An overview of the Projections and Transformations toolset
Last modified November 13, 2008
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When you obtain geographic information system (GIS) data, it often needs to be transformed or projected. Since the data you receive is not always preprocessed, you will often need to place coordinates to your raster image. 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, you must 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 that they come straight from the sensor or scanner—will usually not have any of these coordinates or locations inherent. The transformation tools are responsible for warping the image to the proper location and changing 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 very 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 could maintain the area of a feature but alter its shape.
The following table lists the tools available in the Projections and Transformations toolset and provides a brief description of each. The inputs of the Define Projection and Project tools are feature classes. The inputs of all the other tools listed are raster datasets.
NOTE: Tool availability is determined by the ArcGIS license. Licensing requirements for each tool are listed below.
||Create Custom Geographic Transformation||Creates a transformation method for converting data between two geographic coordinate systems or datums. Available with any ArcGIS license.|
||Define Projection||Records the coordinate system information of the feature class including any associated projection parameter, datum, and spheroid. Available with any ArcGIS license.|
|Feature||Batch Project||Changes the coordinate system of your input feature classes or feature datasets, including the datum or spheroid, and defines the location of points on a planar surface. Available with any ArcGIS license.|
|Feature||Create Spatial Reference||Creates a spatial reference object. For use in ModelBuilder and scripting only. Available with any ArcGIS license.|
|Feature||Project||Changes the coordinate system of your feature class, including its datum or spheroid, and defines the location of points on a planar surface. Available with any ArcGIS license.|
|Raster||Flip||Reorients the raster by flipping it from top to bottom along the horizontal axis, through the center of the raster. Available with any ArcGIS license.|
|Raster||Mirror||Reorients the raster by flipping it from left to right along the vertical axis, through the center of the raster. Available with any ArcGIS license.|
|Raster||Project Raster||Transforms the raster dataset from one spatial reference to another. Available with any ArcGIS license.|
|Raster||Rescale||Resizes the raster by the specified x and y scale factors. Available with any ArcGIS license.|
|Raster||Rotate||Turns the raster around the specified pivot point by an angle specified, in degrees. Available with any ArcGIS license.|
|Raster||Shift||Moves (slides) the raster to a new geographic location, based on x and y shift values. Available with any ArcGIS license.|
|Raster||Warp||Performs a transformation on the raster based on the input control points using a polynomial transformation. Available with any ArcGIS license.|