Transform coordinate systems

A new data frame has no coordinate system. When you add your first layer to it, the data frame takes on that layer's coordinate system. For any subsequent layers that you add, one of two things happens. If the new layer's coordinate system already matches the data frame, the layer is added without question. If the new layer's coordinate system is different, you get a GCS warning and ArcMap changes the coordinate system of the new layer to match the existing one. This operation is called a datum transformation (or coordinate system transformation). For all new layers that you add, ArcMap does these transformations automatically so that layers with different GCSs can be displayed together. (The transformations are done "on the fly," meaning they are applied only inside the data frame; the GCSs of the data sets on disk are not changed.)

What is the exception to this rule?

If the first layer you add to a data frame has latitude-longitude coordinates but no coordinate system information, the data frame becomes GCS_Assumed_Geographic_1 (based on the NAD27 datum). If you then add a second layer with a coordinate system other than NAD27, you'll get a GCS warning message, but no datum transformation will be applied.

It's great that ArcMap does datum transformations for you, but complications arise because there may be multiple versions of a transformation. For example, ArcMap has seven formulas that convert from NAD27 to NAD83. Each works best for a particular region.

Available transformations

Optimal transformation	Location of data
NAD_1927_To_NAD_1983_6	Canada - Quebec
NAD_1927_To_NAD_1983_Alaska	Alaska
NAD_1927_To_NAD_1983_CSRS98_1	Canada - Quebec
NAD_1927_To_NAD_1983_CSRS98_2	Canada - Saskatchewan
NAD_1927_To_NAD_1983_NADCON	Conterminous US (not Alaska or Hawaii)
NAD_1927_To_NAD_1983_NTv2_Canada	Canada
NAD_1927_To_NAD_1983_PR_VI	Puerto Rico - Virgin Islands

Similarly, there are 21 different transformations from NAD27 to WGS84.

The different transformations from NAD27 to WGS84

Optimal transformation	Location of data
NAD_1927_To_WGS_1984_1	Mean for Antigua, Barbados, Barbuda, Caicos Islands, Cuba, Dominican Republic, Grand Cayman, Jamaica, and Turks Islands
NAD_1927_To_WGS_1984_2	Mean for Belize, Costa Rica, El Salvador, Guatemala, Honduras, and Nicaragua
NAD_1927_To_WGS_1984_3	Mean for Canada
NAD_1927_To_WGS_1984_4	Mean for United States (CONUS)
NAD_1927_To_WGS_1984_5	Mean for United States (CONUS East of Mississippi River and MN, MO, and LA)
NAD_1927_To_WGS_1984_6	Mean for United States (CONUS West of Mississippi River)
NAD_1927_To_WGS_1984_7	United States (Alaska)
NAD_1927_To_WGS_1984_8	Bahamas (except San Salvador Island)
NAD_1927_To_WGS_1984_9	Bahamas (San Salvador Island)
NAD_1927_To_WGS_1984_10	Canada (Alberta, British Columbia)
NAD_1927_To_WGS_1984_11	Canada (Manitoba, Ontario)
NAD_1927_To_WGS_1984_12	Canada (New Brunswick, Newfoundland, Nova Scotia, and Quebec)
NAD_1927_To_WGS_1984_13	Canada (Northwest Territories, Saskatchewan)
NAD_1927_To_WGS_1984_14	Canada (Yukon)
NAD_1927_To_WGS_1984_15	Panama (Canal Zone)
NAD_1927_To_WGS_1984_16	Cuba
NAD_1927_To_WGS_1984_17	Greenland (Hayes Peninsula)
NAD_1927_To_WGS_1984_18	Mexico
NAD_1927_To_WGS_1984_21	United States (Alaska - Aleutians East of 180E)
NAD_1927_To_WGS_1984_22	United States (Alaska - Aleutians West of 180E)
NAD_1927_To_WGS_1984_30	Cuba

For each datum transformation, ArcMap has to pick a default formula. For the NAD27 to NAD83 transformation, it uses NAD_1927_To_NAD_1983_NADCON, which is optimized for the continental United States. If your data happens to lie in Alaska or Canada, your features may still not line up properly until you override the default transformation and pick the optimal one.

The default NAD27 to WGS84 transformation is NAD_1927_To_WGS_1984_1, which is the optimal transformation only for Caribbean data.

In this exercise, you will see how ArcMap applies its default datum transformations and you will learn how to change these settings.