to the south from the coniferous forest to the north in North America. Is the “line” really a line at all, or is it better described as a gradual change from deciduous to coniferous as one travels north? Is that vector line then better symbolized as a “zone”, or is vegetation better mapped as a raster data set, with each cell representing the percentage of deciduous and coniferous trees?
How many other data sets do we tend to see as having firm boundaries, when the boundaries are not really firm at all in reality? How does that affect the decisions we make with them? Even the boundary between wetlands and open water were originally interpreted based on land cover data or a satellite or aerial image. As we state in the book, even contour lines were often interpreted originally from aerial stereo pairs. And each data set was collected at a specific scale, with certain equipment and software, at a specific date, and within certain margins of error that the organization established. Maps are representations of reality. They are incredibly useful representations to be sure, but care needs to be taken when using this or any abstracted data.