Talk:Gnomonic projection

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The link http://www.3dsoftware.com/Cartography/USGS/MapProjections/Azimuthal/Gnomonic does not exist. --Jalanthomas 01:40, 5 March 2007 (UTC)

Perspective on the Gnomonic perspective

Geographical 'Gnomonic' projection refers to the use of the gnomon or central, vertical (not axial) rod of a [sun dial]. In a geographic Gnomonic projection, the central point (or gnomon) is chosen on the surface of the earth, to be convenient for the user, and all radii from that point represent '[great circle] routes' thru the central point and its [antipodal point] represented by the entire outer edge of a full earth projection.
In some forms, this can be Azimuthal or a subset of special Conic Projections, with a flat cone, or not, and the apex above or below the central point.
See also Azmuthal Map generation.
The radii (or diameters) represent the Great Circle or shortest route between any two connected points. However the distances along these lines (sometimes represented by parallel circles) might or might not be linear. Any non-radial measurements will vary from '1 times scale' at the central point (or contact circle), to 'infinity' at the antipode or perimeter, at which any land mass appears 'smeared out'. Often to minimize this disconcerting effect, the map is constrained to a small portion of the earth and an altitude needed to connect the points of interest.

As a practical matter, the map is most useful to you only if centered on your own position, because it allows you to aim the trajectory of your airplane, cannon, or antenna more correctly toward any other point on the surface of the earth,without the distortion caused by more conventional mapping projections. Most obviously, east & west directions do not follow the parallels.
Weather, Coriolis, and geomagnetic effects, will all require trajectory compensation.
Wikidity (talk) 20:19, 6 July 2010 (UTC)

rectilinear projection

If I understand rectilinear projection correctly (as exemplified by a pinhole camera), when some 'concentric rings' are equidistant on the spherical surface (globe) they cannot be equidistant on the plane surface (map) - and vice versa. Is this always true of gnomonic projection, or is rectilinear projection a special case of gnomonic projection ? --Redbobblehat (talk) 18:14, 14 February 2011 (UTC)

It is always true for the gnomonic projection that concentric circles on the sphere are not spaced with correspondingly equal distances between them on the planar surface. Strebe (talk) 19:38, 14 February 2011 (UTC)

Replace low-contrast images

I will be replacing images on the various map projection pages. Presently many are on a satellite composite image from NASA that, while realistic, poorly demonstrates the projections because of dark color and low contrast. I have created a stylization of the same data with much brighter water areas and a light graticule to contrast. See the thumbnail of the example from another article. Some images on some pages are acceptable but differ stylistically from most articles; I will replace these also.

The images will be high resolution and antialiased, with 15° graticules for world projections, red, translucent equator, red tropics, and blue polar circles.

Please discuss agreement or objections over here (not this page). I intend to start these replacements on 13 August. Thank you. Strebe (talk) 22:40, 6 August 2011 (UTC)

Deleted PDF file

Note: Moved from my Talk page: Strebe, what is wrong with file I added?(Boss of 105 (talk) 05:51, 3 April 2014 (UTC))

The image does not add anything to the understanding of the article beyond what is already on the page. Strebe (talk) 20:51, 3 April 2014 (UTC)

File:Gnomonic projection SW.jpg to appear as POTD

Hello! This is a note to let the editors of this article know that File:Gnomonic projection SW.jpg will be appearing as picture of the day on September 9, 2015. You can view and edit the POTD blurb at Template:POTD/2015-09-09. If this article needs any attention or maintenance, it would be preferable if that could be done before its appearance on the Main Page. Thanks! — Chris Woodrich (talk) 00:21, 23 August 2015 (UTC)

Picture of the day

A gnomonic projection of a portion of the northern hemisphere, centered on the geographic North Pole. Such projections display all great circles as straight lines, resulting in any line segment on a gnomonic map showing the shortest route between the segment's two endpoints.Map: Strebe, using Geocart

It’s all good. Thanks, Chris Woodrich! Strebe (talk) 03:13, 23 August 2015 (UTC)

Line segment

Probably "any line segment on a gnomonic map" is geometrically correct, but it might be clarifying for a reader to add that this means a straight line. More so because the map (POTD today) shows circles. Even I, while familiar with this projection, had to check the definition of a line segment. (even worse: when I explain the great circle to people, I use the example of flying from the North pole over any meridian etc., which of course is a circle). -DePiep (talk) 08:21, 9 September 2015 (UTC)

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Online viewer

First I saw this article years ago. It had formulas and links, and there were no interactive map that user could scroll or zoom. In later years, I checked this more times and there were no interactive map still. Now, I insert a link for it and it gets retracted like it was spam. So I guess I have to spent 30 minutes of my life to provide justification for a person who reverted it in 5 seconds (like it was spam).

https://www.humus.name/index.php?page=Cubemap&item=Earth

Just because it was on someone's personal site? Well this page is more relevant to article than any other links, which tell same information already existing in the article. Meanwhile the article has Sphaerica link is useless for majority of wikipedia users -- virtually nobody going to install it. I did, and was puzzled - WTF- how is it even relevant to the article?

