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
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
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 |
|
NAD_1927_To_NAD_1983_Alaska |
|
NAD_1927_To_NAD_1983_CSRS98_1 |
|
NAD_1927_To_NAD_1983_CSRS98_2 |
|
NAD_1927_To_NAD_1983_NADCON |
Conterminous US (not |
NAD_1927_To_NAD_1983_NTv2_Canada |
|
NAD_1927_To_NAD_1983_PR_VI |
|
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 |
NAD_1927_To_WGS_1984_2 |
Mean for |
NAD_1927_To_WGS_1984_3 |
Mean for |
NAD_1927_To_WGS_1984_4 |
Mean for |
NAD_1927_To_WGS_1984_5 |
Mean for |
NAD_1927_To_WGS_1984_6 |
Mean for |
NAD_1927_To_WGS_1984_7 |
|
NAD_1927_To_WGS_1984_8 |
|
NAD_1927_To_WGS_1984_9 |
|
NAD_1927_To_WGS_1984_10 |
|
NAD_1927_To_WGS_1984_11 |
|
NAD_1927_To_WGS_1984_12 |
|
NAD_1927_To_WGS_1984_13 |
|
NAD_1927_To_WGS_1984_14 |
|
NAD_1927_To_WGS_1984_15 |
|
NAD_1927_To_WGS_1984_16 |
|
NAD_1927_To_WGS_1984_17 |
|
NAD_1927_To_WGS_1984_18 |
|
NAD_1927_To_WGS_1984_21 |
|
NAD_1927_To_WGS_1984_22 |
|
NAD_1927_To_WGS_1984_30 |
|
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
The
default NAD27 to WGS84 transformation is NAD_1927_To_WGS_1984_1, which is the
optimal transformation only for
In this
exercise, you will see how ArcMap applies its default datum transformations and
you will learn how to change these settings.