Talk:Map projection

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[edit] Map projections image files

I am not sure whether I am right on this one, but these images seem identical with ESRI's poster on map projections that comes with their book "Getting to know ArcGIS". Someone should verify the copyright of these images. The USGS website does not mention any copyright information.


[edit] Map projections from Planet math

I don't know if the articles for projections exist, but it might be worth nabbing those that do from Planet math. Mr. Jones 15:05, 8 May 2004 (UTC)

[edit] Formatting criticism

You should put a ":" before the math marker of the Mercator formula, to indent it. Same with the next line starting with phi. -- Martin Vermeer

Okay, go ahead cleaning it up :-)

[edit] History of Map Projections

12/29/05

It would be a good idea to add a short section on the history of Map Projections - this is an interesting topic, as it mirrors the evolution of human knowledge over the centuries. Great ingenuity has gone into developing the ideas of map projection to make it more and more useful for navigators and other users.

[edit] Benson Projection

I have invented a superior map. Everyone will agree that it is the best. How do I submit it? 70.177.90.238 20:40, 11 January 2006 (UTC)

You don't. If you want, tell people about it, and perhaps one of them will add information about it to the article. See Wikipedia:Autobiography. ¦ Reisio 21:33, 11 January 2006 (UTC)

[edit] map projection or spherical projection?

IMHO spherical projection is the more standard mathematical term for this. i've done a redirect from spherical projection to this article, but if noone objects, then i would suggest switching the redirect to the other direction - i.e. "spherical projection" would be the main article. Boud 08:41, 27 April 2006 (UTC)

But the main use is cartographical. Mathematicians may have a few projections they use frequently and then generalise. But cartographers use all these and more. --Henrygb 09:05, 27 April 2006 (UTC)
Also note that that the surface to be projected in Cartography is not always a sphere, it might be a spheroid (or ellipsoid). On the other hand, the projection process is not necessarily geometric - From the article: "The term "projection" here refers to any function defined on the earth's surface and with values on the plane, and not necessarily a geometric projection." Alvesgaspar 15:49, 27 April 2006 (UTC)

[edit] European or American map

The article uses maps with America at the centre. I though that the Wikipedia standard was to use maps with Europe/Africa at the centre (because they don't cut Asia in half). If so, does this article deviate from that for a specific reason? And are there names for these maps (I can't find an article on it)? DirkvdM 07:06, 17 August 2006 (UTC)

Most of the maps come from the United States Geological Survey. --Henrygb 09:14, 17 August 2006 (UTC)

[edit] Triangular map?

A while back I heard of a type of projection that was made up of a bunch of triangles, and that by unfolding it a certain way, it showed the world as a chain of continents surrounded by water, or a chain of oceans surrounded by land. Does anyone have any information on this? Stale Fries 00:13, 3 March 2007 (UTC)

Nevermind, it's Dymaxion map. Stale Fries 00:16, 3 March 2007 (UTC)

I think placing the Dymaxion Map under the heading "compromise projections" is missleading as it is the only unique map among those listed and the description for the section in no way describes the Dymaxion Map. I think part of the confusion above stems from the poor classification of this map. 71.214.83.247 (talk) 05:46, 3 January 2008 (UTC)

It makes sense there because it certainly is a compromise projection. An alternative might be to introduce a category "decorative" or "novelty" projections (I think that's the term Snyder uses in his reference manual). The Dymaxion projection would fit there, as well as several of the other "neat looking" projections like the butterfly and heart projections. Paul Koning (talk) 12:03, 3 January 2008 (UTC)

[edit] Cylindrical projections

The statement that cylindrical projections have straight meridians is true only for the conventional (projection axis == earth axis) form, not for the transverse or oblique forms. Paul Koning 17:50, 10 May 2007 (UTC)

[edit] Article focuses on whole-earth projections

... whereas most maps show only a small part of the earth. I realize that this is just "a special case" where you only show part of the map, but given the extremely common nature of this issue there should be some specific discussion of what projections are commonly used for smaller maps (and how to choose the parameters). Personally, I'm trying to figure out which projection to use for my map of a country (Guatemala) but the same issue exists even for the smallest, city-scale maps. --190.56.85.26 17:10, 11 July 2007 (UTC)

It's true that a lot of the decorative and PC projections are meaningful only, or mostly, for whole earth maps. But the mainstream ones are applicable in both cases. For example, you might find Mercator, transverse Mercator, polyconic, and gnomonic projections, to name just a few. I can't see a point in worrying about this for city maps but certainly it's a valid concern for country maps such as the one you mentioned. So the question you'd want to answer: what property matters most? Maps ("charts") for some specific professional application tend to pick a specific projection to meet those requirements -- you might find gnomonic or Mercator projections there. (I think marine charts are often in that category.) For others, conformal projections are probably a good starting point. Yes, it might be interesting to talk about this. Snynder could be a good reference, I would think. Paul Koning 18:26, 11 July 2007 (UTC)

