Define vs. Project

When a data set either does not have a CRS/SRS OR the CRS/SRS is incorrect, you need to Define the CRS/SRS. Then, if the data is not in the CRS/SRS that you want it to be in, you need to Transform the data to another CRS/SRS.

This "Transformation" is called "Project" in ArcMap but it will both project, if needed, and perform a datum transform, if needed.

Note that there are three tools in ArcMap for perform these two functions. There is one to define the projection for any data and then separate tools to "Transform/project" the data for each data type, vector (in the "Feature" tools) and raster.

Note: The names and locations of "Project" and "Define" vary somewhat from one version of ArcGIS to another.

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