Cylindric projection equations yield projected graticules with straight meridians and parallels that intersect at right angles.The following illustrations show some of the projected graticules produced by projection equations in each category. As you might imagine, the appearance of the projected grid will change quite a lot depending on the type of surface it is projected onto, how that surface is aligned with the globe, and where that imagined light is held. Moreover, the cylinder is frequently positioned tangent to the equator (unless it is rotated 90°, as it is in the Transverse Mercator projection). The cone is typically aligned with the globe such that its line of contact (tangency) coincides with a parallel in the mid-latitudes. All three are shown in their normal aspects. There are three main categories of map projection, those in which projection is directly onto a flat plane, those onto a cone sitting on the sphere that can be unwrapped, and other onto a cylinder around the sphere that can be unrolled (Figure 2.15 above). That is the amount of distortion we have in the simple projection below (one of the more common in web maps of the world today).Ĭredit: © Penn State University, is licensed under CC BY-NC-SA 4.0 Imagine the kinds of distortion that would be needed if you sliced open a soccer ball and tried to force it to be completely flat and rectangular with no overlapping sections. Even this simplest projection produces various kinds of distortions thus, it is necessary to have multiple types of projections to avoid specific types of distortions. Projections that are more complex yield grids in which the lengths, shapes, and spacing of the grid lines vary. The simplest kind of projection, illustrated below, transforms the graticule into a rectangular grid in which all grid lines are straight, intersect at right angles, and are equally spaced. Inverse projection formulae transform plane coordinates to geographic. The mathematical equations used to project latitude and longitude coordinates to plane coordinates are called map projections. These georeferenced plane coordinates are referred to as projected. The true geographic coordinates called unprojected coordinate in contrast to plane coordinates, like the Universal Transverse Mercator (UTM) and State Plane Coordinates (SPC) systems, that denote positions in flattened grids. Latitude and longitude coordinates specify positions in a spherical grid called the graticule (that approximates the more-or-less spherical Earth).
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