In Geodatabases, the ‘Shape_Area’ field/attribute stores polygon area, and the ‘Shape_Length’ stores the length of the polygon perimeter. In this lecture we will cover how spatial reference information is stored in ArcGIS and how to manage this aspect of shapefiles, geodatabases and coverages. ArcToolbox – tools to make a new version of a dataset in a new spatial reference system, tools to change the spatial reference header information, data conversion tools Changing the spatial reference system header information in a geodatabase (‘Project’ tool) – THIS CHANGES THE COORDINATES IN THE SPATIAL DATASET Changing PROJECTION DEFINITION to another spatial reference system (Define Projection Tool)- THIS DOES NOT CHANGE THE COORDINATES 1) If you load a dataset into an empty ArcMap session, the Data Frame will be set to the spatial reference system of the dataset you loaded. One way to figure out the spatial reference of a dataset is to open a new ArcGIS session and load the dataset into the new data frame. 2) If you load a second dataset to your ArcMap session that has a different spatial reference system from the first, the second dataset will be 3) All datasets should have their spatial reference system defined. If you have a dataset with undefined spatial reference system information, you can use the ‘Define Projection’ tool (under Data Management Tools : Projections and Transformations, in ArcToolbox) 5) You can create a new version of a dataset in a different spatial reference system by using the projection transformation tools. See ‘Project’ under (under Data Management Tools : Projections and Transformations : Features , in ArcToolbox) for vector data projection transformations, or ‘Project Raster’ (under Data Management Tools : Projections and Transformations : Raster , in ArcToolbox) for raster datasets. 6) Changing the projection of a dataset can change the shape of a polygon, thereby changing the polygon area and length of perimeter. In Geodatabases, the ‘Shape_Area’ field/attribute stores polygon area, and the ‘Shape_Length’ stores the length of the polygon perimeter. 7) Note that sometimes a GIS dataset will have a field like ‘Square_km’ or ‘Size’. Recognize that fields like this do not necessarily have the correct area if the spatial reference system has been changed. Source.