The United States Geological Survey has been mapping the country since the 1880s. Over more than a century of work, they produced hundreds of thousands of individual map sheets — each one a precisely bounded rectangle of terrain carefully measured and drawn.
To keep this enormous project organized, the USGS divided the country into a standardized grid. Every map covers exactly one cell of that grid, called a quadrangle. Understanding how that grid works makes it easier to find the map you’re looking for — and helps explain why maps from different series can look and feel so different from one another.
What Is a Quadrangle?
A quadrangle is a rectangular section of the Earth’s surface, defined by lines of latitude and longitude. Latitude runs north and south; longitude runs east and west. Both are measured in degrees, and each degree contains 60 minutes.
A map described as a “7.5-minute” sheet covers 7.5 minutes of latitude and 7.5 minutes of longitude — an area roughly 7 by 9 miles across in the continental US. A 15-minute map covers twice that. A 30-minute map covers four times as much. The number in the name tells you how large a piece of the Earth the sheet captures.
More Area Means Less Detail
A larger area has to fit onto the same size sheet of paper. That means less room for fine detail.
A 7.5-minute map can show individual buildings, fence lines, small trails, and the precise course of a stream. A map covering ten times as much ground has to zoom out — and in doing so, loses that precision. Major roads and rivers remain, but the granular detail that makes a map useful for on-the-ground navigation disappears.
Cartographers describe this relationship as scale. A 7.5-minute map has a scale of 1:24,000 — one inch on the paper represents 24,000 inches on the ground. A map covering more area uses a smaller scale ratio, which means less detail per inch. See our Map Scale guide for a fuller explanation.
Why So Many Different Series?
Different eras and different needs produced different series. In the early twentieth century, 15-minute and 30-minute maps were the workhorse series for most of the country — detailed enough to be useful, practical to produce with the survey methods of the time. As technology improved and demand for precision grew, the USGS shifted focus to the more detailed 7.5-minute series, which gradually became the standard and ultimately covered the entire continental United States by 1992.
The older series were not updated as new coverage was completed. They were superseded and eventually discontinued — leaving behind a rich historical record of how the country was mapped, decade by decade, in successive waves of survey work. How those surveys were actually conducted — from ground crews with measuring chains to aircraft carrying cameras — changed dramatically over the course of that history. See our Survey Methods guide for more on that story.
Series at a Glance
The table below summarizes the series we carry. Each has its own article with more detail on history, characteristics, and what to look for as a collector or buyer.
| Series | Area per Sheet | Scale | Detail Level | Production Years | Quad Count | US Coverage |
|---|---|---|---|---|---|---|
| 60-Minute | 55 × 69 miles | 1:250,000 | Lowest | 1885–1915 | 85 | 10% |
| 30-Minute | 27 × 35 miles | 1:125,000 | Moderate | 1889–1945 | 1,000 | 30% |
| 15-Minute | 14 × 17 miles | 1:62,500 | High | 1890s–1960s | 6,800 | 50% |
| 7.5-Minute | 7 × 9 miles | 1:24,000 | Highest | 1940s–1992 | 54,000 | 100% |
| 100K | 55 × 35 miles | 1:100,000 | Moderate | Mid 1970s–1986 | 1,800 | 100% |
| 1×2 Degree | 110 × 69 miles | 1:250,000 | Low | 1950s–1980s | 500 | 100% |
| Special | Varies | Varies | Varies | Varies | — | Localized |