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The instructions provided describe how to work with raster data coordinate systems and georeferencing information in ArcGIS Desktop. ArcGIS Desktop handles coordinate systems and georeferencing information differently than ArcInfo Workstation and ArcView GIS. Understanding the following concepts improves the ability to use raster data in ArcGIS Desktop as well as ArcInfo Workstation and ArcView GIS.
The georeferencing information of some raster formats can be stored internally in a world file or in an aux/aux.xml file. While ArcGIS Desktop is able to recognize the georeferencing information in all of these options, older applications may not have the same capacity.
By default, the coordinate system of the ArcMap data frame is set to the projection of the first data added to the map.
ArcMap projects any data with a spatial reference that differs from its data frame's coordinate system 'on the fly'. The 'on the fly' projection of the data is for display and query purposes, and does not cause the spatial reference of the data to be modified. Instructions on how to modify the data frame's coordinate system are provided in the following link.
Note: Refer to Knowledge Base article 21327, linked in the Related Information section below, for a list of Geographic Transformations and their areas of use.5. Click OK, and click OK again.
ArcMap does not project data on the fly if the coordinate system for the data has not been defined or if the projection of the ArcMap data frame has not been set.
A data set with an undefined coordinate system is displayed in the map based on the coordinate extent of the data. This results in the data's misalignment, unless the data frame is set to the undefined dataset's native spatial reference. Data with undefined spatial reference should always have its projection definition set to avoid confusion. For more information, refer to Identifying an unknown coordinate system..
Storing Coordinate System Information
The coordinate system information for raster data comes from a variety of sources. Certain raster formats, such as TIFF, MrSID, and GRID, can store spatial reference information internally in their header. ArcGIS Desktop uses an AUX.XML file for images that do not support storing coordinate system information internally. The image header and AUX file inform ArcMap about the coordinate system, including units and projection.
World files and HDR files store the georeferencing information of the image, but they do not store any information about the coordinate system. For example, they do not store the map units used by the georeferencing information.
Note: For further information regarding the format of world files see the Related Information section below.
Priority of Coordinate System Sources
ArcGIS Desktop prioritizes coordinate system and georeferencing sources. If a source does not contain any coordinate system or georeferencing information or if the file does not exist, the next source is used. The priority is:
1) Image Header
2) AUX file (has the same name as the raster, with an .aux extension)
3) World file (for a TIFF: .tfw, for a JPG: .jgw, etc.)
Note: This is a change from ArcInfo Workstation and ArcView GIS. The order of priority for these applications is: 1) World file 2) Header
Due to this priority difference, ArcGIS Desktop may not place an image in the same location as ArcView GIS or ArcInfo Workstation. This causes a problem for some rasters. For example, the raster may overlay other data perfectly in ArcView GIS or ArcInfo Workstation, but in ArcGIS Desktop the image does not line up with other data.
Depending on the situation, there are workarounds available to remedy misaligned data problems.
• Create a New Image
1. Open the image in an image editing software package.
2. Save the image as a new file.
3. Make a copy of the world file or .hdr, whichever is used, and rename it to match the new image.
Note: The new image is stripped of any internal georeferencing or coordinate system information; therefore, ArcGIS Desktop now uses the world file to place the image.