Horizontal Reference Datum
When you need to accurately enter latitude and longitude coordinates in a GIS, the first step is to give it a datum. A geodetic datum uniquely defines all locations on Earth with coordinates.
Because where would you be on Earth without having reference to it?
Because the Earth is curved and in GIS we deal with flat map projections, we need to accommodate both the curved and flat views of the world. In surveying and geodesy, we accurately define these properties with geodetic datums.
We begin modelling the Earth with a sphere or ellipsoid. Over time, surveyors have gathered a massive collection of surface measurements to more reliably estimate the ellipsoid.
When you combine these measurements, we arrive at a geodetic datum. Datums precisely specify each location on Earth’s surface in latitude and longitude. For example, NAD27, NAD83 and WGS84 are geodetic datums.
A Mammoth Collection of Survey Benchmarks
In order to create a geodetic datum, surveyors undertook a mammoth collection of monument locations in the late 1800s. Surveyors installed brass or aluminum disks at each reference location.
Each monument location was connected using mathematical techniques like triangulation.
From the unified network of survey monument, the result of triangulation was the North American Datum of 1927 (NAD 27). After, geodesists developed the more accurate NAD 83, which we still use today. NAD 27 and NAD 83 provide a frame of reference for latitude and longitude locations on Earth.
Surveyors now rely almost exclusively on the Global Positioning System (GPS) to identify locations on the Earth and incorporate them into existing geodetic datums.
For example NAD27, NAD83 and WGS84 are the most common geodetic datums in North America.
What is North American Datum 1927 (NAD27)?
NAD27 stands for North American Datum of 1927. NAD27 is the adjustment of long-baseline surveys. Overall, it established a network of standardized horizontal positions on North America. Most historical USGS topographic maps and projects by the US Army Corps of Engineers used NAD27 as a reference system.
A horizontal datum provides a frame of reference as a basis for placing specific locations at specific points on the spheroid. Geodesists use a horizontal datum as the model to translate a spheroid / ellipsoid into locations on Earth with latitude and longitude lines. Geodetic datums form the basis of coordinates of all horizontal positions on Earth. All coordinates on Earth are referenced to a horizontal datum. The North American Datum of 1927 (NAD27) is one of the main three geodetic datums used in North America.
NAD27 uses all horizontal geodetic surveys collected at this time using a least-square adjustment. This datum uses the Clarke Ellipsoid of 1866 with a fixed latitude and longitude at Meade’s Ranch, Kansas. (39°13’26.686″ north latitude, 98°32’30.506″ west longitude)
Kansas was selected as a common reference point because it was near the center of the contiguous United States. The latitudes and longitudes of every other point in North America were based off its direction, angle and distance away from Meade’s Ranch. Any point with a latitude and longitude away from this reference point could be measured on the Clarke Ellipsoid of 1866.
Surveyors gathered approximately 26,000 stations in the United States and Canada. At each station, surveyors collected latitudes and longitude coordinates. NOAA’s National Geodetic Survey used these survey stations and triangulation to form the NAD27 datum.
As time went on, the number of stations also grew. For example, surveyors benchmarked approximately 250,000 stations. This set of horizontal positions formed the basis for the North American Datum of 1983 (NAD83). In 1983, the NAD27 datum was eventually replaced with NAD83.
What is North American Datum 1983 (NAD83)?
The North American Datum of 1983 (NAD 83) is the most current datum being used in North America. It provides latitude and longitude and some height information using the reference ellipsoid GRS80. Geodetic datums like the North American Datum 1983 (NAD83) form the basis of coordinates of all horizontal positions for Canada and the United States.
The North American Datum of 1983 (NAD 83) is a unified horizontal or geometric datum and successor to NAD27 providing a spatial reference for Canada and the United States.
NAD83 corrects some of the distortions from NAD27 over distance by using a more sense set of positions from terrestrial and Doppler satellite data. NAD83 is a geocentric datum (referenced to the center of Earth’s mass) offset by about 2 meters. Even today, geodesists are continually improving horizontal geodetic datums.
WGS84: Unifying a Global Ellipsoid Model with GPS
It wasn’t until the mainstream use of Global Positioning Systems (GPS) until a unified global ellipsoid model was developed. The radio waves transmitted by GPS satellites enable extremely precise Earth measurements across continents and oceans. Global ellipsoid models have been created because of the enhancement of computing capabilities and GPS technology.
This has led to the development of global ellipsoid models such as WGS72, GRS80 and WGS84 (current). The World Geodetic System (WGS84) is the reference coordinate system used by the Global Positioning System.
