Geography is the study of spatial distribution of phenomena. We all use maps every day. They show the concrete and abstract representation of things in a 3D world. Maps show us relationships between places and the things in those places. Maps describe what is where, when, and why. Sounds easy enough, right? Just take the data you have and put it on the map and voila, you have results.
Well it’s not that simple when accuracy counts. When accuracy matters, mapping is profoundly complex and requires extraordinary professional skill. Some of the most important aspects of an accurate map are never seen on the final product even though they are fundamental to an accurate one. There are three fundamental things underlying all maps and enabling them to accurately convey information; a datum, a projection, and a coordinate system.
A datum is a shift in the ellipsoid relative to the geoid to achieve the best fit between the two at the desired location. A projection transforms locations on a ball to locations on a flat surface. A coordinate system provides a reference framework to represent the locations of features. This “secret sauce” of any map makes up the fundamentals needed to ensure things are located in the right spot on a map.
Choosing the correct “sauce” and manipulating these mathematical abstractions of complex data is not for the faint of heart. Manipulations of the datum, projection, and coordinate system are complex and subtle. Very poor choices can produce beautiful, but wrong, maps. For example, will the data be displayed on a local or global scale? Certain datums are good for global mapping while others only make sense locally. When choosing a projection one must understand the intended use of the data. Projections have a profound impact on the accuracy of your data as they take the 3D curved earth surface and stretch, tear, and distort it into a 2D flat surface. We can’t eliminate distortions, only minimize them. Finally, the proper coordinate system cannot be applied until you have a deep understanding of the data being mapped and its intended use. The most commonly used coordinate system is latitude and longitude but these are inappropriate for many applications.
These three map concepts conspire together to make sense of complex data and support quality and accuracy. The sound analysis of mapping data cannot ensue without first ensuring its accuracy will support that analysis.