Debunking map myth #1: All coordinates are in “Latitude/Longitude”

Yesterday I published a post called Falsehoods programmers believe about maps, where I listed 18 falsehoods I believe or have seen programmers believe about maps and spatial data.

What I did not do was provide any backing for my claims that these are in fact falsehoods, and after this was pointed out in the comments I figured “here’s material for some follow-up posts”. I’m not sure I’ll go through all the falsehoods, and it might be that I have to revise some of my claims, but I’ll start with the first falsehood in this post.

So: the first falsehood is: “All coordinates are in “Latitude/Longitude””. Why isn’t it?

TLDR: look at a search for coordinate systems for Norway: “Found 247 valid records”. This is far more than 1, so case closed.

In depth: We separate between geographic coordinate systems, which is (to simplify a bit) angles of latitude and longitude on an ellipsoid that represents the earth. These coordinate systems are also known as “unprojected coordinate systems”. What makes things even more difficult is that there is several reference ellipsoid models used (specified by ellipsoid parameters a and f) and also different datums (specifies how the actual earth is aligned with the ellipsoid).

Thus: given pair of longitude/latitude coordinates, there are several options for ellipsoid and datums. True enough: when people refer to lat/lon they usually refer to the ellipsoid/datum-combination most widely used: WGS84. This is the system used by GPS, by Google Maps and in most cases where “laypeople” refer to “latlon”. Still, there are a lot of other datums (such as ED50, still used by the Norwegian petroleum industry).

In addition to the unprojected geographic coordinate systems we also have a bunch of projected coordinate systems. These usually have meters (or feet for the strange americans) as units, and one can actually use (with some minor exceptions) the Pythagorean theorem to calculate distances between coordinates. The downside with projected coordinate systems is that going from a 3- to 2D-representation _will_ introduce some errors (see Mercator). One common solution is to make a projected coordinate system valid for only “small” areas of the earth (such as “norway”). There are a lot of techniques for projecting a map, but the most commonly used is the Universal Transverse Mercator (UTM). This is not a single projection, but a series of zones.

Zones 32N, 33N and 35N covers Norway, and for web mapping purposes it’s ok to use zone 33N to cover the whole of Norway (there will be errors in the edges, but these are negligible for purposes of screen display). The UTM projection is usually used in combination with the WGS84 datum, but can also be used with other datums.

So: in conclusion: there are a lot of ways to express where on the earth you are, and when given a “latlon”-pair you cannot be sure what coordinate system it’s in (although a fair guess is WGS84)

2 thoughts on “Debunking map myth #1: All coordinates are in “Latitude/Longitude”

  1. Lars Marius Garshol

    I knew there were other forms of coordinate systems than lat/long, so the first myth I knew about, but this filled in lots of detail that was new to me. Thank you.

    I’ve been told that one of the benefits of UTM is that the frame of reference is the Eurasian plate, so that the positions keep their coordinates even as the tectonic plates move. I’ve always wondered if that’s actually true. (Also strikes me that it wouldn’t work very well for Iceland.)

  2. Atle Post author

    Well, that would not be UTM, as that’s just a system for projecting (from 3 to 2D). If you meant WGS84 that can’t be true either, as it’s the _world_ geodetic system.

    What you might have heard about is ETRS89 (http://en.wikipedia.org/wiki/European_Terrestrial_Reference_System_1989), referred to as EUREF89 in Norway. Euref89 was defined in 1989 to be equal to WGS84 at that time and is “tied” to Eurasian plate to avoid drifting due to continental drift. (for more info (in norwegian) see http://www.statkart.no/Kunnskap/Kart-og-kartlegging/Referanseramme/Referanserammer-for-Norge/ and http://www.milvang.no/gps/datum/datum.html).

    And: for the record: I’m no expert at these topics (but had some courses on them 5-6 years ago), so don’t take my words for the correctness of all the details here, altough I’m rather confident that the big picture presented is right.

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