Never before have we’ve been able to estimate the ellipsoid with such precision because of the global set of measurements provided by GPS. It comprises of a reference ellipsoid, a standard coordinate system, altitude data and a geoid. Similar to NAD 83, it uses the Earth’s center mass as the coordinate origin. The error is believed to be less than 2 centimeters to the center mass.
Geodetic Datums: NAD83 versus NAD27
NAD83 corrects some of the distortions from NAD27 over distance by using a more dense set of positions from terrestrial and Doppler satellite data. Approximately 250,000 stations were used to develop the NAD83 datum. This compares to only 26,000 used in the NAD27 datum.
One of the primary difference is that NAD83 uses an Earth-centered reference, rather than a fixed station in NAD27. All coordinates were referenced to Kansas Meade’s Ranch (39°13’26.686″ north latitude, 98°32’30.506″ west longitude) for NAD27 datum. The National Geodetic Survey relied heavily on the use of Doppler satellite to locate the Earth’s center of mass. However, NAD83 is not geocentric with an offset of about two meters.
North American Datum of 1983 is based on the reference ellipsoid GRS80 which is physically larger than NAD27’s Clarke ellipsoid. The GRS80 reference ellipsoid has a semi-major axis of 6,378,137.0 meters and a semi-minor axis of 6,356,752.3 meters. This compares to the Clarke ellipsoid with a semi-major axis of 6,378,206.4 m and semi-minor axis of 6,356,583.8 meters.
The Varying Historical Accuracy of the Ellipsoid
Is the Earth Round? Earth bulges out more at the equator than at the poles by about 70,000 feet.
And since the beginning of the 19th century, the dimensions of the ellipsoid have been calculated at least 20 different times with considerably different accuracy.
The early attempts at measuring the ellipsoid used small amounts of data and did not represent the true shape of the Earth. In 1880, the Clarke ellipsoid was adopted as a basis for its triangulation computations. The first geodetic datum adopted for the United States was based on the Clarke ellipsoid with its starting point in Kansas known as Meades Ranch.
One Datum with Many Versions and Abbreviations
Since 1986, geodesists have made several updates to NAD83. Actually, because of these changes, there are more than one version of NAD83. For example, the National Geodetic Survey has adjusted the NAD83 datum four times since the original geodetic datum estimation in 1986.
- NAD83 (1986): This version was intended to be geocentric and used the GRS80 ellipsoid.
- NAD83 (1991, HARN, HPGN): High Accuracy Reference Network (HARN) and High Precision Geodetic Network reworked geodetic datums from 1986-1997
- NAD83 (CORS96): Continually Operating Reference Stations (CORS) are composed of permanently operating Global Positioning System (GPS) receivers
- NAD83 (CSRS, CACS): Canadian Spatial Reference System and Canadian Active Control System with GPS processing.
- NAD83 (NSRS 2007, 2011): National Spatial Reference System and current survey standard using multi-year adjusted locations based on GNSS from the CORS.
The Importance of Datum Transformations
The coordinates for benchmark datum points are typically different between geodetic datums. For example, the latitude and longitude location in a NAD27 datum differs from that same benchmark in NAD83 or WGS84. This difference is known as a datum shift.
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.
It’s important to note that the physical location has not changed. To be clear, most monuments have not moved. Datum shifts happen because survey measurements improve. Also, it happens when 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 maps were created in different geodetic datums throughout history, datum transformations are often necessary. Especially, this is true when using historical data. For example, USGS topographic maps generally were published using a NAD27 datum. You would need to apply a datum transformation when working with NAD83 data.
When Do We Need Datum Transformations?
A coordinate transformation is the conversion from a non-projected coordinate system to a coordinate system. A coordinate transformation is done through a series of mathematical equations.
The geodetic datum is an integral part of projections. All coordinates are referenced to a datum. A datum describes the shape of the Earth in mathematical terms. A datum defines the radius, inverse flattening, semi-major axis and semi-minor axis for an ellipsoid. The North American datum of 1983 (NAD 83) is the United States horizontal or geometric datum. It provides latitude and longitude and some height information.
Unfortunately, NAD 83 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 in an effort 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 to common geodetic datums.
Projected coordinate systems are based on geographic coordinates, which are in turn referenced to a datum. For example, State Plane coordinate systems can be referenced to either NAD83 and NAD27 geodetic datums.
NAD27 Datum vs NAD83 Datum
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
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.
For any type of work where it’s important for coordinates to be consistent with each other, it is critical that you use the same geodetic datum. If you are marking property or land boundaries or building roads or planning for coastal inundation scenarios, you must know about and use the correct geodetic datums.
Image credit: NADCON – North American Datum Conversion Utility