Geographic datum transformations

A geographic datum transformation is a calculation that is used to convert between two geographic coordinate systems to ensure that data is properly aligned. When the geographic coordinate systems used by images differ from the coordinate system of the project, a transformation is required.

Geographic coordinate systems describe how locations on the earth are placed on a hypothetical reference spheroid. They use angular units, such as degrees, to assign locations to coordinates on a reference spheroid. There are many types of geographic coordinate systems because each is meant to fit certain portions of the earth. This is necessary because the earth is a lumpy, irregular oval rather than a perfect sphere. The transformation is a calculation to convert the geographic coordinate system of the layers to match the geographic coordinate system of the map as the map draws to achieve alignment. The data is not changed by a transformation. This real-time transformation is sometimes referred to as projecting on the fly.

The best geographic coordinate system to use for a project depends on where and how much of the earth's geography you are mapping. In ArcGIS Drone2Map, the best transformation for a map is suggested based on the data and the extent, but you can specify a different system.

In Drone2Map, both maps and their layers have coordinate systems, and they are not always the same. Each coordinate system can be either geographic or projected. Projected coordinate systems always include an underlying geographic coordinate system. A projection transforms the angular coordinates (such as latitude and longitude) from the reference spheroid to distance units (such as meters) on a flat surface. For example, the projection may describe how the spheroid coordinates will map to a flat rectangle hypothetically wrapped around the reference spheroid as a cylinder.

Transformations relate only to the underlying geographic coordinate systems. A transformation is applied only when the geographic coordinate systems are not identical. If layers have a different projected coordinate system than the map, but both the layers and the map reference the same underlying geographic coordinate system, a transformation is not needed.