Q: How do I define or change map projection in ArcGIS?

Answer

Individual shapefiles have a projection, and the data frame has a projection. Both of these need to be defined for your data to display properly. Note that if there is no projection defined, you must first define it before you try to change it. To define a projection:

1. Open ArcToolbox and select Data Management Tools > Projections and Transformations > Define Projection.
2. Choose the appropriate coordinate system. You have to define it as what it is, not as what you want it to be. If you do not know what projection to choose, you may need to look at the layer's metadata or contact the publisher.
3. Once the projection is defined correctly, the projection can be changed. Choose the Project tool under either the Raster or Feature menu (depending on whether your data is raster or vector), still within the Projections and Transformations toolset.

To define or change the projection of the dataframe:

1. Right-click in the map and select Properties > Coordinate System.
2. Select the Coordinate System tab.
3. Choose a coordinate system from the tree or click the Import button and browse to a data source that is defined with the coordinate system you want to use.
4. Once you have chosen a coordinate system, its parameters are displayed in the Current coordinate system box. Click OK. All layers in the data frame will now be displayed with that coordinate system.

Further Resources:

• ESRI: Fundamentals of a shapefiles's coordinate system
• ESRI: Specifying a coordinate system (in the data frame)

See Also

• ArcGIS Landing Page

Created by MIT GIS Services. Email gishelp(at)mit.edu for more assistance.