The Projections and Transformations toolset contains tools for converting geographic data between coordinate systems. There are additional tools for transforming raster datasets, such as shift, rescale, and rotate.
When you obtain GIS data, it often needs to be transformed or projected. Since the data you receive is not always preprocessed, you may need to place coordinates in raster images. The transformation tools in the Projections and Transformations toolset can be used to rectify these issues. Whether you treat the earth as a sphere or a spheroid, you must transform its three-dimensional surface to create a flat map sheet. This mathematical transformation is commonly referred to as a map projection.
To understand how transformations work, keep in mind that all places on the earth have a location, and spatial data corresponds to one of these locations. Imagery and raster data that are not preprocessed—meaning they come directly from the sensor or scanner—typically do not have any of these coordinates or locations inherent. You can use the transformation tools to move the image to the proper location and change the image to the proper orientation.
Altering spatial properties using map projections can be described as shining a light through the earth onto a surface called the projection surface. Imagine that the earth's surface is clear, with the graticule drawn on it. Wrap a piece of paper around the earth. A light at the center of the earth will cast the shadows of the graticule onto the piece of paper. You can now unwrap the paper and lay it flat. The shape of the graticule on the flat paper is different from what it was on the earth because the map projection has distorted the graticule.
A spheroid can't be flattened to a plane any more easily than a piece of orange peel can be flattened; it will rip. Representing the earth's surface in two dimensions causes distortion in the shape, area, distance, or direction of the data. A map projection uses mathematical formulas to relate spherical coordinates on the globe to flat, planar coordinates.
Different projections cause different types of distortions. Some projections are designed to minimize the distortion of one or two of the data's characteristics. A projection can maintain the area of a feature but alter its shape.
| Tool | Description |
|---|---|
Converts coordinate notations contained in one or two fields from one notation format to another. | |
Creates a transformation definition for converting data between two geographic coordinate systems or datums. The output of this tool can be used as a transformation for any tool with a parameter that requires a geographic transformation. | |
|
Overwrites the coordinate system information (map projection and datum) stored with a dataset. This tool is intended for datasets that have an unknown or incorrect coordinate system defined. | |
Projects spatial data from one coordinate system to another. |
An overview of the Raster toolset
The following table summarizes the tools in the Raster toolset (a toolset under the Projections and Transformations toolset). These tools are for projecting and transforming rasters.
| Tool | Description |
|---|---|
|
Transforms a raster dataset from one coordinate system to another. |