science

North American Datum

North American Datum is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand North American Datum rather than just read about it. In short: The North American Datum (NAD) is the horizontal datum used to define latitude and longitude (horizontal coordinates) in North America. A datum is a formal description of the shape of the Earth along with survey markers serving as "anchor" points for the coordinate system.

Key takeaways

North American Datum belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect North American Datum to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of North American Datum from memory before moving on to harder problems.

Reference excerpt

The North American Datum (NAD) is the horizontal datum used to define latitude and longitude (horizontal coordinates) in North America. A datum is a formal description of the shape of the Earth along with survey markers serving as "anchor" points for the coordinate system. In surveying, cartography, and land-use planning, two North American Datums are in use: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). Both are geodetic reference systems based on slightly different assumptions and measurements. Vertical measurements, based on distances above or below Mean High Water (MHW), are calculated using the North American Vertical Datum of 1988 (NAVD 88). NAD 83, along with NAVD 88, is scheduled to be replaced in 2026 with the Datum of 2022, a new geometric reference frame and geopotential datum based on GPS and gravimetric geoid model.

First North American Datum of 1901 In 1901 the United States Coast and Geodetic Survey adopted a national horizontal datum called the United States Standard Datum, based on the Clarke Ellipsoid of 1866. It was fitted to data previously collected for regional datums, which by that time had begun to overlap. In 1913, Canada and Mexico adopted that datum, so it was also renamed the North American Datum.

North American Datum of 1927 As more data were gathered, discrepancies appeared, so the datum was recomputed in 1927, using the same spheroid and origin as its predecessor. The North American Datum of 1927 (NAD 27) was based on surveys of the entire continent from a common reference point that was chosen in 1901, because it was as near the center of the contiguous United States as could be calculated: It was based on a triangulation station at the junction of the transcontinental triangulation arc of 1899 on the 39th parallel north and the triangulation arc along the 98th meridian west that was near the geographic center of the contiguous United States. The datum declares the Meades Ranch Triangulation Station in Osborne County, Kansas to be 39°13′26.686″ north latitude, 98°32′30.506″ west longitude. NAD 27 is oriented by declaring the azimuth from Meades Ranch to Waldo Station (also in Osborne County, about 4.5 mi (7.2 km) northwest of Waldo, Russell County) to be 255°28′14.52″ from north. The latitude and longitude of every other point in North America is then based on its distance and direction from Meades Ranch: If a point was X meters in azimuth Y degrees from Meades Ranch, measured on the Clarke Ellipsoid of 1866, then its latitude and longitude on that ellipsoid were defined and could be calculated.

These are the defining dimensions for NAD 27, but Clarke actually defined his 1866 spheroid as a = 20,926,062 British feet, b = 20,855,121 British feet. The conversion to meters uses Clarke's 1865 inch-meter ratio of 39.370432. The length of a foot or meter at the time could not practically be benchmarked to better than about 0.02 mm. Most USGS topographic maps were published in NAD 27 and many major projects by the United States Army Corps of Engineers and other agencies were defined in NAD 27, so the datum remains important, despite more refined datums being available.

North American Datum of 1983 Because Earth deviates significantly from a perfect ellipsoid, the ellipsoid that best approximates its shape varies region by region across the world. Clarke 1866, and North American Datum of 1927 with it, were surveyed to best suit North America as a whole. Likewise, historically, most regions of the world used ellipsoids measured locally to best suit the vagaries of Earth's shape in their respective locales. While ensuring the most accuracy locally, this practice makes integrating and disseminating information across regions troublesome. As satellite geodesy and remote sensing technology reached high precision and were made available for civilian applications, it became feasible to acquire information referred to a single global ellipsoid. This is because satellites naturally deal with Earth as a monolithic body. Therefore, the GRS 80 ellipsoid was developed for best approximating the Earth as a whole, and it became the foundation for the North American Datum of 1983. Though GRS 80 and its close relative, WGS 84, are generally not the best fit for any given region, a need for the closest fit largely evaporates when a global survey is combined with computers, databases, and software able to compensate for local conditions.

Comparing NAD 27 to NAD 83

A point having a given latitude and longitude in NAD 27 may be displaced on the order of many tens of meters from another point having the identical latitude and longitude in NAD 83, so it is important to specify the datum along with the coordinates. The North American Datum of 1927 is defined by the latitude and longitude of an initial point (Meades Ranch Triangulation Station in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The North American Datum of 1983 is based on a newer defined spheroid (GRS 80); it is an Earth-centered (or "geocentric") datum having no initial point or initial direction. NOAA provides a converter between the two systems. Some examples of discrepancies between the two datums are that if you use a modern GPS device set to work in NAD 83 or WGS 84 to navigate to NAD 27 coordinates (as from a topo map) near Seattle, you would be off by about 95 meters (not far enough west). You would be about 47 meters off near Miami (not far enough north-northeast), whereas you would be much closer for points near Chicago.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with North American Datum

Start with the simplest possible case. Write down what North American Datum claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to North American Datum before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about North American Datum ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of North American Datum

In research: North American Datum appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses North American Datum in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: North American Datum is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geodetic datums, Surveying of Canada, Surveying of the United States, so understanding it makes those chapters shorter.
In everyday life: Look for North American Datum outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study North American Datum in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what North American Datum means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain North American Datum out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is North American Datum in simple terms?

Why does North American Datum matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study North American Datum?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on North American Datum.