Geographic Phenomena: Spatial Dimensions

Geographic phenomena are often classified according to the spatial dimension best used to describe their nature. These include points, lines, polygons, and volumes (3D). As you likely remember, we used the spatial dimension of map elements (e.g., line vs. point) in a previous lab to decide how to symbolize and apply feature labels to our maps.

Points exist in a singular location. Points are usually specified using a coordinate pair (x, y or latitude and longitude), though they occasionally include a z-value (height). Points are most appropriate in situations where the specific geometry of a feature is unimportant, or if the scale of the map is too small to usefully or accurately render the geometry of a feature. Points are also useful in cases where you are trying to minimize the amount of visual information being presented in a map. Points are used to map point locations such as weather recording stations, control points, or stream gages.

Figure 6.2.1: This map uses point symbols to show bike share stations in the city of Milwaukee.

Credit: city.milwaukee.gov/

Lines are one-dimensional spatial features defined by a sequence of at least two pairs (x, y) of coordinates. A third dimension, z (height), can also be assigned to lines, but this is uncommon. Lines are used to map geographic phenomena that are best conceived of as linear features, including features that have greater dimensionality in reality (e.g., streams are defined by surface area and volume). There are also linear features that do not visibly exist in the real world (e.g., property lines). Often, someone else has decided for you whether or not a given feature should be encoded as a line rather than a polygon, but if you’re trying to make this determination, you could think in terms of how many dimensions are needed to sufficiently present the geographic phenomenon. For example, Figure 6.2.2 is drawn at a scale such that the width of the Blue Ridge Parkway would be difficult to represent, and road width would be an immaterial variable anyway– the goal of this map isn’t furthered by that data (the map reader doesn't need to be able to accurately measure the road's width). We only need to know the path of the road (where it exists), so a line is the appropriate choice for representation here (the thickness of which is irrelevant).

Figure 6.2.2: A map that uses linear features to encode roads and hiking trails.

Credit: nps.gov/

Polygon features, also called area features, are represented by a sequence of (x, y) points that form a boundary that encloses a space. Areal phenomena can include natural features like lakes and islands, as well as human-defined locations like cities or census blocks.

Figure 6.2.3: A map that uses area features to show buildings, parking lots, so on.

Credit: health.act.gov.au/

2-½ and 3-D features are sometimes grouped together, but the distinction between them is important. 2-½D features define a continuous surface—they have an x, y, and a z at every location. A good example is elevation, which varies continuously across the landscape. Therefore, a topographic map is a common depiction of 2-½D phenomena.

Figure 6.2.4: A Topographic Map, which depicts a 2½ D surface.

Credit: USGS

True 3D maps have an x, y, and z, plus an additional data value, at every location and height. Imagine, as an example, a map of elevation like the one above; but at every point along the terrain surface, there are additional measurements being taken at various depths of that surface. Thus, rather than depicting a continuous 2D surface, true 3D maps depict a continuous volume.

Figure 6.2.5: A 3D map showing sea depth and temperature.

Credit: NOAA

As mentioned earlier, the scale of your map has significant influence on what spatial dimension will best represent the phenomenon you intend to map. Cities, for example, are often drawn as polygons on large-scale maps, but may appear as points on smaller-scale maps. Rivers are usually drawn as lines on small-scale maps but are better represented as areas on large-scale maps. We will discuss this more during discussions of cartographic generalization later in the course.