Coordinate Conversions for Dummies

Map projections

It's out considered opinion that, if you don't know jack about how to do something, you probably shouldn't be writing a set of instructions for doing that thing. That's our opinion, a belief that clearly was not shared by the founders of and contributors to the website formerly known as eHow.com.¹ Hundreds of self-appointed "professional freelancers" pounded out tens of thousands of ignorant and sometimes downright dangerous posts. Today's DotD nominee falls on the ignorant end of that spectrum: meet Rose Kerr, who penned "How to Convert XY Coordinates to Longitude and Latitude" for Sciencing.com. Huh.

Let's get the answer out of the way up front: you can find dozens, if not hundreds, of coordinate transformation widgets online. Here's one for UTMs, for instance. But not everyone knows that, apparently including Kerr. Rose started out wrong-footed, botching the concept of coordinate transformations in her introduction:

"The position of an object in XY coordinates is converted to longitude and latitude to get a better and clear idea about the spot of the object on the surface of the earth."

We suppose that, if one squinted long and hard enough, that would make sense; but it might make more time than it's worth! Once she's shown her inadequacy for discussing the topic, Rose launches into a half-assed set of instructions. We said "half-assed," given that her first step is:

"Ensure that x, y and z values are specified in Cartesian coordinate system. The formula used is derived with the assumption that x, y and z values are defined in Cartesian coordinate system."

The utter stupidity of that statement should be obvious, but we'll explain anyway. First, it is highly unlikely that any set of XY coordinates offered for conversion to Lat-Long are Cartesian. And second, where the hell did that Z coordinate come from?

Kerr then proceeds to smash up some formulas she scraped off a forum at Stack Overflow (eHow.com forbade the use of forums as references a couple of years later). Her unfamiliarity with high-school math led her to include such computational "hints" as,

• "Asin is arc sin, which is a mathematical function, and atan2 is a variation of the arc tangent function..."
• "Here √ indicates square root, here square root of (x²+y²)..."
• "In this formula, sin and cos are mathematical functions..." [No, they're trigonometric functions!]

Regardless of Kerr's innumeracy, this post is a sham. Given that the OQ wanted to know how to convert XY to LatLong, not xyz, it's obvious that the desired answer would address one of the multitude of projected coordinate systems, very likely the familiar UTM coordinates. In that case, Step 1 should have been to determine the projection of the data or map. Rose, however, was ignorant of all that and instead dumped out an answer that would have most likely proved useless to most people interested in the topic. Even for those who are interested in a Cartesian Coordinate to Lat-Long would want to know the origins of X, Y, and Z; but that's not forthcoming. That's most likely because our Dumbass of the Day was completely ignorant of the concept.

¹ The owner of the website has parceled out most of the content into niche websites in an attempt to look more authoritative. Confidential to Leaf Group: it ain't workin'...

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