Plate-Packet-Introduction - Sloan Digital Sky Survey 3D Mapping

Aug 21, 2026

Sloan Digital Sky Survey (SDSS) maps the night sky in three dimensions and explains why modern astrophysics relies on distance measurement to uncover the universe’s large-scale structure beyond the Milky Way. The text contrasts flat mapmaking with topographic elevation data, then transfers the idea to astronomy using redshift. It distinguishes Doppler shift from cosmological redshift, links cosmological expansion to wavelength stretching over time, and shows how redshift enables converting observational 2D views into more accurate 3D distributions and velocity maps of galaxies.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages4

Reading Time10 min

AccessFree to download

Mapping the Universe in Three Dimensions

A Different Kind of Map

Making a Three-Dimensional Map of Space

Redshift

From Redshift to Distance

Mapping Velocity Curves of Galaxies

Mapping the Universe in Three DimensionsYour Piece of the Sloan Digital Sky The Sloan Digital Sky Survey is now in its 4th generation of mapping the night sky in three dimensions. Why do scientists care so much about mapping the universe? How DO you map objects in space, and what does this aluminum plate with all the holes in it have todo with it? With your own piece of the SDSS survey in your possession, keep reading and exploring to discover what modern astrophysics and engineering are doing to uncover the secrets of the universe from here on Earth. A Different Kind of Map Our familiar understanding of a map, whether we use a phone, GPS unit, or a large paper versio, is as a tool forgetting from point A to point B. It tells us what lies inbetween points A and B and sometimes describes the terrain, but if we want to get a sense of the land elevations and geographic features, we need more information. This information is found on a topographic map. In order to describe the additional Google Maps dimension of elevation on a two-dimensional surface, we need elevation information at many points. Points of equal elevation are connected to one another; these connections create contour lines that reveal information from which geologists can see the geography of an area and infer its geologic history. The same types of maps exist in astronomy. If you want to know where in the sky to point a telescope, a two-dimensional view is all you need. If, however, you want to know where objects are actually placed in three-dimensional space, you need to collect distance information, and then you need to devise a way to display that information. Making a Three-Dimensional Map of Space In the same way that cartographers measure elevation information to create a topographic map on Earth, astronomers need distance information to stars and galaxies to make three-dimensional maps of space. The Sloan Digital Sky Survey (SDSS) is concerned with a specific kind of map, one that reaches out beyond our Milky Way to the large-scale structure of the universe itself. For this we need a long distance measuring tool called redshift . Redshift When light from astronomical objects is observed, the wavelengths are often measured as being longer than expected. Red has the longest wavelength in the visible spectrum, so this effect is called redshift. There are two ways to think about redshift: Doppler shift and Cosmological Redshift . These two effects both lengthen wavelengths that are emitted from an astronomical object, but in two very different ways. Doppler Shift Doppler shift is the effect that you can hear when a fast car passes by you. As it moves towards you, the sound waves in front of the car are compressed (making the pitch higher), whereas when it moves away from you, the sound waves are stretched out, making the pitch lower. The same effect can be seen with waves of light: if an object is moving quickly towards you then the wavelengths will be compressed, making the object appear more blue, similarly, if an object is moving away from you at a velocity comparable to the speed of light then the wavelengths will stretch out, making the object appear more red. Everyday objects such as cars do not move fast enough for us to see a shift in the wavelength of their light. Galaxies, however, do move fast enough relative to Earth that we can measure a redshift. Cosmological redshift The other effect that can cause wavelengths to be lengthened is cosmological redshift, this is an effect due to the expansion of the universe. The Universe is expanding isotropically. This means that, on a large enough scale, everything in the universe is moving away from everything else. In 2D, this is analogous to an infinite rubber sheet being stretched, with the galaxies represented by coins glued onto this sheet. As the sheet expands, all of the coins will move away from each other. The diagram below shows a section of the universe (the grey

FAQs

What is the Sloan Digital Sky Survey and what does “three-dimensional mapping” mean here?

The SDSS is the fourth generation effort to map the night sky in three dimensions. It reconstructs spatial placement in the universe by using distance information derived from redshift measurements.

How is redshift related to mapping objects in space?

Observed light wavelengths are measured as longer than expected, and redshift provides a link between what the light tells us and how far away an object is. With knowledge of the universe’s expansion, redshift helps infer travel time and distance.

What is the difference between Doppler shift and cosmological redshift?

Doppler shift results from motion toward or away from the observer, changing wavelengths like the pitch of a sound from a passing car. Cosmological redshift arises from the universe’s expansion stretching wavelengths as light travels through space over time.

Aug 21, 2026

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