The following is a glossary of terms used in Map-a-Planet, the About MapMaker page, and the related help pages. 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. 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 , 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. 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 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. 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 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. A curve formed by the intersection of a fictitious cutting plane parallel to the equatorial plane. A latitude (east-west) line. 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 The number of picture elements (pixels) used to represent one degree of latitude or longitude on a planet (pixels/degree). 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 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 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.’) . 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. 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. Source.