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Spatial
Referencing - Data Projections
Concepts
data projection
reprojecting data
geographic transformation
Data projection
The data
projection is the coordinate system in which a data layer is
stored. This can be different from the map projection, which is the
projection in which an actual map is made (more on that in the next
section). However, in most cases they are the same. In
Maine, this is usually UTM Zone 19N, NAD83/GRS80. You can
determine the data's projection (coordinate system) in ArcMap by
looking at the 'Source' tab of the layer properties, or by looking at
the metadata in ArcCatalog.
Reprojecting data
If you have data layers which use different projection systems, ArcMap
will 'know' this and automatically reproject one to match the
other. However, in large or complex data layers this can take a
long time. Also in some cases you may want the coordinates of
features to be in the same coordinate system as other layers.
There are simple tools available in ArcMap to convert data from one
coordinate system to another. The steps to do this are:
1.) Activate ArcToolbox 
2.) If ArcMap does not already 'know' the coordinate system of
the data, then set the projection under Data Management Tools -
Projections and Transformations - Define Projection.
3.) Reproject the data using the 'Project' tool under Data
Management Tools - Projections and Transformations - Feature - Project.
"Define Projection" tool is used to assign a projection to data whose
projection is unknown.
"Project" tool is used to reproject data.
You can find the UTM, Zone 19N NAD83 coordinate system by clicking on
the button  , then clicking  , and then looking under:
Geographic Transformation
If you are reprojecting data between datums, you will need to also
specify a method for transformation.
The transformation just tells ArcMap how to recompute the coordinates
for the features. With NAD27-to-NAD83 conversions, this is
NAD_1927_To_NAD_1983_NADCON. With WGS84-to-NAD83 conversions,
this is NAD_1983_To_WGS_1984_1.
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