Just found [1] which is much more "useful" from my perspective than this article, as it organizes by purpose rather than by mathematical criteria. I would suggest that this article should keep the initial explanations as is, but the actual projections should be reorganized along the lines of this source.

ps. For my purposes, I've chosen LCC - appropriate for a "taller" country and atlas-style "just looking" maps. Now, I just have to figure out how to enter the parameters into GRASS - it asks for "central", "first", and "second" parallels, which default to 23,33,and 45; I try 15,13,and 17 for a map which will extend 13N to 18N and it doesn't accept them... maybe I need to use negative rather than positive numbers for my false northing and false easting...

pps. Sure it makes hardly any difference for city maps - but you still have to choose one. --190.56.85.26 18:48, 11 July 2007 (UTC)

[edit] "Modern notation"

Many articles on projections, such as equirectangular projection, use some kind of "modern notation" in favor of a more understandable x = x(λ, φ), y = y(λ, φ) type of thing that would be easier to understand and is more standard in terms of notation.

This is a notation occasionally used for functions/mappings, but it may not be the best choice here, the shorthand really doesn't make it shorter and only obscures it.

If no one objects I'm intent on fixing all map projection articles that use this convention.

Edit: sorry, forgot to sign. -Ben pcc 22:43, 27 July 2007 (UTC)

I favor this proposal. The so-called "modern notation" is not normal in the professional geographic map projections' literature; nor does it help novices understand what's going on.

Strebe 01:10, 29 July 2007 (UTC)

Done. Whew. -Ben pcc 17:20, 7 August 2007 (UTC)

[edit] Please explain: Polar projection is not a projection; it's an "aspect" of any projection

I was dealing with an edit to mariner's astrolabe (since moved to its talk page) that referenced "polar projection" on an astrolabe. The link provided by the author was to polar and [[map projection|projection]]; I changed it to polar projection only to discover no such link existed. I looked at this article and saw no polar projection mentioned.

Not being a cartographer, I looked up polar projection and found it described as a map projection and added a section here with an image. It was removed with the comment as in the section headline. Please explain the difference - it's clear that there is some ambiguity in other references that only a cartographer is likely to explain.

Since the description on talk:mariner's astrolabe is lacking detail on what polar projection means specifically, does anyone know what map projection is appropriate to describe? Thanks for your help. Michael Daly 05:40, 18 October 2007 (UTC)

The current talk:mariner's astrolabe links the words "polar projection" to Map_projection#Azimuthal (projections_onto_a_plane) which is right. "Polar" implies a projection related to a point, specifically one of the poles. Azimuthal projections are projections around a point, which may be any point on the globe. "Azimuthal" isn't a specific projection, it's a category -- specific examples include stereographic, gnomonic, and many others. So "polar projection" would be a shorthand for "an azimuthal projection centered on the pole". That's still a category, or aspect (where it is centered). If you wanted to describe a specific projection in its polar aspect, you might have "gnomonic projection centered on the pole" (or perhaps more succinctly "polar gnomonic projection"). Paul Koning 18:34, 18 October 2007 (UTC)
Thanks for that. I put that link there as the closest to where I had added "polar projection" to this article. I wasn't sure whether gnomonic would be the correct term in this instance. Michael Daly 19:03, 18 October 2007 (UTC)
I mentioned gnomonic just as an example. Chances are it isn't that one, but I don't have a good guess about which azimuthal projection one might find on an astrolabe. You might be able to figure it out by examination. Or it may be that the "projection" is really just a diagram, centered on the pole but with no well defined scale or geometry. Paul Koning 21:05, 18 October 2007 (UTC)
It would be well-defined. The astrolabe described was used for navigation and other astrolabe types tend to be quite good - they knew their astronomy when they built them. Actually, my initial guess was gnomonic since it's simple and an old projection. However, I only know a few projections, so that assumption is based as much on ignorance as knowledge. I'll leave it until someone, preferably the person who added that info to the article, explains what that astrolabe is all about. Then, with your explanation, I'll have an easier time figuring out a better link to this article. However, I suspect the info will be removed permanently, since it sounds like a "we invented everything" from someone who has done that before in another article. In the meanwhile, I now know the difference between "polar projection" and the general azimuthal projections. Michael Daly 01:19, 19 October 2007 (UTC)