Why are the NAD 83 position values so far from the NAD 27 values? Were the old coordinates wrong?

The old coordinates were not wrong, just different.

Positions obtained using the North American Datums of 1927 (NAD 27) and 1983 (NAD 83) are based on different earth shapes--or ellipsoids--and used the best technology available at the time. Mathematically, NAD 83 is a stronger datum because all previously existing horizontal stations and newer GPS surveyed stations were adjusted simultaneously. The positions within NAD 27 were adjusted in arcs, as the networks progressed across the country. Errors between stations adjusted in different arcs could have been substantial.

This issue is of declining importance and is seldom relevant to anything other than historical USGS maps (generally meaning maps published before 1990). All modern maps and GIS data are cast on NAD 83 or WGS 84, which are equivalent datums at map scales of 1:5,000 and smaller.

All federal agencies will replace NAD 83 and NAVD 88 (a vertical datum) with a new datum in 2022.

Learn more: Datum Shifts and Digital Map Coordinate Displays


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Why are USGS historical topographic maps referenced to outdated datums?

Many different horizontal reference datums exist, but in the United States only three datums are commonly used: The North American Datum of 1927 ( NAD27 ) uses a starting point at a base station in Meades Ranch, Kansas and the Clarke Ellipsoid to calculate the shape of the Earth. The North American Datum of 1983 ( NAD83 ) was developed when...

How large is the North American Datum (NAD) 27 to NAD 83 shift?

Within the conterminous 48 states, the North American Datum of 1927 (NAD 27) to the North American Datum of 1983 (NAD 83) shift of the latitude/longitude graticule (lines showing parallels of latitude and meridians of longitude for the earth) is in the range of 10-100 ground meters. Changes to UTM values are generally larger, around 200 meters,...

How are different map projections used?

The method used to portray a part of the spherical Earth on a flat surface, whether a paper map or a computer screen, is called a map projection. No flat map can rival a globe in truly representing the surface of the entire Earth, so every flat map misrepresents the surface of the Earth in some way. A flat map can show one or more--but never all--...

Why do all of the coordinates (latitude and longitude) in the Geographic Names Information System seem incorrect?

Coordinates that seem to be incorrect in the Geographic Names Information System might just be projected on a different datum from the datum used on your map or your positioning system (GPS). Most USGS maps published approximately 1940-1995 are projected on the North American Datum of 1927 (NAD27) . Later maps are projected on the North American...
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Finding Yourself Outdoors

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Great strides made toward a national hydrography framework with release of the initial USGS NHDPlus High Resolution datasets.

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November 18, 2010

PubTalk 11/2010 — Silicon, Software, and Science

Monitoring the Earth's Landscape with Low-Cost High-Tech

by Rian Bogle, Remote Sensing Specialist


  • The USGS is one of the world's largest providers of remote sensing data, employing the best tools and techniques to expand our knowledge of the Earth.
  • Working with low-cost field and aerial imaging technologies,
November 18, 2004

PubTalk 11/2004 — From Plane Tables to Pixels

The Revolution in Mapping at the U.S. Geological Survey

by Susan P. Benjamin, Research Geographer

  • Mapping the United States in the 19th century was arduous, dangerous work; flash floods, bears, and bandits were just a few hazards
  • By the mid-20th century, aerial photography, photogrammetry, and stereophoto pairs, allowed