Working
with coordinate systems in ArcGIS
All
geographic datasets have a geographic coordinate system (
If all the
data you want to display on a map is stored in the same geographic coordinate
system, you can just add it to the map—the layers will overlay properly. If
some of the datasets also have projected coordinate systems, even if they are
different, you can also just add them to the map without data alignment
worries—ArcMap will automatically make the layers overlay using a process
called "on-the fly projection." The geographic coordinate system is
the common language. ArcMap can convert the geographic coordinate system to any
projected coordinate system and it can convert any projected coordinate system
back to the geographic coordinate system.
An issue
arises when you want to display datasets that have different geographic
coordinate systems on the same map. The first layer you add to an empty data
frame determines the coordinate system for the data frame. If that layer has a
projected coordinate system, the data frame will have that same projected
coordinate system. If you add a layer that has the same geographic coordinate
system but a different projected coordinate system (or no projected coordinate
system at all), ArcMap will perform an on-the-fly projection and convert the
data to the data frame's projected coordinate system.
The layers will overlay properly.
If,
however, you try to add a layer that has a different geographic coordinate
system, ArcMap will display a warning message telling you that it may not be
able to properly align the data. ArcMap can still project the data on the fly,
but it can no longer guarantee perfect alignment. (For
perfect data alignment, you need to apply a transformation to make the
geographic coordinate systems match—transformations are beyond the scope of
this course.)
How do you
know what coordinate system your data is stored in? You can view the coordinate
system information for a dataset in ArcCatalog™, in its metadata. If a dataset
has no coordinate system information in its metadata (it's missing), you may
not be able to display the data in ArcMap. You may need to do some research to
find out the coordinate system, then define the
coordinate system using the ArcGIS tools provided. You will do this in the
exercise coming up.
What happens when coordinate system
information is missing?
When you add a dataset to ArcMap that is missing
coordinate system information, ArcMap will try to read the coordinates of the
data and determine whether they have been projected. If the coordinates are in
the range of longitude-latitude values (x = ±180, y = ±90), ArcMap will add the
data to the map and project it on the fly, although there may be inaccuracies
because ArcMap cannot determine the geographic coordinate system for the data.
If the coordinates are not in the range of
longitude-latitude values, ArcMap will display a warning. It will still add the
data to the map, but it cannot project it on the fly. The result is usually
that the data doesn't "fit" in the same coordinate space as the rest
of the data, and either doesn't display or has serious
alignment problems. In this case, you'll have to enter the necessary coordinate
system information yourself in order to display the data properly on a map.