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The following is a glossary of terms used in Map-a-Planet, the About MapMaker page, and the related help pages.
B
- Band
- The color images used by Map-a-Planet are made of 3 grayscale image files which are combined in the red, blue, and green channels of the color image to produce the color map. The color of each pixel is determined by the relative proportion of the red, green, and blue values in the resulting color image. Each of the grayscale images is referred to as a band, and generally has attributes associated with it depending on the data type (such as the spectral bandwidth the pixel values represent for images collected by cameras, elevation ranges represented by the pixels for topography data, etc.) For example, in the Viking color data, the green band in the color images was synthetically generated from the violet and red spectral image observations of the planet.
C
- Contrast
- Contrast is the visible difference in brightness from the darkest color to the lightest.
D
- Data Set
- A Data Set refers to a single image product type, which may be data collected by a particular instrument on a particular mission, a product created by specialized processing of one of these mission data sets, or a product that combines two or more data sets to create a new data set. Map-a-Planet uses several Data Sets, such as the Mars Viking color imagery, the Mars MDIM, and the Mars merged Viking color with the MDIM. For more information about the data sets available on Map-a-Planet, see the Map-a-Planet Data Sets page.
- Digital Number (DN)
- Each pixel in an image contains a numerical value, often referred to as the digital number (DN). Low DNs typically show up as black in the image and high DN as white.
G
- Grayscale
- Grayscale refers to an image which is composed of shades between black and white (inclusive).
M
- Mercator Projection
-
Mercator projection works very poorly in polar regions and becomes
undefined at the north and south poles. Historically Mercator is
interesting because it is one of the oldest map projections to be used.
Christopher Columbus used the Mercator projection in his travels to the
new world. This projection is often used in navigation because any
straight line is a rhumb line (a line of constant direction). Parallels
of latitude and longitude are straight. Features increase in size as the
map approaches the poles. Areas and shapes of large areas are
distorted. Distortion increases away from the equator and is extreme in
polar regions. However, being a conformal projection, angles and
shapes within any small area are essentially true.
For more information about map projections, see the entry for Projection.
![[Mercator Projection Example]](images/merc.jpg)
Mercator Projection Example. Note this image does not extend all the way to the poles: it was trimmed to ±85° latitude due to the distortion near the poles.
- Meridian
- Lines of longitude representing north-south direction on a globe or map.
P
- Parallels
- A curve formed by the intersection of a fictitious cutting plane parallel to the equatorial plane. A latitude (east-west) line.
- Pixel
- An image is made up a two-dimensional array of "cells" called picture elements or pixels for short. Each pixel has its own value, called the pixel value (or digital number), which appears as a level of brightness or darkness when the image is displayed or printed.
- Projection
- The orderly transfer of positions on the surface of a planet to corresponding points on a flat surface, i.e. a map. Since the surface of a sphere cannot be laid flat on a plane without distortion, the process of transformation requires a degree of approximation and simplification. For more about the map projections supported by Map-a-Planet, see the glossary entries for the Sinusoidal, Simple Cylindrical, and Mercator projections.
R
- Resolution
- The number of picture elements (pixels) used to represent one degree of latitude or longitude on a planet (pixels/degree).
- Rhumb line
- A line of constant compass direction or bearing.
S
- Scale
- In Map-a-Planet (as well as PDS and Isis image labels) Scale is a particular way of stating the resolution of an image in terms of the dimensions of a single pixel as a measured distance on the surface of the planet or satellite represented in the image. In Map-a-Planet, the units for Scale are kilometers per pixel. Thus, if the scale is 10 kilometers per pixel, each pixel represents an area on the surface 10 kilometers high by 10 kilometers wide. (It is important to note Scale here does not refer to the traditional meaning "the ratio of the measurement on a map versus the measurement on the actual planet it represents.")
- Simple Cylindrical Projection
-
This projection works well for maps of the mid-latitude regions of the
planet, distortion of the shapes of features increases as the map
approaches the poles. Features on a planet, such as round impact
craters, become flattened at the higher latitudes. As the name implies,
this projection represents perhaps the simplest of map projections. In
this projection, lines of latitude and longitude are straight lines where
the X-coordinate (left-to-right direction) on the map is proportional to
the map longitude and the Y-coordinate (top-to-bottom direction) is
proportional to the latitude.
For more information about map projections, see the entry for Projection.
![[Simple Cylindrical Projection Example]](images/simp.jpg)
Simple Cylindrical Projection Example
- Sinusoidal Equal-Area Projection
-
In this projection, parallels of latitude are straight lines, with
constant distances between equal latitude intervals. Lines of constant
longitude on either side of the projection meridian are curved since
longitude intervals decrease with the cosine of latitude to account for
their convergence toward the poles. The Sinusoidal Equal-area projection
is characterized by a projection longitude which is the center meridian of
the projection. Areas remain equal size in this projection but feature
shapes are distorted the further one goes from the central meridian and at
the poles. In order to minimize map distortion, Map-a-Planet always uses
a center meridian at the center of the image map unless specified
otherwise in the advanced options.
For more information about map projections, see the entry for Projection.
![[Sinusoidal Equal-Area Projection Example]](images/sinu.jpg)
Sinusoidal Equal-Area Projection Example
- Spherical
- Having the shape of a round ball.
Z
- Zoom
- When using Map-a-Planet, Zoom refers to changing the relative resolution or scale of the image. Zoom In increases the resolution of the image - features will appear larger and closer after zooming-in. Conversely, Zoom Out decreases the resolution of the image, and features will appear smaller and farther away after zooming-out.