There are several different types of coordinate chart which are adapted to this family of nested spheres, each introducing a different kind of distortion. The best known alternative is the Schwarzschild chart, which correctly represents distances within each sphere, but (in general distorts radial distances and angles). Another popular choice is the isotropic chart, which correctly represents angles (but in general distorts both radial and transverse distances). A third choice is the Gaussian polar chart, which correctly represents radial distances, but distorts transverse distances and angles. In all three possibilities, the nested geometric spheres are represented by coordinate spheres, so we can say that their roundness is correctly represented.
In a Gaussian polar chart (on a static spherically symmetric spacetime), the line element takes the form
Depending on context, it may be appropriate to regard f, g as undetermined functions of the radial coordinate. Alternatively, we can plug in specific functions (possibly depending on some parameters) to obtain an isotropic coordinate chart on a specific Lorentzian spacetime.
Gaussian charts are often less convenient than Schwarzschild or isotropic charts. However, they have found occasional application in the theory of static spherically symmetric perfect fluids.
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