FAQ: Map Options and Features

Q:Where did the Blue Marble maps come from?

A:TheBlue Marbleimages were created byNASAusing data from NASA'sModerate Resolution Imaging Spectroradiometer(MODIS), an instrument aboard theTerra satellite.Please see the Great Circle Mappercredits for more information.

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

A:NASA released theoriginal Blue Marbleimagery in 2002, using data from the MODIS instrument(seeprevious question).Two years later, NASA releasedBlue Marble: Next Generation,again using data from MODIS but with improved image processingtechniques and observations spanning an entire year. The newer imageryalso featured higher resolution, with a single pixel representing asquare 15 arc-seconds on a side versus 1 arc-minute (60 arc-seconds) forthe 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 pathSFO-DFW,SFO-IAHwith the default path color (red). Changing the color for justSFO-IAH to navy blue can be done using the pathSFO-DFW,color:navy,SFO-IAH.You can shortencolor: to justc:and can change colors as many times as you like.

Valid colors include those offered for any of the color optionsin the Map Controls. In addition, colors can be specified usinghexadecimal colors using a syntax like that used by CSS. Forexample,c:#ff0000 is the same asc:red.

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

A:The Great Circle Mapper software has no architectural limit and is onlylimited by memory on the server (and patience). On the development servera 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 amaximum URL length of 2,083 characterswhich is probably the most restrictive limit.Most other browersare less restrictive, in which case the limit becomes the length of arequest line accepted by theApache HTTP Serversoftware. Thislimit is less than 8,190 bytes.

Work is underway to allow map definitions to be saved on theGreat Circle Mapper web site which will allow more complex mapswithout exceeding HTTP's limits on URL lengths. (There is not yeta 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 ontoa plane. All map projections distort the result in one way or another anda 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 treatedas simple rectangular coordinates of pixels in the map image. Although itmay 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 theperspective of a point infinitely far away. Photographs of Earth from themoon are examples of this projection. Using this projection, any geodesicpath to/from the center of the projection is a straight line. These mapsmay 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 projectionbut displays the entire body, somewhat like splaying out the entire peel of anorange. The point opposite the map's center point is not a point but theentire rim of the map. Like the orthographic projection, any geodesicpath to/from the center of the projection is a straight line, but for theazimuthal equidistant projection the distance along that line is proportionalto the distance along the path.

Best: Rectangular or Polar

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

For a more humorous look at map projections, seexkcd's commentary onWhat 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 thusmaps can be redendered more quickly. It also makes it far easier to rendermaps based on photographs or other imagery, such the the Blue Marble maps.For these reasons, the Great Circle Mapper favors this map projection, thoughothers are offered for cases where the rectangular projection is unsuitable.

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

Q:How does the "best" projection work?

A:The "best" projection uses either a polar-aspect orthographic projectionor the rectangular projection depending on what is being mapped. If all pointson the map, including points along any paths, are on the same side of theequator (or on the equator), and any points are in the Arctic or Antarcticpolar region, then the orthographic projection centered on the appropriatepole 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 ofsmall radius as measured in degrees, used to visualize a projection'sdistortion at a single point. In common use, Tissot's indicatrices areplaced at the intersections on a graticule.

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