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NASA measures a lunar crater’s size by combining calibrated images with elevation data—not by counting pixels without context. For an image measurement, scientists define which edge counts as the crater boundary, measure across it, and convert that image distance to a ground distance using the image scale. For broader catalogs and studies of crater shape and depth, NASA also uses elevation measurements from the Lunar Orbiter Laser Altimeter (LOLA).
How an image measurement becomes a real-world diameter
A ruler or pixel count gives a distance on an image, not the crater’s diameter on the Moon. To calculate a ground distance, the image must have a known scale, such as meters per pixel or kilometers per millimeter on a printed page.
- Establish the image scale. Find the ground width represented by a known image distance, using the image’s stated resolution or another documented measurement.
- Choose and mark the boundary. Decide which visible edge will define the crater’s diameter, then measure across the crater from one side to the other using that same boundary consistently.
- Convert the image distance. Multiply the measured image distance by the ground scale. The result is a diameter in meters or kilometers, depending on the scale used.
NASA’s Lunar Math, Version 3 gives a classroom example: an image representing 16.6 kilometers across measures 134 millimeters on the page, which works out to about 124 meters per millimeter. A measured distance of 5 millimeters across a feature in that image would therefore represent about 620 meters on the Moon. The conversion is only as meaningful as the image scale and the boundary selected.
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Why the crater edge changes the answer
A crater does not always have one crisp line that every observer would identify as its edge. A report might measure the rim crest, the inner rim, or the outer wall edge; each boundary can produce a different diameter. A useful size report should state which edge was measured rather than presenting a number as if the boundary were self-evident.
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NASA’s Lunar Math answer key illustrates the difference with Alphonsus. In that image-based educational exercise, one edge choice gives an estimate of about 102 kilometers, while measuring to the outermost crater-wall edge gives about 132 kilometers. These are classroom estimates from the exercise’s image, not a replacement for an authoritative catalog measurement.
What image resolution and field width tell you
Image scale helps readers judge what a measurement can show. A narrow view can reveal local features in detail, while a wider view provides more landscape context; the stated resolution or ground width is essential for interpreting either one.
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- NASA describes an LROC Narrow Angle Camera frame of Concentric Crater as 510 meters wide. NASA’s image page identifies the frame’s width.
- A NASA LROC Wide Angle Camera mosaic of Mendeleev Crater is described as 100 meters per pixel and 313 kilometers across. Those figures belong to that specific mosaic and crater. See NASA’s Mendeleev image page.
These examples use different image products and scales. The numbers should not be treated as a single standard resolution for all lunar imagery.
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Images show visible surface features; LOLA provides elevation measurements. NASA uses LOLA topography to support work that goes beyond the visible outline, including large-crater catalogs and analysis of crater shape and depth-to-diameter relationships.
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NASA’s “Counting Craters on the Moon” visualization depicts large lunar craters above 20 kilometers in diameter and says LOLA data were used for a comprehensive catalog of large craters. That threshold describes the visualization’s scope, not a limit on which lunar craters can be studied.
NASA’s Drygalski Crater page describes high-resolution LOLA topography as a way to refine depth-to-diameter relationships and constrain crater shape. Such relationships vary with crater type and planetary-body properties, so a single depth-to-diameter ratio should not be assumed to apply to every lunar crater.
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Diameter and depth are different measurements
Diameter describes a distance across the crater’s surface; depth describes its vertical relief. They can rely on different evidence. For a specific example, NASA reported that images of the Falcon 9 upper-stage impact site showed a rim that helped scientists measure the crater at 60 feet wide, while its depth was estimated from shadow length at less than 10 feet. Those figures refer to that impact and that shadow-based estimate, not to a universal NASA procedure for measuring crater depth. See NASA’s August 18, 2026 report.
How to interpret a reported crater size
When comparing crater measurements, check what the number actually describes. A diameter measured from an image, a topographic analysis, and a classroom estimate may answer related but different questions.
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- Method: Was the size inferred from a calibrated image, or derived using elevation data?
- Boundary: Does the measurement use the rim crest, inner rim, outer wall, or another mapped edge?
- Scale: What ground width or resolution applies to the image?
- Dimension: Is the figure a surface diameter or width, or a depth inferred from topography or, in a particular case, a shadow?
- Scope: Is it one crater measured in an image, an educational exercise, or part of a catalog?
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