The importance of datum transformations
As the name implies, datum transformations transform your data from one datum to another.
Depending on where you are in North America, NAD27 and NAD83 may differ in tens of meters for horizontal accuracy. The average correction between NAD27 and NAD83 is an average of 0.349″ northward and 1.822″ eastward.
In this case, you would need to perform a datum transformation.
The history of datums
The first thing you should be careful of is that the physical locations have not changed. Most survey monuments have not moved.
Datum shifts happen because survey measurements improve, there are more of them, and methods of geodesy change.
This results in more accurate geodetic datums over time. The horizontal datums that form the basis of coordinates of all horizontal positions in North America improve.
Because we created maps using different geodetic datums throughout history, datum transformations are often necessary when using historical data. For example, USGS topographic maps generally were published using a NAD27 datum. You would have to perform a datum transformation if you’re using data in NAD83.
When do you need a datum transformation?
We reference all coordinates on Earth to a datum. A datum describes the shape of the Earth in mathematical terms. A datum defines the following parameters for an ellipsoid:
- Inverse flattening
- Semi-major axis
- Semi-minor axis
The North American datum of 1983 (NAD83) is the current horizontal or geometric datum for the United States. It provides latitude and longitude and some height information.
Unfortunately, NAD83 is not the only datum you’ll encounter. Before the current datum was defined, many maps were created using different starting points. And even today, people continue to change geodetic datums to make them more accurate.
A common problem is when different coordinate locations are stored in different reference systems. When combining data from different users or eras, it is important to transform all information into common geodetic datums.
Projected coordinate systems are based on geographic coordinates, which are in turn referenced to a datum. For example, the State Plane Coordinate System can be referenced to either NAD83 and NAD27 geodetic datums.
The NAD27 datum was based on the Clarke Ellipsoid of 1866:
- Semi-major axis: 6,378,206.4 m
- Semi-minor axis: 6,356,583.8 m
- Inverse flattening: 294.98
The NAD83 datum was based on the Geodetic Reference System (GRS80) Ellipsoid:
- Semi-major axis: 6,378,137.0 m
- Semi-minor axis: 6,356,752.3 m
- Inverse flattening: 298.26
What is a datum transformation?
For any type of work where coordinates need to be consistent with each other, you must use the same geodetic datum. If you are marking property boundaries, building roads, or planning for coastal inundation scenarios, you must know about and use the correct geodetic datums.
The challenge is to get points in different datums to align. After you get two sets of coordinated in a common datum, there are three steps to get them in alignment:
- Origin Alignment: Position the coordinates in close proximity
- Axis Rotation: Rotate the axis until coordinates align
- Scale Factor: Scale axes until coordinates match
Try for yourself
Remember next time in ArcMap when you use the “Project” tool…
When you transform NAD83 and NAD27 geographic coordinates to projected State Plane coordinates, it is the same projection method.
However, because the geodetic datums were different, the resulting projected coordinates will also be different.
In this case, a datum transformation is necessary. Be mindful of geographic transformations and how to choose the right one.