Prior to GPS and InSAR, scientists used techniques from geodesy, a branch of Earth science that deals with Earth's shape. Thanks to numerous ancient Greek scholars from around 2,500 years ago, we know that Earth isn't flat (really!). And thanks to the work of countless geodesists since then, we know that Earth isn't exactly round, either. Our planet is actually a bumpy oblate spheroid—squashed at the poles and swollen at the equator as a result of its spin, and slightly irregular as a result of its uneven distribution of mass.

Earth's bumpy shape even changes over time in response to subsurface processes like magmatism and mountain building. Geodesists refer to the changes as deformation.

Leveling is a geodetic technique used to measure vertical deformation – surface uplift or subsidence. It's decidedly low-tech compared to GPS or InSAR, but it played a pivotal role in our understanding of Yellowstone.

Using a surveying instrument called a level and a pair of graduated staffs (imagine two very long rulers held vertically with the level positioned on a tripod between them), a leveling crew measures elevation differences between a starting benchmark and a series of temporary marks, keeping track of the total until the next benchmark is reached and its elevation is established. By repeating the process, elevations of an entire network of benchmarks are determined. At some later date, a second survey can detect any changes that occurred in the interim.

The first leveling survey in Yellowstone was done almost a century ago, in 1923. Some of the original benchmarks still survive, and in all there are now more than 500 benchmarks in Yellowstone National Park.

More than 50 years passed before the next leveling survey was done in response to the 1975 magnitude 6.1 Yellowstone Park earthquake. An astounding pattern emerged, much beyond anything that could be explained by the quake. The central part of the caldera had been uplifted more than 72 centimeters (28 inches) since the 1923 survey, at an average rate of 1.4 centimeters per year (0.5 inch per year). Yellowstone was rising!

Annual leveling surveys starting in 1983 showed that uplift continued until 1985, when a strong earthquake swarm near the northwest rim of the caldera coincided with a reversal to subsidence. Scientists attributed the subsidence and swarm to magmatic or hydrothermal fluids leaking out of the caldera, relieving pressure that built during the preceding decades of uplift. Subsidence at rates of 9–35 millimeters per year (0.3–1.4 inches per year) continued for the next decade.

Starting in 1995, things got more complicated and a lot more interesting. Part of the caldera floor started rising again while another part continued to sink. A new area of uplift was discovered and for a time it rose faster than ever measured before or since. And scientists began to recognize connections between various sources of deformation, and between deformation episodes and earthquake swarms.

But those discoveries were made using GPS and InSAR, and will describe those revelations in future articles (so stay tuned!). The last Yellowstone leveling survey was completed in 2007. Leveling's heyday had passed, but not before advancing our understanding of one of Earth's most dynamic calderas.