Do you suggest anyone wishing to take a look at gnomonic Earth map install some proprietary software, or write their own implementation? Don't like the link? Find a better one, then delete it. Google maps isn't going to add option to use gnomonic projection anytime soon.Alliumnsk (talk)

Thank you for prompting me to look at the other links. The Sphaerica link is also not appropriate, and neither is the Radical Cartography link. I have deleted them as well. The Cubemap link has at least three problems:

• It is a link to a site of a person of unknown credentials. This violates WP:LINKSTOAVOID.
• The visualization’s interactivity does not function on common mobile devices.
• The linked page provides no information about the display. It is not merely an interactive representation of a gnomonic map; it seems to be an orthographic(?) view of a cube whose faces are gnomonic projections…? It is not educational if the viewer cannot even tell what they are seeing, and in any case, the cube is a complicating factor that impedes understanding of the projection itself. It’s even worse given that two different projections seem to be involved. Strebe (talk) 15:33, 20 September 2020 (UTC)

it seems to be an orthographic(?) view of a cube whose faces are gnomonic projections do you really mean it or is it a typo? Alliumnsk (talk) 17:57, 21 September 2020 (UTC)

No typo. Strebe (talk) 02:08, 22 September 2020 (UTC)

For a person who seems to have interest in cartography, this statement is very strange. This is pretty much how orthographic view of a cube whose faces are gnomonic projections (sic!) looks like: [1] It is finite, whereas gnonomic projection of Earth surface is infinite. The Humus link I posted indeed displays interactive gnomonic map, if you having doubts, you could try actually try to rotate/zoom it and overlay over contour map examples taken from the article. Cubemap texture used internally for computational reasons. The Humus link doesn't teach theory, but this is what wikipedia article is while the wikipedia article lacked interactive map which is very much needed. What kind of map geek you are if you can't recognize gnomonic map without Reliable Source saying, that it is, indeed so? Alliumnsk (talk) 18:26, 23 September 2020 (UTC)

@Alliumnsk:If you’re going to be uncivil, you should at least be right. Instead, you’re an ill tempered fool. The link is useless: No parameterizations of the gnomonic projection yield results anything like what that website give, and that is obvious to the trained eye. But I didn’t just look at it; I experimented using an actual map projection program. You’re a time-waster. The reason Wikipedia has the rules I cited is so that these kinds of pointless arguments about unverifiable content don’t happen. 06:18, 27 September 2020 (UTC)

"Centered on"

The article notes that a gnomonic projection is centered on a point on the surface of the Earth. So far, so good. However, some of the illustrations are described as being "centered on" a line, such as the Equator or the 40th parallel. These are not points, and it's been bothering me that the captions are leaving it at that. Ideally, these captions should identify the point at which the projection is tangent to the Earth's surface (e.g., the North Pole). If that level of precision is not possible, the caption should say something like "a point on the Equator" or "a point near 40°N 75°W" to be completely accurate. Piledhigheranddeeper (talk) 16:26, 30 October 2023 (UTC)

What we have seems fine to me. “Centered on” doesn’t have to mean the center is given for every coordinate; it can mean centered on the coordinate given in the description, which is the salient coordinate, with the other coordinates unspecified. We do this in life all the time: “Center the desk between the two walls” without specifying vertical position or how far from the wall. Even the image that says “centered on the north pole” is incomplete, since which meridian appears vertical and below center is not given. Strebe (talk) 17:27, 30 October 2023 (UTC)

This seems unhelpfully pedantic to me. The precise center point isn't the point of the caption: these maps are primarily showing the projection of the graticule with a traced coastline for some context, but are too small to be of practical use as maps in any context where knowing the exact center matters. (On the other hand, if you want to add more complete descriptive information to the image description page, go for it.) –jacobolus (t) 19:47, 30 October 2023 (UTC)