Polar Anomalies: why is topography usually missing from the poles in GIS data?
For today’s mapping challenge, my theme was Null – or no data. After some consideration, I decided to get to the bottom of a question that had been bugging me for a while: why do a lot of the basemaps that come native in Esri software seem to erase small areas directly over the North and South poles? I don’t encounter this a lot, as it is usually not noticeable unless you work in a projection that highlights the polar regions. For example, in today’s map I chose to incorporate the World From Space projection into my project because it allow you to clearly see the areas of the poles that have no data.
Usually, when I encounter this issue, I draw over the affected area to try to “hide” it, but I was never sure why the issue presented itself in the first place. I figured the reason had to rest with how the data was collected and after some investigation, that turned out to be a correct assumption (though I also came across some far more amusing, if more conspiratorial, hypotheses during the course of my Googling). The gap in polar topographic data does have to do with how satellite imagery and topographic data gets collected.
Many imagery satellites follow what’s known as a Sun-synchronous orbit (SSO), a near polar orbit that allows them to pass over any given point of the Earth’s surface at the same local mean solar time. The effect of this is that every time the satellite is overhead, the surface illumination angle below remains consistent, which is useful in the collection and use of aerial imagery. Due to the orbital trajectories of many of these satellites, the poles are often not captured. It should be noted that there are elevation datasets for the poles that are freely available and ready to explore. If you have ever encountered a map and saw that Antarctica and/or the North Pole had a circular area missing from it, it has to do with data collection methods. If you want to find out what’s there, peruse the datasets linked above. That will lead you to a website hosted by the University of Minnesota where you can access Digital Elevation Models (DEM) of both poles. Not a cartographic conspiracy, unfortunately. Just a side effect of how data is collected.