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Cartographic Q & A

Here are some of the more common cartographically-related web searches that point folks to IDVUX…

Q: datum versus elevation

A: Elevation just means how high or low a particular place on the surface of the Earth is compared to some reference point, like mean sea level.  Elevation is different than datum, but related.  Earth is a pretty bumpy place, and a datum’s goal is to provide an overall average elevation (or overall bulge-at-the-waist factor) to describe a reference spheroid used as the model of the earth, upon which coordinate systems are referenced.  If you are Himalayan and your maps only care about that area, then you would use a datum who’s bulge amount met up with your general elevation.  The problem is, your datum would be fantastically inappropriate for most other places in the world (but you wouldn’t care).  The trick is picking a datum that best fits your needs: local precision vs global compatibility.  These days, as global mapping systems are more prevalent, datums well suited for local areas are giving way to a datum that is designed for the whole world: WGS 1984.  It is less locally precise, but more globally standardized.  That kind of trade-off is well understood in the software industry.

Image courtesy ESRI via the University of Washington


Q: Mercator projection + cold war

A: Looks like someone is writing a paper.  Remember: State your thesis in the first paragraph and try to work in a joke early.  When I was a kid I would look at my classroom’s big Mercator wall map and fear that massive matchless landmass labeled USSR.  A key attribute of the Mercator projection is that it exaggerates the relative size of things in the more extreme latitudes.  And the Soviet Union was up there in a pretty extreme latitude.  A lot of folks think that Mercator maps were particularly popular in the cold war era because it enhanced the perceived size (and therefore threat) of the Soviet Union.  I was also petrified of Canadians -but for the same reason?  I don’t know if there was a concerted Cold War cartographic strategy, it sounds pretty conspiratorial but why not?

Recommended reading: Mark Monmonier’s How to Lie With Maps -a dose of healthy skepticism that teaches us how to evaluate maps with a critical eye.



Q: 0.001 degrees to miles

A:  It depends on if you want the distance for Latitude Degrees or Longitude Degrees.  And if you are talking longitude, it depends on how far poleward you are (longitude degrees are variable depending on…your latitude).  At the equator, the spacing of the lines of longitude is wider than near the poles.  Longitude lines pinch together at the poles so the distance represented by a degree approaches the infinitely small.

For a breakdown of the ground distance of decimal degrees at the equator: http://idvux.spaces.live.com/blog/cns!2EB6AAF6C3AC1EBE!708.entry



John Nelson / IDV Solutions / john.nelson@idvsolutions.com

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