Understanding projections and datums

A projection represents the irregular three-dimensional surface of the Earth as a flat surface. A map projection is used to transform the locations of features on the Earth's surface to locations on a two-dimensional plane. A variety of map projections exist, based typically on one of the following three basic types:

For example, the Transverse Mercator projection is a variation of the cylindrical projection.

A datum is a mathematical surface used to make geographic computations. An ellipsoid approximates the size and shape of all or part of the Earth. The datum includes parameters to define the size and shape of the ellipsoid used, and its position relative to the center of the Earth. Geographic coordinate systems use different datums to calculate positions on the Earth.

If you compare the same point using two different datums or projections, the coordinates of the point will be different. Referencing a project's coordinates to the wrong datum or using the wrong projection may result in features being offset by significant distances.

Different projections and datums introduce different distortions or warping into the image. You must select the projection and datum that will provide the results you expect for your project. If you are using data from multiple projections, OrthoEngine can only reproject the coordinates correctly when the projection and datum are set properly.

For more information on projections and datums, refer to the following publications:

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