FAQ: Map Options and Features

Q: Where did the Blue Marble maps come from?

A: The Blue Marble images were created by NASA using data from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS), an instrument aboard the Terra satellite. Please see the Great Circle Mapper credits for more information.

Q: What is the difference between Blue Marble and Blue Marble: Next Generation?

A: NASA released the original Blue Marble imagery in 2002, using data from the MODIS instrument (see previous question). Two years later, NASA released Blue Marble: Next Generation, again using data from MODIS but with improved image processing techniques and observations spanning an entire year. The newer imagery also featured higher resolution, with a single pixel representing a square 15 arc-seconds on a side versus 1 arc-minute (60 arc-seconds) for the orignal Blue Marble imagery.

The Great Circle Mapper's Blue Marble maps use the Next Generation imagery.

Q: How can I make different paths use different colors?

A: Consider the path SFO-DFW,SFO-IAH with the default path color (red). Changing the color for just SFO-IAH to navy blue can be done using the path SFO-DFW,color:navy,SFO-IAH. You can shorten color: to just c: and can change colors as many times as you like.

Valid colors include those offered for any of the color options in the Map Controls. In addition, colors can be specified using hexadecimal colors using a syntax like that used by CSS. For example, c:#ff0000 is the same as c:red.

Q: How many points/paths can be on a map?

A: The Great Circle Mapper software has no architectural limit and is only limited by memory on the server (and patience). On the development server a map with 84,532 paths was generated with no problem.

The length of a URL does limit the complexity of maps which can be requested, however. Internet Explorer has a maximum URL length of 2,083 characters which is probably the most restrictive limit. Most other browers are less restrictive, in which case the limit becomes the length of a request line accepted by the Apache HTTP Server software. This limit is less than 8,190 bytes.

Work is underway to allow map definitions to be saved on the Great Circle Mapper web site which will allow more complex maps without exceeding HTTP's limits on URL lengths. (There is not yet a target date for when this work will be completed.)

Q: What map projections are available?

A: A map projection is a system for projecting points of an ellipsoid onto a plane. All map projections distort the result in one way or another and a projections that works well for one map may be a poor choice for another. The Great Circle Mapper therefore offers a choice of map projections:

Rectangular (Plate Carrée)

Sometimes called unprojected maps, latitude and longitude are treated as simple rectangular coordinates of pixels in the map image. Although it may appear to be similar to a Mercator projection it is not the same thing; see "Why use the rectangular (Plate Carrée) projection?"

Orthographic

This projection shows one hemisphere as viewed from space, from the perspective of a point infinitely far away. Photographs of Earth from the moon are examples of this projection. Using this projection, any geodesic path to/from the center of the projection is a straight line. These maps may in the polar aspect (see below) or oblique.

Polar-Aspect Orthographic

This is the orthorgraphic projection centered above one of the poles, as opposed to an oblique orthographic projection.

Azimuthal Equidistant

The azimuthal equidistant projection resembles the orthographic projection but displays the entire body, somewhat like splaying out the entire peel of an orange. The point opposite the map's center point is not a point but the entire rim of the map. Like the orthographic projection, any geodesic path to/from the center of the projection is a straight line, but for the azimuthal equidistant projection the distance along that line is proportional to the distance along the path.

Best: Rectangular or Polar

The "best" projection uses either a polar-aspect orthographic projection or the rectangular projection depending on what is being mapped. If all points on the map, including points along any paths, are on the same side of the equator (or on the equator), and any points are in the Arctic or Antarctic polar region, then the orthographic projection centered on the appropriate pole is used. Otherwise, the rectangular projection is used.

For a more humorous look at map projections, see xkcd's commentary on What your favorite Map Projection says about you.

Q: Why use the rectangular (Plate Carrée) projection?

A: The rectangular or Plate Carrée projection is computationally easy and thus maps can be redendered more quickly. It also makes it far easier to render maps based on photographs or other imagery, such the the Blue Marble maps. For these reasons, the Great Circle Mapper favors this map projection, though others are offered for cases where the rectangular projection is unsuitable.

See Mercator vs. well...not Mercator (Platte Carre) for a good discussion of why online mapping tools often use the rectangular or Plate Carrée projection.

Q: How does the "best" projection work?

A: The "best" projection uses either a polar-aspect orthographic projection or the rectangular projection depending on what is being mapped. If all points on the map, including points along any paths, are on the same side of the equator (or on the equator), and any points are in the Arctic or Antarctic polar region, then the orthographic projection centered on the appropriate pole is used. Otherwise, the rectangular projection is used.

Q: What is a graticule?

A: A graticule is a grid of lines on a map representing parallels and meridians (lines of latitude and longitude, respectively).

Q: What is a Tissot's Indicatrix?

A: A Tissot's Indicatrix is an ellipse projected on a map from a circle of small radius as measured in degrees, used to visualize a projection's distortion at a single point. In common use, Tissot's indicatrices are placed at the intersections on a graticule.

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